BACKGROUND

Technical Field

The invention relates to a mirror unit according to the preamble of claim 1 , a multi-mirror array comprising multiple mirror units and an optical system comprising at least one multi-mirror array.

Prior Art

Microlithographic projection exposure methods are used for producing semiconductor components and other finely structured components, e.g. masks for microlithography. Use is made of masks (reticles) or other patterning devices carrying or forming the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object plane of the projection lens and illuminated with an illumination radiation provided by the illumination system. The projection lens images the pattern onto the substrate to be exposed, which is coated with a radiation-sensitive layer and the surface of which lies in the image plane of the projection lens, said image plane being optically conjugate with respect to the object plane.

In an effort to produce ever finer structures, optical systems have been developed in recent years which operate with moderate to medium numerical apertures and achieve high resolution capabilities substantially by means of the short wavelength of the used electromagnetic radiation from the extreme ultraviolet range (EUV), in particular having operating wavelengths in the range of between 5 nm and 30 nm.

Radiation from the extreme ultraviolet range cannot be focused or guided with the aid of refractive optical elements, since the short wavelengths are absorbed by the known optical materials that are transparent at higher wavelengths. Therefore, mirror systems are used for EUV lithography. Mirrors with multilayered coatings (multilayer mirrors) are used for normal or almost normal incidence.

In general, depending on the type of structures to be imaged, different illumination modes (so- called illumination setting) are used, which can be characterized by different local intensity distributions of the illumination radiation in a pupil surface of the illumination system. Determining the angular distribution of the illumination light in the illumination field. In general, an illumination system has a pupil shaping unit for receiving radiation from a primary radiation source and for generating a two-dimensional intensity distribution that can be set in a variable fashion in the pupil surface of the illumination system.

Some concepts provide pupil shaping units including one or more controllable mirror arrays in the form of a multi-mirror array (MMA) having a multiplicity of individual mirror elements which can be tilted independently of one another to set and change the angular distribution of the radiation incident on the totality of the mirror elements in a targeted manner such that the desired spatial illumination intensity distribution arises in the pupil surface.

Examples are disclosed in US 2017/0363861 A1. The document discloses multi-mirror-arrays (MMA) composed of multiple mirror units each having an individual mirror element. A mirror unit comprises a displacement device for pivoting the mirror element with two degrees of freedom of pivoting. A mirror unit includes an electrode structure having actuator electrodes configured as comb electrodes arranged in a common plane and forming a direct drive for pivoting the mirror element. A suspension system supporting the mirror element has Cardan-type properties providing a high stiffness in non-actuated degrees of freedom and low stiffness of the actuated degrees of freedom. The displacement device may be designed as a micro-electro-mechanical system (MEMS).

The demands of the semiconductor market require further development of the EUV microlithographic projection exposure systems. One seeks to continuously improve throughput and resolution. The first challenge can be achieved by employing higher power EUV sources and by increasing the transmission. An improved resolution may be obtained by EUV systems with higher NA (such as NA order of 0.33 or even 0.55) and increased flexibility of the illumination settings.

Therefore, there is a need for controllable mirror arrays and components thereof capable of operating with high precision and sufficient flexibility even under the conditions prevailing in high resolution, high throughput systems currently being developed

OBJECT AND SOLUTION

It is an object of the invention to provide a mirror unit in accordance with the preamble of claim 1 capable of operating in an EUV optical system with high precision and sufficient flexibility even under the conditions prevailing in high resolution, high throughput systems currently being developed. A large tilt range shall be provided, preferably with maximum tilt angles exceeding 100 mrad. A further object is to provide a mirror unit having high reflectivity which essentially does not degrade with time of use.

As a solution, the invention according to one formulation provides a mirror unit comprising the features of claim 1 , a multiple-mirror array (MMA) according to claim 11 and an optical system according to claim 12.

Advantageous developments are specified in the dependent claims. The wording of all the claims is incorporated by reference in the content of the description.

According to one formulation of the invention, the invention provides a mirror unit comprising a base element, a mirror element, an actuation system and a flexible suspension system arranged between the base element and the mirror element. The mirror element comprises a mirror substrate and a reflective coating configured to reflect extreme ultraviolet (EUV) radiation on a first surface of the mirror substrate. Preferably the reflective coating comprises a reflective multilayer system. The flexible suspension system provides a mechanical connection between the base element and the mirror element and allows relative movement of the mirror element thanks to the built-in flexibility of the suspension system. The actuation system is configured to tilt the mirror element relative to the base element in response to control signals.

The flexible suspension system is configured to allow tilting the mirror element relative to the base element essentially in two degrees of freedom only. The degrees of freedom are predominantly or exclusively rotational degrees of freedom allowing pivotal or tilting movements of the mirror element relative to the base element about respective (virtual) tilt axes. In an ideal system only two rotational degrees of freedom about mutually orthogonal fixed axes would be desirable. Practically, the suspension system is configured to provide a relatively high stiffness in undesired degrees of freedom and a relatively low stiffness in desired degrees of freedom so that the allowed degrees of freedom are favoured over the “forbidden” degrees of freedom when actuating forces and moments act on the mirror element. Only parasitic movements in undesired degrees of freedom are possible, such as a translational movement of the mirror element. In other words, a flexible suspension system according to the claimed invention shall provide high stiffness contrast. Flexible suspension systems featuring these properties are also denoted as Cardan-type suspension since both allow relative rotation of connected parts about two mutually orthogonal rotational axes only while blocking other degrees of freedom. Movements of the mirror element relative to the base element can be generated by an actuation system configured to tilt the mirror element relative to the base element in response to control signals. Once a mirror element is tilted away from its equilibrium position the flexible suspension system generates restoring forces effective to bring the mirror element back into equilibrium position.

The suspension system comprises first connector elements connected to the base element, second connector elements connected to a second surface of the mirror element (which may be opposite to the first surface) and elastic spring portions arranged between the first and second connector elements.

According to a formulation, an elastic spring portion of the suspension system is made of an elastic solid composite material combining at least one volume portion made of a first material and at least one volume portion made of a second material comprising a thermal conductivity differing from the thermal conductivity of the first material, wherein one of the first and second material is crystalline silicon (cr-Si).

An “elastic solid composite material” is a material which is produced from two or more constituent materials which, in the present case, have notably dissimilar properties at least regarding thermal conductivity. The materials are merged to create a spring portion material with properties unlike that of the individual constituents. In a composite structure, the individual constituent materials remain separate and distinct, unlike mixtures or solid solutions. In other words, a composite material is a structure made of two or more materials, which occupy different space, such as in a multi-layered structure.

In this application the phrase “the material is crystalline silicon” is to be interpreted as a material which substantially consists of crystalline silicon. This includes pure crystalline silicon (including only unavoidable impurities) as well as modifications of crystalline silicon purposely including higher amounts of one or more second material(s), e.g. obtained by doping the crystalline silicon for the purpose of modulating its electrical and structural properties. In any case mechanical properties of crystalline silicon dominate the elastic properties of this material.

Selecting crystalline silicon as one of the constituents of the solid composite material provides advantages both from a manufacturing point of view and from the point of view of relevant properties of the flexible suspension system. Utilizing crystalline silicon as one constituent facilitates manufacturing a mirror unit using established manufacturing processes known in the field of micro-electro-mechanical system (MEMS). Further, utilizing crystalline silicon may contribute to a desirable high thermal conductivity and reasonable stiffness of the spring portions. Combining crystalline silicon with at least one other material provides manufacturing degrees of freedom to optimize the properties of the suspension system both with respect to thermal conductivity and mechanical properties.

The (at least one) crystalline silicon volume portion may be dimensioned and designed so as to dominate the elastic properties of the composite suspension. Using crystalline silicon as one of the materials in the composite can prevent creep (plastic deformation), which would likely appear in metal-only suspensions. This improves long term stability of suspension properties. A composite suspension system according to this aspect can combine both elastic softness (low stiffness, low Young’s Modulus) in the allowed degrees of freedom and high thermal conductivity, thereby providing desirable properties for use of the mirror units in mirror systems where high tilt ranges are desirable and the mirrors are subject to significant thermal load.

Materials (one or more) to be combined with cr-Si to form a composite material are partly selected based on their thermal conductivity. The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by or k. The defining equation for thermal conductivity is q = - VT where q is the heat flux, is the thermal conductivity, and VT is the temperature gradient. Therefore, thermal conductivity is typically expressed in units W/m.K. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. The reciprocal of thermal conductivity is called thermal resistivity.

In this application, a material exhibiting a thermal conductivity higher than the thermal conductivity of monocrystalline silicon under otherwise identical conditions will be denoted as “high thermal conductivity material”, abbreviated as HTC material, or as low thermal resistance material. A material exhibiting lower mechanical stiffness than monocrystalline silicon under otherwise identical conditions will be denoted as “low stiffness material” abbreviated as LS material, or as “softer material”

A composite suspension may combine elastic softness (low stiffness, low Youngs Modul) in directions defining desired degrees of freedom and high thermal conductivity. The following remarks may help to better understand the background of this aspect of the invention.

Inventors carefully analyzed current and expected future conditions posing restraints on developing components for high performance EUV mirror systems. As mentioned above, there is no refractive optics for 13nm or other EUV wavelengths. The only way is to use EUV mirrors, which nevertheless absorb a significant amount of the in-band irradiation and large portion of the accompanying IR light, resulting in a large thermal load at the mirrors. The EUV radiation sources are continuously developing, the EUV power increases and the thermal load for the future systems will also increase. EUV optics need to be operated in vacuum or under low- pressure H2 atmosphere, since gasses absorb EUV. The dominating cooling mechanism is the solid-state heat conduction. The fight for higher NA and the demand for flexibility for the illumination settings require relatively large two-dimensional tilt ranges for tiltable mirror elements. Tilting ranges greater than 100 millirads (mrad) or even greater than 120 mrad (1 radian = 1 rad = 1807K) are considered desirable.

Focusing on developing suitable mirror units the above boundary conditions impose challenges regarding appropriate actuator and suspension systems. The small size of the single mirror elements and the need for independent actuation of the mirror elements limit the options for actuators: they must fit in the available space and provide sufficient force/torque. There is a preference for the actuation to be linear, as side-effects like hysteresis, creep and temperature dependence are undesired. Feasible candidates for actuation systems appear to be the electrostatic combs (see e.g. US 2017/0363861 A1) and probably thin-layered piezo actuators with additional temperature sensing and compensation.

Inventors first conclusion is that one key to a proper solution is to make the mirror's suspension soft enough to achieve the necessary tilt range with the available actuators' force.

However, thermal challenges need to be considered, too. The mirror surface is typically coated by special EUV-reflective coating, such as a multilayer system, which shall provide high reflectivity for the lifetime of the scanner (many years). Higher temperatures tend to accelerate the formation of defects and loss of reflectivity, thus the mirror should be kept below some reasonable temperature limit. As mentioned above, solid-state conduction is the dominating mechanism for cooling of the mirror. The absorbed heat has to propagate a long path up to parts of the MMA which can be actively cooled to a fixed low temperature. It is contemplated that the flexure (spring portion) is the mirror's part with highest thermal resistance acting as a “bottle neck” for heat flow.

Therefore, inventors second conclusion is that it is desirable to reduce as much as possible the thermal resistance of the mirror's suspension to be able to conduct the absorbed heat away from the tiltable mirror element. A mirror unit with a low thermal resistance enables minimizing the temperature of the reflective layer thereby preventing degradation. The requirements of (i) providing a reasonably soft suspension system which, at the same time,

(ii) provides good heat conduction somehow contradict each other. In other words: inventors identified as a general problem posed by the large tilt range mechatronic systems, which must work under high thermal load (like in the novel EUV lithographic systems), is how to design an appropriate suspension. The suspension preferably should be:

(i) soft enough to assure the tilt range. This would favor a structure to be long, thin and narrow, possibly with lower Young's modulus and higher fracture strength (stress at which it breaks)

(ii) well thermally conductive. This would favor a structure to be short, thick and wide, and with higher thermal conductivity

(iii) made from EUV compatible material (not prohibited for being used in an EUV environment, e.g. low outgassing), and its structuring technique should be compatible with established MMA fabrication technology

(iv) stable to keep the elastic properties for the Scanner lifetime, which might exclude most metals exhibiting creep (plastic deformation after long deformation)

A suspension system including composite elastic spring portions including volume portions made of crystalline silicon (cr-Si) can provide better overall performance in view of these requirements.

Choice of material combination and structural layout of the spring portions should be made in view of the desired Cardan-type functionality of the suspension system. The suspension system of Cardan-type is designed to provide a high stiffness in non-actuated degrees of freedom and low stiffness in actuated degrees of freedom. Specific conditions regarding the shape of the spring portion can provide am major contribution to achieving this functionality.

In preferred embodiments a shape of the composite elastic spring portion is flat with a length measured in a length direction, a width measured in a width direction perpendicular to the length direction and a thickness measured between a first surface and a second surface of the flat shape in a thickness direction perpendicular to a plane defined by the length and width direction, the thickness being less than 10%, preferably less than 1% of the length and width dimensions. In absolute terms, the thickness may be 5 pm or less or 1 pm or less. The length may be significantly shorter than the width, e.g. by a factor of two or more, or five or more.

In mirror units having square mirror elements about 1 mm in edge length spring portions may have a width in the range from 100 pm to 400 pm and/or a length in the range from 20 pm to 100 m and/or a thickness in the range from 0.5pm to 3 pm, for example. Other mirror sizes are possible, edge lengths may be in the range from 0,5 mm to 10 mm, for example

It has been found beneficial if the elastic spring portion includes one or two very thin layers of cr-Si. A cr-Si layer may have a thickness of less than 1.5 pm, for example 1.0 pm or less. The thickness may be 0.1 pm or more, or 0,2 pm or more. A total thickness variation should be ± 5% or less, if possible. Preferably, a total thickness variation (TTV) is 10% or less.

The usage of ultra-thin Si layer(s) without substantial thickness variation allows further optimization of the spring portions with respect to durability (long term stability, no fatigue), flexibility (strong bending possible without breakage) and flexural softness (facilitating large tilt angles), for example.

A flat elastic spring portion may act as an elastically deformable flexure providing a rotational degree of freedom about an axis in the width direction while higher mechanical resistance exists in other degrees of freedom. The elastic spring portion may be configured as and function as a flexure bearing. Preferably, the flexure bearing is engineered to be compliant substantially in one angular degree of freedom only. The flexure may be compliant in the length direction while being stiffer in the width direction. A rotational degree of freedom provided by the spring portion may be oriented parallel or essentially parallel to the width direction, while other degrees of freedom of movement are essentially blocked.

An elastic spring portion may also be integrated into the suspension system so as to act as a torsion spring.

Connector elements may be formed integral with the elastic spring portion. A connector element may be a portion having the same width and thickness dimensions as the spring portion. Preferably, a connector element is thicker than the spring portion to allow keeping a distance between the spring portion and the component connected to the connector element. A connector element may be a rigid body of material providing a fixed positional relationship between an end of a spring portion and he connector element attached thereto.

The flexible suspension system may include a plurality of spring portions, e.g. a total of four spring portions. A spring portion connecting elements fixed at its longitudinal ends may be split into two or more sub-spring portions acting in parallel. In preferred embodiments the suspension system comprises two connecting element groups each comprising: a base connecting element fixedly connected to the base element, a first spring portion fixed with one end to the base connecting element and with an opposite end to a rigid interconnection element, the rigid interconnection element, a second spring portion fixed at one end to the rigid interconnection element and at an opposite end to a mirror connection element fixedly connected to the mirror element. The first spring portions provide first rotational actuation axes and are formed and arranged such that the first rotational actuation axes are coaxially aligned with each other, and the second spring portions provide second rotational actuation axes and are constructed and arranged in such a way that the second rotational actuation axes are aligned coaxially with one another and orthogonally with respect to the first rotational actuation axes.

Crystalline silicon has unique physical properties: a crystal with cubic symmetry, semiconductor with possibility for integration of ICs and other electric/electronic devices, with high thermal conductivity, high elasticity, high fraction strength, long lifetime practically without exhaust or degradation of the electrical and mechanical properties, no creep or hysteresis by deformations, for example. There are developed methods for structuring by deposition, changing of the doping/conductivity, isotropic and anisotropic etching of thin layers and in bulk with (sub)-nm accuracy.

The microstructure of the crystalline silicon material may be polycrystalline. In many embodiments the crystalline silicon material is monocrystalline silicon (sr-Si). Due to the absence of grain boundaries the thermal conductivity of the monocrystalline material may be significantly higher than that of the polycrystalline silicon, e.g. by a factor of about three, thereby reducing heat flow resistance.

Alternatively or in addition it is preferred if a difference in thermal conductivity between the first material and second material is at least 50 W/m.K, preferably more than 100 W/m.K. This enables obtaining high overall thermal conductivity of the composite structure forming the composite spring portion. The HTC material may be a metal, preferably a noble metal such as Au, Ag, or Cu or an alloy having the respective metal as the major constituent.

In some embodiments a material distribution of dissimilar materials (cr-SI and at least one other elastic thermally conductive material) within the elastic solid composite of the elastic spring portion is symmetric with respect to a center plane of the elastic spring portion between the first surface and the second surface. This contributes to obtaining a self-balanced layout not exhibiting active bending when temperature changes occur. In some embodiments the elastic spring portion comprises a sandwich structure comprising a middle layer, a first enveloping layer on a first side of the middle layer and a second enveloping layer on a second side of the middle layer opposite to the first side of the middle layer.

In some embodiments having sandwich structure the middle layer is made of crystalline silicon and each of the enveloping layers is made of material other than crystalline silicon. This type of structure may be briefly denoted as (x| cr-Si | x) structure in this application, where “x” represents a layer material other than crystalline silicon (cr-Si).

In alternative embodiments having sandwich structure the middle layer is made of a material other than crystalline silicon and each of the enveloping layers is made of crystalline silicon. This type of structure may be briefly denoted as (cr-Si | x | cr-Si) structure in this application, where “x” represents a layer material other than crystalline silicon (cr-Si).

According to another formulation of embodiments with sandwich structure the elastic spring portion may include a middle layer of elastic thermally conducting material, and two enveloping layers of another elastic and thermally conducting material, one of which materials is crystalline silicon.

A multilayer structure designed as a sandwich structure including at least three layers with dissimilar materials may be designed to have relatively stable geometry even under high thermal load and under temperature changes. In contrast, bimorph combinations (two-layer composites) are expected to curve and bend upon change of temperature due to the material and stress mismatch. Thus, a bimorph system changes its stiffness in the desired and the parasitic directions, the eigenfrequencies. All these parasitic effects, especially when the geometrical and fabrication tolerances are considered, may have negative influence on the pointing direction and stability. A sandwich structure may be designed to largely avoid disadvantages of bimorph systems.

In some embodiments the sandwich structure is configured symmetrically with respect to a center plane of the middle layer. This contributes to obtaining a self-balanced layout not exhibiting active bending when temperature changes occur. Symmetry may be obtained particularly if the first enveloping layer and the second enveloping layer are made of the same material and have the same thickness. In some embodiments the first enveloping layer directly contacts a first surface of the middle layer and the second enveloping layer directly contacts a second surface of the middle layer opposite to the first surface. A three-layer sandwich (sandwich consisting of exactly three layers) is thereby obtained. Manufacturing may be facilitated if this simple structure is used.

As an alternative, one or more intermediate layers may be interposed between the middle layer and an enveloping layer, for example selected to enhance adhesion and/or to create an electrical connection and/or to provide a thermal shortcut. A thickness of an intermediate layer may be small compared to the thickness of the middle and enveloping layers, for example 20% or less or 10% or less of the thickness of the middle layer. Intermediate layers may be thin enough so that the overall properties of the sandwich are still dominated by the nature of the middle layer and enveloping layers. An intermediate layer material may be selected from the group consisting of chromium (Sr), titanium (Ti), gold (Au), Platinum (Pt), Copper (Cu), Nickel (Ni), for example.

Various modification of sandwich structures having specific advantages are possible.

In some embodiments the middle layer is made of crystalline silicon and the first and second enveloping layers are made of a material having a thermal conductivity higher than the thermal conductivity of crystalline silicon. The material of the enveloping layer may be selected from a group consisting of Au, Al, Cu, Ag, poly-diamond, nano-diamond, for example.

In other embodiments the middle layer is made of a material having a thermal conductivity higher than the thermal conductivity of crystalline silicon and the first and second enveloping layers are made of a made of crystalline silicon, wherein preferably the material of the middle layer is selected from a group consisting of Au, Al, Cu, Ag, poly-diamond, nano-diamond.

When the material combined with cr-SI provides thermal conductivity better than that of cr-Si (HTC material), the high thermal conductivity material works as a thermal shortcut, conducting a larger portion of the thermal load through it. Thus, the thermal resistance of such composite suspension will be less and a maximum temperature also lower than in reference suspension having the same dimensions but made of crystalline silicon only. The high thermal conductivity material may be selected from the group consisting of Au, Al, Cu, Ag, poly- or nano-diamond, etc. or alloys based on Au, Al, Cu, Ag. The concept of utilizing spring portions made of a composite material provides further constructional degrees of freedom allowing to taylor the mechanical stiffness of the spring portions.

In some embodiments, a mechanical stiffness of the material to be combined with cr-Si is lower than the mechanical stiffness of crystalline silicon (low stiffness material, LSM). In other words: the “partner” to cr-Si may be selected by its Young's modulus. The Young modulus E = o / E (or the modulus of elasticity in tension) is a mechanical property that measures the tensile stiffness of a solid material. It quantifies the relationship between tensile stress o (force per unit area) and axial strain E (proportional deformation) in the linear elastic region of a material. Young's moduli are typically expressed in gigapascals (GPa).

If the material partnered with crystalline silicon is also softer (smaller Young's modulus), the composite suspension with lower temperature resistance will have also less stiffness in comparison with a reference suspension having the same dimensions but made of Si only. In a sandwich structure, the flexure softening is relatively small when the softer material is in the middle and stronger when used for the outer enveloping layers.

Alternatively, or in addition, the composite suspension may be designed thicker than a cr-Si only reference one, or shorter than the reference one. Thus, its stiffness would be the same as that of the reference one, while the thermal resistance will be even lower (i) due to the better conducting material and (ii) due to the higher thickness I shorter thermal path.

Even if the material combined with cr-Si is harder (higher Young's modulus), but an extremely good thermal conductor, like e.g. film of poly- or nano-diamond (A=7001000 W/m.K), or graphene (A-5000 W/m.K), the composite suspension may be designed thinner. Thus, it will have comparable or slightly smaller stiffness, but significantly improved thermal behavior when compared to a cr-Si only reference suspension.

Using a composite suspension with cr-Si sublayer, thinner than that of a Si-only reference suspension will result in less stress in the Si sub-layer (the stress is proportional to the thickness I length ratio). Thus, the working conditions are more removed from the fracture limit, compared to that of the reference spring, and the lifetime will be longer.

Alternatively, or in addition, the composite suspension may be designed shorter, keeping the stress in the silicon the same as by the reference suspension, but achieving simultaneously less stiffness and less thermal resistance. The invention further relates to a multi-mirror-array (MMA) comprising multiple mirror units made according to the claimed mirror unit, and to an optical system comprising at least one multi- mirror-array of this type.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further features emerge not only from the claims but also from the description and the drawings, wherein the individual features can be realized in each case by themselves or as a plurality in the form of sub-combinations in an embodiment of the invention and in other fields. Exemplary embodiments are illustrated in the drawings and are explained in greater detail below.

Fig. 1 shows optical components of an EUV microlithographic projection exposure apparatus in accordance with an embodiment and a detail of a facet mirror comprising multiple mirror units;

Fig. 2A, 2B show in a perspective view of a mirror unit according to an embodiment (in Fig. 2A) and a perspective view of the suspension system in Fig. 2B;

Fig. 3 shows a schematic section of an elastic spring portion having a three-layer sandwich structure,;

Fig. 4A, 4B show in Fig. 4A a schematic of a reference spring portion made of Cr-Si oly and in 4B a three-layered spring portion with a composite sandwich structure according to an embodiment, both with relevant dimensions used in calculations;

Figs 5, 6 and 7 show schematically alternative design options of spring portions with composite structure;

Fig. 8 shows an exemplary process flow for MEMS-MMA with Si/x/Si suspension;

Fig. 9 shows an exemplary process flow for MEMS-MMA with x/Si/x suspension;

Fig. 10 shows an exemplary process flow to generate a mirror element. DETAILED DESCRIPTION OF EMBODIMENTS

Aspects of the disclosure relate to a flexible suspension system for high-tilt, high thermal load mechatronic systems, such as MEMS micro-mirrors for EUV illumination systems, and to mirror units including a suspension system of this type. Multiple mirror units may be combined to form a multi-facetted EUV mirror (multi-mirror array (MMA)) in which multiple mirror elements are arranged alongside one another and jointly form a composite mirror surface of the multi-mirror array.

EUV mirrors of this type can be used in various optical systems, e.g. in the field of EUV microlithography. Figure 1 shows by way of example optical components of an EUV microlithographic projection exposure apparatus WSC in accordance with an embodiment. The EUV microlithographic projection exposure apparatus serves for the exposure of a radiation-sensitive substrate W arranged in the region of an image plane IS of a projection lens PO with at least one image of a pattern of a reflective patterning device or mask M, said pattern being arranged in the region of an object plane OS of the projection lens.

The Cartesian xyz coordinate system facilitates description of respective positional relationships between the components illustrated in the figure. The projection exposure apparatus WSC is of the scanner type. During the operation of the projection exposure apparatus, the mask M and the substrate are moved synchronously in the y-direction and thereby scanned.

The apparatus is operated with the radiation from a primary radiation source RS. An illumination system ILL serves for receiving the radiation from the primary radiation source and for shaping illumination radiation directed onto the pattern. The projection lens PO serves for imaging the structure of the pattern onto a light-sensitive substrate.

The primary radiation source RS can be, inter alia, a laser plasma source or a gas discharge source or a synchrotron-based radiation source or a free-electron laser. Such radiation sources generate a radiation RAD in the EUV range, in particular having wavelengths of between 5 nm and 15 nm, such as 13.5 nm in the example. In order that the illumination system and the projection lens can operate in this wavelength range, they are constructed with components which are reflective to EUV radiation.

The radiation RAD emerging from the radiation source RS is collected by means of a collector COL and guided into the illumination system ILL. The illumination system comprises a mixing unit MIX, a telescope optical unit TEL and a field forming mirror FFM. The illumination system shapes the radiation and thereby illuminates an illumination field situated in the object plane OS of the projection lens PO or in the vicinity thereof. In this case, the shape and size of the illumination field determine the shape and size of the effectively used object field OF in the object plane OS.

A reflective reticle (mask M) or some other reflective patterning device is arranged in the object plane OS during the operation of the apparatus.

The mixing unit MIX substantially consists of two facet mirrors FAC1, FAC2. The first facet mirror FAC1 is arranged in a plane of the illumination system which is optically conjugate with respect to the object plane OS. Therefore, it is also designated as a field facet mirror. The second facet mirror FAC2 is arranged in a pupil plane of the illumination system that is optically conjugate with respect to a pupil plane of the projection lens. Therefore, it is also designated as a pupil facet mirror.

With the aid of the pupil facet mirror FAC2 and the imaging optical assembly which is disposed downstream in the beam path and which comprises the telescope optical unit TEL and the field forming mirror FFM operated with grazing incidence, the individual mirroring facets (individual mirrors) of the first facet mirror FAC1 are imaged into the object field.

The spatial (local) illumination intensity distribution at the field facet mirror FAC1 determines the local illumination intensity distribution in the object field. The spatial (local) illumination intensity distribution at the pupil facet mirror FAC2 determines the illumination angle intensity distribution in the object field.

The projection lens PO images the pattern arranged in the object plane OS of the projection lens in a reduced scale into the image plane IS that is optically conjugate to the object plane and lies parallel thereto. This imaging is performed by means of electromagnetic radiation from the extreme ultraviolet range (EUV) around an operating wavelength A = 13.5 nm.

The exemplary projection lens PO has six mirrors M1 to M6 having mirror surfaces which are arranged in a projection beam path PR between the object plane OS and the image plane IS in such a way that a pattern arranged in the object plane or in the object field OF can be imaged into the image plane or the image field IF by means of the mirrors M1 to M6. Other mirror numbers are possible, e.g. four mirrors or eight mirrors. The mirrors (EUV mirrors) M1 to M6 having a reflective effect for radiation from the EUV range each have a substrate, on which is applied a multilayer arrangement having a reflective effect for radiation from the extreme ultraviolet range and comprising a large number of layer pairs comprising alternately relatively low refractive index and relatively high refractive index layer material. The mirrors M1 to M6 each have curved mirror surfaces, such that each of the mirrors contributes to the imaging. The projection lens consists of two partial lenses. In this case, the first four mirrors M1 to M4 form a first partial lens, which generates an intermediate image IMI in the ray path between the fourth mirror M4 and the fifth mirror M5.

Projection exposure apparatuses and projection lenses having this or similar construction are disclosed for example in patent US 7,977,651 B2. The disclosure of said patent is incorporated by reference in the content of this description.

The field facet mirror FAC1 and the pupil facet mirror FAC2 each comprise a multiplicity of individual mirrors elements or groups of individual mirrors elements.

For example, the field facet mirror FAC1 is designed as a multi-mirror array (MMA) wherein the multi-mirror array (MMA) may include individual mirrors realized as micromirrors. In the example, The field facet mirror FAC1 is formed as a microelectromechanical system (MEMS) and incudes a multiplicity of individual mirrors units MU arranged in a matrix-like manner in rows and columns in a mirror array (see detail in Fig. 1). Other arrangements are possible, e.g. a hexagonal arrangement.

In some embodiments, the second facet mirror FAC2 arranged in a pupil plane of the illumination system may be designed as an MMA with tiltable micro mirrors.

Fig. 2A shows a perspective view of an exemplary mirror unit MU according to an embodiment with a part of the mirror substrate SUB cut away to show parts of the flexible suspension system FSS and actuation system AS. Fig. 2B shows a perspective view of the suspension system in Fig. 2A. Except for the layout of spring portions or bending springs the mirror unit may have similar design as the mirror unit shown in Fig. 10 of US 2017/0363861 A1, incorporated herein by reference.

Each mirror unit MU comprises a base element BE, a mirror element ME comprising a mirror substrate MS and a reflective coating comprising a reflective multilayer system RMS configured to reflect extreme ultraviolet (EUV) radiation arranged on a first surface S1 of the mirror substrate. The reflective multilayer system forms the mirror surface MS and comprises a large number of layer pairs comprising alternately relatively low refractive index and relatively high refractive index layer material (e.g. Mo-Si).

A flexible suspension system FSS is arranged between the base element BE and the mirror element ME. An actuation system AS comprising plural comb electrodes arranged between base element BE and mirror element ME is an integral part of the mirror unit and provides forces and moments to tilt the mirror element relative to the base element in two rotational degrees of freedom. Examples of suitable actuation systems are disclosed in US 2017/0363861 A1, which is hereby incorporated by reference regarding features of the actuation system. Other actuators may be used in addition or instead those examples.

The mirror units MU may be arranged on a common carrying structure making it possible to arrange essentially any number of mirror units or mirror elements such that an overall reflection surface is formed by the totality of all mirror elements. The MMA may include 1000 or more and/or 1000000 or less mirror units, or a number outside this range. The reflective faces of the mirror elements may be arranged side by side in such a way that they provide a substantially gap-free tessellation of a plane or curved surface.

The individual mirror elements ME are designed to be continuously tiltable by the actuator system AS. The tilt angles of the individual mirror elements ME are adjusted for changing the illumination settings. The tilt angles may have a displacement range of 50 mrad or more, or about 100 mrad or more. An accuracy of better than 0.2 mrad or better than 0.1 mrad or better than 0.05 mrad or better than 0.02 mrad may be achieved when setting the tilt position of the individual mirror elements ME, particularly when using a controlled system including a tilt angle sensor.

The flexible suspension system FSS is arranged between the base element BE and the mirror element ME and is mechanically connected to both. The flexible suspension system is configured to allow tilting of the mirror element ME relative to the base element BE in two rotational degrees of freedom predominantly about two mutually orthogonal tilting axes RA1 and RA2. The tilting action is initiated by the actuation system AS in response to control signals of a control unit controlling action of the facet mirror (MMA). The suspension system FFS is constructed to provide low stiffness (low mechanical resistance) in the desired degrees of freedom, whereas high stiffness is provided in those degrees of freedom which are not desired.

This layout makes it possible to tilt the mirror element ME about two mutually orthogonal rotational axes (or tilt axes) RA1 and RA2 located in the plane which also includes the suspension system FES and the actuation system AS and which cross at a center point CP of the mirror unit. Described in the more general way, the suspension system FSS comprises two first connector elements BC1, BC2 connected to the base element BE, second connector elements MC1, MC2 connected to the second surface (underside) of the mirror element ME, and total of four elastic spring portion SP arranged between first and second connector elements. More specifically, the suspension system FSS comprises two connecting element groups CEG1, CEG2 each basically consisting of five different functional portions attached to each other. The first connecting element group CEG1 comprises a base connecting element BC1 which is fixed to the top side of the base element BE and extends with its longer side essentially in a radial direction. The first spring portion SP1 is fixed with one of its longitudinal ends to the base connecting element. The opposite end of the first spring portion SP1 is fixed to an interconnecting element IE1 which is a plate-shaped element having a thickness to provide rigidity and two adjacent edges, orthogonal to each other. The first spring portion SP1 is attached to one of those edges. The other edge extending basically parallel to first rotational axes RA1 is fixedly connected to one longitudinal end of the second spring portion SP2. The opposite end of the second spring portion SP2 is fixedly attached to a first mirror connecting element MC1 which extends basically parallel to the tilting (or rotational) axes RA1 and attached, with its topside, to the second surface of the mirror element.

A second connecting element group CEG2 is arranged opposite to the first connecting element group CEG1 with respect to a plane diagonal between the actuation axes and is of similar construction so that a total of four spring portions are present in the suspension system.

Each of the spring portions SPx functions as an elastically deformable flexure providing a rotational degree of freedom about an axis parallel to its width direction W oriented radially to the center point CP. The length direction L is measured orthogonal to the width direction between the longer sides of the spring portions. The width of the spring portion is substantially larger than the length, for example by a factor of five or more. The spring portions are relatively thin, having a thickness measured in the thickness direction (T-direction) less than 1% of the length and width dimensions. In an example having an edge length of the mirror element ME of one 1 mm, the width may range between 100 pm and 400 pm, the length may range for example between 20 pm and 100 pm and the thickness may range from 0,5 pm to 3 pm, for example. Due to these dimensional conditions these spring portions SP are very soft and compliant to allow a rotation about the respective tilt axes, while high stiffness is provided in any direction other than the allowed tilting axes. Fig. 3 shows a schematic section of an elastic spring portion SP2 in Fig. 2A.. Each elastic spring portion is generally shaped as a flat rectangular spring having a length L (measured in radial direction between the central portion CP and the connector), a width W in a lateral direction perpendicular to the length direction and a thickness T in the direction perpendicular to the length and width direction. In some embodiments, length and width dimensions may be in the order of tens to hundreds of micrometers (pm) (for example between 20 pm and 400 pm) whereas the thickness may be a at least an order of magnitude smaller, for example only a few micrometers, such as between 0.5 pm and 10 pm. Owing to these dimensional conditions, each spring portion forms an elastic flexure being elastically compliant in only one angular degree of freedom (easy bending about an axis parallel to the width direction) and provides high stiffness in other degrees of freedom (e.g. rotation about an axis parallel to the width direction).

The development of multi-mirror arrays prepared by MEMS-technology has identified crystalline silicon (cr-Si) as a material suitable to manufacture suspension systems. For example, US 2017/0363861 A1 discloses suspension systems including torsion springs produced from highly doped monocrystalline silicon in a manufacturing process compatible with established MEMS manufacturing systems and providing high thermal conductivity and good electric conductivity.

Inventors contemplate that use of cr-Si alone may not be sufficient in future high throughput systems which may require increased flexibility with respect to illumination settings, for example. While high throughput may be obtained by using higher power EUV sources, high power EUV radiation will cause increased thermal loads on components including mirror units for MMAs. It may be difficult to keep mirror temperatures below 100°C, for example. Expected increase in temperature may induce additional negative physical effects resulting from the fact that the thermal conductivity of crystalline Si (4s/) decreases at higher temperatures. For example, Asi=149 W/m.K at room temperature (20°C), but about 120 W/m.K at 100°C and about 100 W/m.K at 200°C. Therefore, allowing the mirror temperature to increase unnecessary^ will worsen the thermal situation.

In view of anticipated challenges in constructing flexible suspension systems for high-tilt high thermal load mechatronic systems (like MEMS micro-mirrors for EUV illumination) it has been found beneficial to deviate from established manufacturing concepts for flexible suspension systems. As illustrated in Fig. 3, the elastic spring portion SP of the suspension system FSS is made of an elastic solid composite material combining (at least) two dissimilar materials in a multi-layer structure (including layers of at least two physically different materials). In the embodiment, the sandwich structure comprises a middle layer ML, a first enveloping layer EL1 arranged on a first side of the middle layer ML and a second enveloping layer EL2 arranged on a second side of the middle layer opposite to the first side of the middle layer. Although intermediate layers may be arranged between the middle layer ML and an enveloping layer, for example to increase adhesion, there is a direct contact between each of the enveloping layers EL1 , EL2 and the middle layer ML so that the sandwich structure is a three-layer structure.

In the embodiment, both the first and the second enveloping layers EL1 , EL2 have the same thickness and are made of the same material (within manufacturing tolerances) so that the sandwich structure has a layer sequence and material distribution mirror-symmetrical with respect to a center plane CTP of the middle layer ML. This provides for a self-balanced behaviour at any temperature and upon temperature changes, meaning that, in the absence of external forces, the flat layer structure is straight (not bent) and does not bend upon temperature change.

In the embodiment, the middle layer ML is made of monocrystalline silicon, while each of the enveloping layers EL1, EL2 is made of a metal or metallic alloy with a thermal conductivity significantly higher than that of monocrystalline silicon. For example, the enveloping layers may be made of copper (Cu), gold (Au) or aluminium (Al) or alloys thereof having the element as the main constituent. The enveloping layers EL1, EL2 thereby function as thermal shortcuts conducting a larger portion of the thermal load along the length direction of the spring portion through it. Thus, the thermal resistance of such composite suspension system in the region of the composite spring portions will be less and a maximum temperature will be lower than in a reference system having the same dimensions but made of crystalline silicon only.

In order to further explain the concept and beneficial effects thereof, elastic and thermal properties of a three-layered bending spring (elastic spring portion with three-layer sandwich structure) will be described and compared to those properties with a reference spring portion having the same dimensions but made of monocrystalline silicon only.

The reference spring portion is abbreviated by REF in the following nad may have similar dimensions as a bending spring 69, 70 taken from Fig. 10 of US 2017/0363861 A1.

Fig. 4 shows in Fig. 4A a schematic of a reference spring portion REF made of cr-Si and in 4B a three-layered spring portion SP with a composite sandwich structure according to an embodiment, both accompanied with parameters used for the calculations. In each case, index (0) indicates parameters for the reference spring REF, while corresponding parameters without index 0 refer to the three-layered sandwich structure. The tilt for the reference flexure (Fig. 4A) is described by: where M is the torque, a is the tilt angle, ko is the reference tilt stiffness, Eo is the Yc ^) modulus (130GPa for (100) Si), Lo is the length along the thermal path (e.g. 50pm) and Io is me moment of inertia:

Wo ■ tp 0 “ 12 where wo and to are the spring's width (72pm) and thickness (1 ,5pm).

The factor 2 in (1) expresses that 2 single springs in parallel are bent by a 1 D (one-dimensional) tilt. Thus: (3)

E _o ■ w ₀ ■ k° ⁼ - T 6 ii -o

The heat transfer through the reference flexure is described by: where P _m = P _m ■ a ² is the absorbed power per mirror with size a, with P _m as absorbed power density, A _o is the thermal conductivity of cr-Si (149 W/m.K at room temperature), and R° _h denotes the thermal resistivity of the reference flexure. Therefore:

(5)

Therefore, AT would be the temperature difference at one bridge, if it alone conducts a power P _m . But following the heat path, we find two bridges in parallel and two in serial, thus the whole thermal resistance of the complete flexure is equal to that of one bridge and AT is really the temperature difference between the mirror and the monolithic mirror's body below (at the reference suspension considered 40°C). Any thermal resistance of the remaining structures along the heat path is neglected.

When the reference spring bends due to point-applied or linear distributed force or torque, it curves approximately as an arc with radius Ro and angle a. (6)

Tp — ^o/ ^a

The mechanical stress o in the lever is zero in its middle plane and increases linear in radial direction, becoming tensile (sign +) and maximal at the upper surface, and compressive (sign -) and thus minimal at the lower surface:

(7)

The tilt stiffness for the 3-layerd flexure from Fig. 4B is described by:

In the above relation the cumulatively of the product E.l is used. Thus, from a monolithic beam with thickness t and elasticity E _ou t is extracted the middle layer with thickness (t-2h) and with elasticity E _ou t and added there a middle layer with the same thickness (t-2h) and elasticity Emdi.

By determining the thermal resistance R _t h three parallel resistors are calculated in accordance to (5), as the middle is that of the middle layer ML, and the other two correspond to the outer layers (enveloping layers EL1 , EL2), as the subscripts of (5) have to be substituted accordinalv. The full thermal resistance is then:

The maximal stress in the middle and outer layers can be found from (7) with an appropriate subscript change: and as the stress "jumps" between the layers at z=(t-2h)/2 by:

It is possible to use different approaches in evaluating the system. One way is to look at the suspension system as a composite suspension with adjusted vertical structure. A way to prove the advantages of a current concept is to compare analytically a composite (or "sandwich") suspension, having the same width and length as the reference suspension REF (i.e. w=wo and L=Lo), and having cr-Si as one material of the composition, but varying gradually the vertical structure (i.e. t and h).

Having similarity in some dimensions, it is worthy to define relative changes in the tilt stiffness k and the temperature difference AT as:

As a limiting condition the maximum stress in the Si part shall not approach the fraction strength level.

Metallic materials such as Cu, Au, Al and non-metals like poly-diamond are chosen due to their high thermal conductivity - either as an outer or middle material. The full thickness of the sandwich flexure is varied from 1 to 3pm, with a step of 0.1 pm, while outer layer thickness for every t is increased in steps of 1/8 pm from 0 (0 corresponds to a flexure from the inner material only) up the last multiple of 1/8 pm but below t/2 (0<2h<t).

Table 1 below lists physical properties of some candidate materials for composite springs

(source: Wikipedia) The calculations explained above allow identifying combinations of material, full thickness and height of the outer layer, which, in combination, provide for the following: a) relative change of the tilt stiffness dk _sw < 1 equal (or if not possible- also smaller) than that of the stiffness of the reference system REF), which means, that the maximal tilt range of the reference system can be achieved. b) AND relative change of the temperature difference dAT _sw <1 , showing, that the mirror is sustained at lower temperature that by the CCM design c) and stress in the Si layer < 1.5GPa (with 50 % safety margins included), ensuring a safe work without fraction in the Si layer, which determines the elastic behavior of the while suspension

The following assumptions have been made in the investigation:

• these most-promising results are achieved at the above chosen step for t and h. When the steps are decreased, somehow better results appeared in-between

• the thin spring part only is considered, as the thermal resistance of the remaining parts is neglected

• symmetrical sandwich case: top layer == bottom layer

• no temperature or thickness dependence of thermal conductivity of crystalline Si is currently considered in thus sub-section

• additional layers at or close to the springs, e.g. wires, passivation, thin (native) oxide, metal/metal bonding pads, are not considered

• 1 GPa is taken as safe fraction stress level for Si (1.5GPa with margin), despite in the literature a fracture strength limit of 2GPa is cited

• The basic MMA temperature is taken as T_basic=40°C. Thus, T_ _m irror = T_basic+AT.

• It is supposed that crystalline, metallic and/or poly- or nano-diamond layers can be prepared and structured at any reasonable height from sub-pm to pm thickness

• The tolerances and thickness variations and their influence are currently neglected.

Table 2 below lists a number of material combinations identified as useful for making composite springs.

Some preferred combinations are listed below. In each case a caclulated thermal benefit in terms of lower mirror temperature compared to the reference suspension (e.g. -45% lower mirror temp) is listed together with the layer layout of the spring portion in terms of materials and layer thickness.

-45% lower mirror temp by 1.5pm spring with 0,5/0.5/0.5pm Cu/Si/Cu

- 40% lower mirror temp by 1.5pm spring with 0,625/0.25/0.625pm Cu/Si/Cu

<60% lower mirror temp by 1.7pm spring with 0,375/0.95/0.375pm Au/Si/Au

- 63% lower mirror temp by 1.8pm spring with 0,5/0.25/0.8pm AI/Si/AI

-45% lower mirror temp by 1.6pm spring with 0,25/1.1/0.25pm Si/Au/Si

-60% lower mirror temp by 1.7pm spring with 0,125/1.45/0.125pm Si/AI/Si

<70% lower mirror temp by 1.7pm spring with 0,25/1.2/0.25pm Si/AI/Si

-42% lower mirror temp and -93% less stiffness by 1.1 pm spring with 0,25/0.6/0.25pm Si/poly- dia/Si

Composite spring portions having multilayered sandwich structure are not the only options of realizing the benefits of the invention. The sandwich structures described above represent a design solution compatible with MEMS fabrication processes and complying with lifetime requirements.

Fabrication by MEMS technology and the operation conditions both impose restrictions to materials that can be used. The former to avoid contaminations (e.g. elements diffusing into silicon and thus changing electrical properties, like Au) and to assure sufficient adhesion between the sandwich layers, latter e.g. to avoid gaseous contamination within the machine and structure damage over lifetime.

Furthermore, it must be considered, that a “thermal conductivity” material may be weak towards mechanical stress. This can lead to a reduced stiffness for unwanted parasitic movements of the device on one hand, but also to a plastic deformation within the material over lifetime.

Figures 5, 6 and 7 show schematically alternative design options which may mitigate those challenges. Fig. 5 and 6 illustrate the concept of “Sealing” of high thermal conductivity material, HTCM. In the embodiment of Fig. 5 plural metallic lines of HTC material arranged in a common plane at equal mutual lateral distances are embedded into an envelope made of cr-Si. The crystalline silicon material fills the gaps between the metallic lines, covers the metallic lines on the short sides and on the top and bottom (here similar to the envelope layers in the sandwich structure). Therefore, the metallic HTC material is protected on all sides by cr-Si, which also stabilizes this composite structure mechanically.

In the embodiment of Fig. 6, the composite spring portion includes a single flat layer HTCM of HTC material surrounded on all sides by cr-Si. In other words: the HTC material forms a flat, heat conducting core and the cr-Si forms a cover enveloping the core circumferentially thereby stabilizing the composite structure and protecting the core from all sides.

The concept can be used in other suspension system layouts. For example, a spring portion may be a flexure type portion bending about an axis perpendicular to its length direction (see e.g. Fig. 2. However, some or all spring portions may be designed as torsion springs where both longitudinal ends are rotated relative to one another about axis in the longitudinal direction. Except for the internal layout of spring portions the mirror unit may have similar design as the mirror unit shown in Fig. 9 of US 2017/0363861 A1 , incorporated herein by reference.

Fig. 7 shows a variant of the concept shown in Fig. 6 obtained by introducing interconnect structures 1ST to increase stiffness by reduced volume (e.g. cylinders, building locks or filled honeycomb structures) and to get rid of adhesion dependence. The interconnects structures run in the thickness direction from the first to the second free surface.

Next, exemplary process flows for realizing (x/Si/x) and (Si/x/Si) suspension systems for MEMS- MMA are described. The process flows are schematic, shorted and show only some of all steps (step numbers (1), (2), (3) etc.) of exemplary chains of manufacturing steps.

An example process flow for MEMS-MMA with Si/x/Si suspension is shown in Fig. 8.

The suspension requires initially two Silicon-on-lnsulator (SOI) wafers with crystalline silicon device layer Si as thick as the outer Si layers of the sandwich suspension. One of these SOI wafers may be pre-structured (see (1)), e.g. having formed TSVs (through-Si vias). Copper or highly doped poly-Si are typically used to create the conductive columns as vertical wiring, while the insulation is made of thermally grown SiO2.

Next, the middle material (material selected to form the middle layer, here denoted by “x”) is deposited on both layers of device Si (2-3), as the combined thickness of both layers shall give the thickness of the suspension's middle layer. Metals like e.g. Cu, Au or Al can be used as middle material and in the same tome to define a connection interface for the two SOI wafers. The above three metals are the most established materials for thermocompression bonding because of their high diffusion rates. Further, the two wafers are face-to-face bonded, e.g. by thermo-compression or eutectic process (4). The next step is grinding and polishing away the holder wafer and etching of the BOX (buried oxide) of the second SOI wafer (5).

Step (6) includes structuring the bonding pad BP for the mirror element. Au/Au, AI/AI, or Al/Ge eutectic or thermo-compression bonding may be used to attach the mirrors; thus, the lower bond-pad can be made from Au, Al or Ge. Shaped metallic pads are created by metal deposition, photolithography and etching (wet chemical or plasma) of the metal, unprotected by photoresist (PR). Similar, the suspension has to be defined by another PR mask and "cut" by consequent selective reactive ion etching (RIE) of Si by F-based plasma (CHF3+CF4) with etch stop on the middle material and next RIE by Cl-based plasma with stop on the lower Si level (7).

The following step (8) show schematically already formed the remaining structures like actuators' and sensor's parts, contacts, planar wiring, etc, employing etching of Si and/or metal and/or deposition and structuring of metals and dielectrics. At this stage the basic wafer is ready to be aligned and bonded (thermo-compression or eutectic) with a wafer with pre-structured mirror elements ME (9-10). The mirror wafer is being prepared in a parallel process. There are different options for its manufacturing. The figures show how the composite suspensions can be integrated within a complete fabrication flow for the MEMS-MMA.

The finalization of the MMAs require the suspension to be released to a membrane (11). It is done by photolithographic definition of the release cavity from the back side and DRIE (deep reactive ion etching of Si) employing a gas chopping, also called Bosch process. This process consists of consequent repetition of short steps of RIE (by SFe + Ar plasma) and passivation (C4F8). After the PR stripping the holder wafer which keeps the mirrors is polished or etched away, followed by etching of the BOX and deposition of the EUV coating. Step (12) shows the finalized mirror unit MU

An example process flow for MEMS-MMA with (x/Si/x) suspension is shown in Fig. 9.

This suspension requires initially a Si wafer with TSVs- Cu or doped poly-Si (1) and a SOI wafer with Si device layer as thick as the middle Si layers of the sandwich suspension. Films from the outer material (Cu, Au, or Al) are deposited on the face sides of the wafers (2), as the summary thickness of the two layers results in the thickness of one outer suspension's layer, and bonded face-to-face (3-4). Again, thermo-compression or eutectic process can be used. The bulk of the SOI wafer is grinded and polished away and the BOX is etched (5). The suspension requires one more deposition of a layer of the outer material with the same thickness as that below the thin Si film (6). The upper suspension's part can be structured according to the design of the integrated sensors and actuators by selective RIE of metal by Cl-based plasma with etch stop on Si (7). Optionally, further RIE of Si by F-based plasma with stop on lower outer film can be applied. Step (8) shows schematically formation of the remaining structures like actuators' and sensor's parts, contacts, planar wiring, etc., and the bonding pad for the mirror. Their microstructuring includes a combination of etching of Si and/or metal and/or deposition and structuring of metals and dielectrics.

Similar to the previous flow (Fig. 8), next the fabrication go through bonding (thermocompression or eutectic) of the basic and mirror wafers (9-10), back-side DRIE for release of the suspension membrane (11), polishing I etching of the mirrors' holder wafer, etching of the mirrors' BOX and deposition of the EUV coating (12). As mentioned above, for a complete understanding of the fabrication of MEMS-MMAs with composite suspensions, it is worthy to look at the structuring of the mirrors' wafer to generate the mirror element (or substrate thereof), schematically shown in Fig. 10.

Initially, a double SOI wafer with a middle device layer as thick as the mirror plate and lower device layer- as the mirror's post MP, is provided (1). If desired, such wafers can be ordered or may be prepared by bonding of oxidized SOI and Silicon wafers together. Next, a small hole at the place of the future post must be etched through the lower device Si and the lower BOX. When it is refilled by doped poly-Si (2) and planarized (e.g. by CMP- chemical-mechanical polishing), it assures an electrical connection to the mirror plate. Then, the upper bonding pad for the mirrors is formed by micro-structuring of Au, Al or Ge etc. depending on the choice of bond process and materials. An additional protection by a film of SiO2 may be preferred (3). A good solution is to deposit PECVD (plasma enhanced chemical vapor deposition) SiC>2 and structure it by lithography and RIE of SiC>2. Further, a photolithography defines the mirror plates, as only the gap between the mirrors is left without PR (4). The next process is DRIE of Si, masked by the PR, which stops on the lower BOX: this step is followed by RIE of the exposed in that way SiO2 (5). Now, when the PR is stripped, the only masked area remains at the bond pad covered by SiO2. Hence, the next DRIE "cuts" vertically the mirrors' gaps up to the upper BOX, separating the mirrors from aside. At the same time this second DRIE "cuts" vertically also the lower SOI layer, forming the mirrors' plates and leaving the post unetched (6). A final RIE of SiO2 finalizes the mirrors' wafer, removing the undesired oxides (7).

Preferred embodiments of the sandwich springs include ultra-thin SOI wafers with small TTV (total thickness variation) ~ 5%, methods of fabrication of SOI wafers with ultra-thin Si layer (<1.5 urn) and/or SOI wafers with Si layer below 2pm. Process steps similar to those provided by commercially available technologies can be used. For example, Soitec’s (www.soitec.com) proprietary wafer-bonding and layer-splitting technology commercialized as “Smart Cut™ “ makes it possible to transfer a thin layer of crystalline material from a donor substrate to another substrate. This technology makes use of both implantation of light ions and wafer bonding to define and transfer ultra-thin single-crystal layers from one substrate to another. It works like an atomic scalpel and allows active layers to be managed independently from the supporting mechanical substrate.