CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of German Patent Application DE 10 2016 21 1 51 1 .1 filed on June 27, 2016. The content of this application is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to a microlithographic illumination unit for post-exposure of a photoresist provided on a wafer in a microlithography process.

Prior art

Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. The image of a mask (reticle) illuminated by means of the illumination device is in this case projected by means of the projection lens onto a substrate (for example a silicon wafer) coated with a light- sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate. It is known here to perform in the microlithography process a structure-less illumination (i.e. without use of a structured mask) of the wafer or of the photoresist located thereon in each case after the individual lithography steps for increasing the sensitivity of the photoresist for the purposes of efficient utilization of the electromagnetic radiation. Only by way of example, it is possible to use UV radiation with a wavelength of 365nm in the process, wherein the respective illumination of the photoresist which is performed without transferring mask structures is typically effected with an intensity distribution which either is homogeneous or varies over comparatively large spatial wavelengths (e.g. of less than 1 mm) with only small amplitude fluctuations (e.g. less than 10%).

As a result, it is possible due to this additional illumination (which can also be referred to as "post-exposure" of the photoresist) to obtain better utilization of the (e.g. EUV) radiation that is radiated in in the actual lithography steps and thus increased throughput of the microlithographic projection exposure apparatus.

With respect to the prior art, reference is made merely by way of example to S. Tagawa et. al: "Super High Sensitivity Enhancement by Photo-Sensitized Chemically Amplified Resist (PS-CAR) Process", Journal of Photopolymer Science and Technology, 26, 6, (2013), 825.

In order to be able to perform the above-described additional illumination or "post-exposure" of the photoresist, an additional illumination unit is necessary, the configuration of which with respect to the existing requirements in terms of intensity, uniformity and dose stability of the electromagnetic radiation that is coupled into the photoresist during this post-exposure represents an ambitious challenge. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a microlithographic illumination unit which, with comparatively simple construction, allows a possibly par- ticularly homogeneous post-exposure, which is as controlled as possible, of a photoresist situated on a wafer for increasing the throughput in the microlithography process.

This object is achieved by way of the features of independent Claim 1 .

A microlithographic illumination unit according to the invention for postexposure of a photoresist provided on a wafer in a microlithography process includes: at least one light source, and - a light-guiding and light-mixing element for coupling the electromagnetic radiation generated by the light source into the photoresist, wherein this light-guiding and light-mixing element has a first pair of mutually opposite side faces, the maximum spacing of which has a first value, wherein multiple reflections of the electromagnetic radiation on these side faces take place; wherein the light-guiding and light-mixing element has a second pair of mutually opposite side faces, the maximum spacing of which has a second value; and wherein the maximum extent of the light-guiding and light-mixing element in the light propagation direction of the electromagnetic radiation has a third value, wherein this third value is greater than the first value and is smaller than the second value.

According to one embodiment, the second value is greater than the first value at least by a factor of two, in particular at least by a factor of five, more particularly at least by a factor of ten. The invention is based in particular on the concept of implementing an illumination unit for the post-exposure of the photoresist in the lithography process or after the individual lithography steps in a manner such that the electromagnetic radiation used for this post-exposure is coupled in via a light-guiding and light- mixing element, the dimensions of which are selected to be appropriate especially for obtaining the desired (specifically both light-guiding and light-mixing or homogenizing) effect. The effect of the element according to the invention as a lightguide is here in particular that the working distance between the light source and the wafer that is to be observed during said post-exposure can be selected to be comparatively large, with the result that firstly undesired introductions of heat, owing to the power dissipation into the wafer or photoresist produced in the light source (e.g. an LED arrangement), and also undesired influences on or changes in the microelectronic circuits, produced on the wafer, due to electric fields of the driver electronics of the light source are avoided.

Another advantage of the configuration according to the invention lies in the avoidance or decrease of undesired contamination of the wafer during the post-exposure according to the invention, since reliable separation between the light source (e.g. LED arrangement) used for producing the electromagnetic radiation and the wafer becomes implementable. According to the invention, an effect is effectively attained that corresponds to a "constriction" of the radiation used for post-exposure along the comparatively short extent of the light-guiding and light-mixing element, without comparatively complicated and costly fixing of additional optical elements on the light source or light sources becoming necessary herefor.

As a result, the illumination unit according to the invention is used to achieve a combination of the two desired effects of light guiding to the wafer and also the light mixing or light homogenization, wherein undesired influences on the already (at least partially) structured wafer are avoided at the same time.

According to one embodiment, the electromagnetic radiation is not reflected at the side faces of the second pair.

According to one embodiment, the light-guiding and light-mixing element is configured in the form of a solid block made of a material that is transparent for the electromagnetic radiation.

According to one embodiment, the multiple reflections at the side faces of the first pair comprise at least one total internal reflection.

According to one embodiment, the side faces of the first pair are mirror surfac- es.

According to one embodiment, the side faces of the first pair are not arranged so as to be parallel with one another. According to one embodiment, the illumination unit has at least one intensity sensor for measuring the electromagnetic radiation.

According to one embodiment, the illumination unit has a light source arrangement composed of a plurality of light sources.

According to one embodiment, the light-guiding and light-mixing element has a light exit surface that is provided with refractive power.

According to one embodiment, the light-guiding and light-mixing element has a light exit surface that is provided with a diffractive or refractive structure.

The invention furthermore relates to a microlithographic projection exposure method, wherein the method comprises the following steps: providing a substrate on which at least partially a photoresist is applied; providing a microlithographic projection exposure apparatus having an illumination device and a projection lens; projecting in each case one mask structure onto a region of the photore- sist by way of the projection exposure apparatus in a plurality of projection steps; and wherein after at least one of these projection steps, a post-exposure of the photoresist using an illumination unit having the previously described features takes place.

Further configurations of the invention can be gathered from the description and the dependent claims.

The invention is explained in greater detail below on the basis of exemplary embodiments illustrated in the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS In the figures:

Figures 1 -6 show schematic illustrations for explaining various embodiments of the invention; and Figure 7 shows a schematic illustration for explaining the possible construction of a microlithographic projection exposure apparatus designed for operation in the EUV range. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 7 shows a schematic illustration of a projection exposure apparatus that is given by way of example and is designed for operation in the EUV range.

According to Fig. 7, an illumination device in a projection exposure apparatus 10 designed for EUV comprises a field facet mirror 3 and a pupil facet mirror 4. The light from a light source unit comprising a plasma light source 1 and a col- lector mirror 2 is directed onto the field facet mirror 3. A first telescope mirror 5 and a second telescope mirror 6 are arranged in the light path downstream of the pupil facet mirror 4. A deflection mirror 7 is arranged downstream in the light path, said deflection mirror directing the radiation that is incident on it onto an object field in the object plane of a projection lens comprising six mirrors 21 - 26. At the location of the object field, a reflective structure-bearing mask 31 is arranged on a mask stage 30, said mask being imaged with the aid of the projection lens into an image plane in which a substrate 41 coated with a light- sensitive layer (photoresist) is situated on a wafer stage 40. Different embodiments of an illumination unit according to the invention that is used to perform a post-exposure (already explained in the introductory part) of the photoresist or wafer after the individual lithography steps will now be described below. By way of this "post-exposure", it is possible to obtain better utilization of the (e.g. EUV) radiation that is radiated in in the actual lithography steps and thus increased throughput of the microlithographic projection exposure apparatus.

First, the construction and function of an illumination unit according to the invention in a first embodiment will be explained with reference to the schematic images of Fig. 1 and Fig. 2a-b.

According to Fig. 1 , an illumination unit 100 according to the first embodiment has a light-guiding and light-mixing element 1 10, which, in the exemplary em- bodiment, but without the invention being limited thereto, is configured in the form of a block which is sufficiently transparent for the wavelength range of the electromagnetic radiation used in the post-exposure according to the invention (that is to say e.g. for a wavelength of 365nm).

In the exemplary embodiment, this block or the light-guiding and light-mixing element 1 10 is made of quartz glass (Si0 ₂ ). In further embodiments, a different, suitable material that is transparent for the relevant wavelength, e.g. calcium fluoride (CaF ₂ ), can also be used.

The illumination unit 100 furthermore has a light source arrangement 120, which in the exemplary embodiment is in the form of an arrangement of a multiplicity of LEDs 121 , wherein these LEDs 121 in turn in the exemplary embodiment are mounted on a side face, representing the light entry surface, of the block that forms the light-guiding and light-mixing element 1 10.

As can be seen best from Fig. 2a-b, the electromagnetic radiation that is emitted by the LEDs 121 and enters the transparent block is guided by (nearly loss- free) total internal reflection in the direction of the substrate or wafer W.

The block that forms the light-guiding and light-mixing element 1 10 is here arranged with respect to the light propagation direction or the position of the wafer W in a manner such that it has a spatial extent in one spatial direction (according to Fig. 2a, in the x-direction) which is greater relative to the distance between the light source 120 or LEDs 121 and the wafer W, and a spatial extent that is shorter relative thereto in the spatial direction which is perpendicular thereto (according to Fig. 2b, in the y-direction).

This configuration causes the divergence of the light rays emerging from the LEDs 121 of the light source arrangement 120 (which can correspond, merely by way of example, to a half aperture angle of 30°) in the spatial direction (y- direction), which corresponds to the comparatively short extent of the block that forms the light-guiding and light-mixing element 1 10, via total internal reflection on the side faces of the block that are mutually opposite one another in this direction to result in an efficient or substantially loss-free light guidance up to the proximity of the wafer W according to Fig. 2b. In contrast, in the spatial direction having a comparatively large extent of the block (x-direction), no reflection takes place on the side faces of the block that are mutually opposite in this direction, wherein to this extent, good mixing of the rays emerging from the individual LEDs 121 according to Fig. 2a is achieved.

The invention is not limited to the above-described implementation of the light- guiding and light-mixing element 1 10 as a transparent block. For example, the relevant element 1 10 in further embodiments can also be configured in the form of a housing made of mutually opposite side walls that form an enclosure for light which passes through, wherein the side walls responsible for the reflection according to Fig. 2b are reflective.

The illumination unit 100 furthermore has a sensor arrangement 130 which in the exemplary embodiment is formed from a multiplicity of photodiodes 131 which are arranged along a bevelled face or chamfer 1 15 that is provided on the block that forms the light-guiding and light-mixing element 1 10 and serve to monitor the intensity of the electromagnetic radiation emitted by the respective LEDs 121 . In this way, the brightness of the individual LEDs 121 can be adjusted, independently of one another, in accordance with the respective current requirements using an additional controller.

This controller makes it advantageously possible in particular to switch off the LEDs 121 (and thus e.g. to avoid undesired scattered light) in phases of interruption of the lithography process (e.g. during a wafer change), because con- stant maintenance of a thermal equilibrium of the LEDs 121 over a continuous operation of the LEDs 121 is not necessary. Another advantage of the previously described monitoring of the brightness of the LEDs 121 is that any defects of individual LEDs can be immediately detected, with the result that, if necessary, the post-exposure according to the invention can be interrupted to avoid inhomogeneous illumination and any asso- ciated damage to the wafer W.

The invention is not limited to the implementation of the light source arrangement 120 in the form of the previously described arrangement of LEDs 121 . In further embodiments, other suitable light sources, such as e.g. discharge lamps, can also be arranged in a similar fashion. Furthermore, a single (e.g. strip-shaped) light source can also be used.

Fig. 3a-b show a further embodiment of the invention, wherein components which are analogous or substantially have the same function are denoted by reference signs increased by "200" in relation to Fig. 1 .

The embodiment of Fig. 3a-b differs from that of Fig. 1 and Fig. 2a-b in that the light exit surface 340, facing the wafer W, of the block that forms the light- guiding and light-mixing element 310 is provided with refractive power. In the exemplary embodiment of Fig. 3a-b, the light exit surface 340 is to this end provided with a cylindrical geometry. In further embodiments, the desired refractive power can also be implemented by way of a cylindrical asphere or by configuring the light exit surface 340 in the form of a Fresnel lens or with a dif- fractive structure.

The refractive power at the light exit surface 340 results in collimation of the electromagnetic radiation, which exits the block that forms the light-guiding and light-mixing element 310, along the y-direction with the result that it is possible to further increase the ratio of the working distance between light exit surface 340 and wafer W and the extent of the illumination in the y-direction (according to the displacement direction of the wafer). It is thus possible to increase the distance of the illumination unit 300 from the wafer W without a need to further widen the lighting in the y-direction. Fig. 4a-b show a further embodiment, wherein, once again, components which are analogous or substantially have the same function are denoted by reference signs increased by "100" in relation to Fig. 3a-b.

The embodiment of Fig. 4a-b differs from that from Fig. 1 and Fig. 2a-b in that the two side faces 410a, 410b of the light-guiding or light-mixing element 410 that are mutually opposite one another in the y-direction are not aligned to be mutually parallel, but tilted about a finite angle. The result of this configuration is that with each reflection of the electromagnetic radiation at one of the side faces 410a, 410b, the angle of the electromagnetic radiation relative to the z- direction is changed, with the result that a collimation of the beam path in the y- direction is likewise achieved and thus, analogously to Fig. 3a-b, an increase of the working distance of the light exit surface of the element 410 from the wafer W is made possible.

The diagram shown in Fig. 6, which shows the result of a simulation of the lighting of a photoresist or wafer with an illumination unit according to the invention, is intended to indicate that in principle, any desired intensity distribu- tions are settable with an illumination unit according to the invention in accordance with the previously described embodiments.

Here, curve "A" shows a Gaussian distribution with which the intensity distribution produced by a single LED of the light source arrangement along the x- direction can be described. Curve "B" (likewise plotted along the x-direction) shows an exemplary profile which is attainable by superposition or summation of the intensities of all the LEDs in the light source arrangement along the x- direction (i.e. over the "long side of the illumination slit"), wherein the recognizable local minima were adjusted in a targeted fashion by dimming individual LEDs. Without such dimming, it is also thus possible to implement a substantially constant intensity profile over the photoresist. Fig. 5 shows a schematic illustration of an illumination unit 500 according to a further embodiment, wherein, once again, components analogous or substantially functionally identical to Fig. 4a-b are designated by reference numerals increased by "100".

The embodiment according to Fig. 5 differs from that from Fig. 1 and Fig. 2a-b in that a structure 550, which scatters along the x-direction (one-dimensionally), is formed on the light exit surface, facing the wafer W, of the block that forms the light-guiding and light-mixing element 510, as a result of which lighting of the wafer W can be smeared along the x-direction or the spatial direction with greater extent of the element 510 and thus further homogenization of the lighting can be obtained. The structure 550 serving as a diffusing screen can be configured as a diffractive structure or from e.g. small lens elements (according to a refractive effect).

Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments are apparent to the person skilled in the art, e.g. by combination and/or exchange of features of individual embodiments. Accordingly, it goes without saying for the person skilled in the art that such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is restricted only within the meaning of the accompanying patent claims and the equivalents thereof.