The present invention relates to a process of deep pattern micro-lithography suited, for example, for use in the formation of microscopic structures, or microstructures, such as waveguides, gratings and micromechanical components having dimensions measured in microns up to millimetres.

A process known as the LIGA (lithographic, Galvanoformung and Abformung) process is known for the manufacture of microstructures having the dimensions described above. The LIGA process includes a lithographic step in which a resist material is selectively exposed through a mask to radiation. Exposed or non-exposed material, depending upon the type of resist, is subsequently removed in a development step. Electroforming and moulding steps are carried out on the three dimensional resist formation to produce the required microstructure.

A particular problem with the LIGA process is the formation of deep structure, by which is meant a structure having a depth typically in the range 100-1000μm Conventionally, the radiation used to penetrate deeply into the resist is hard x-rays, for example, above 10KeV photon energy or less than 0.1 nm wavelength. A commonly used resist is PMMA (polymethylinethacrylate). A penetration depth of 500μm in this resist material requires a radiation wavelength of less than 0.3nm and for greater depths even shorter wavelengths are required.

The use of intense and hard x-rays in the LIGA process presents a number of problems including degradation of structural edge definition caused by secondary electron emission; and the need for the mask used to be of gold and of a thickness in the range 20-30μm for example, which makes the masks both difficult and expensive to produce and reduces the resolution of the process because of the large aspect ratio of the mask features. Moreover, sources of hard x-rays are restricted and

involve, for example, a synchrotron which results in high costs and long exposure times.

The LIGA process is also used in the formation of complex microstructures which have features at more than two levels in the resist, for example step-like discontinuous structures. In order to form these complex microstructures the LIGA process is performed to expose, develop and electroform those regions of the resist defining the lower features of the microstructure, new resist is then added to form a new layer of resist over the top of the electroformed resist before the LIGA process is repeated using a different mask. The desired complex microstructure is thus formed once the new resist has been exposed, developed and electroformed. By repeating the LIGA process in this manner and adding new resist in between, structures can be built up which have a height measured in hundreds of microns. However, new resist must be added after the initial exposure and development step which makes the procedure time consuming and costly.

The present invention seeks to address the difficulties described above with respect to the LIGA process and seeks to provide a lithographic process capable of forming deep pattern microstructures quickly and with good definition and moreover a process which enables complex microstructures to be formed.

In a first aspect the present invention provides a process of lithographically forming a microstructure comprising selectively exposing a radiation sensitive resist to radiation, developing and removing the exposed resist and subsequently exposing to radiation new regions of the same radiation sensitive resist revealed by the removal of exposed resist and developing and removing said new regions.

Thus, with the present invention during the lithographic process exposed regions of the resist are removed revealing underlying regions of the resist which are then irradiated. In this way the incident radiation is

required to penetrate only a short depth into the resist whilst still enabling large, for example > 80μm, features of the microstructure to be formed. In this way the problem of deep penetration of radiation and the associated need for hard x-ray radiation and thick masks is avoided enabling the use of soft x-ray radiation. Also, where hard x-ray radiation is employed, with the present invention process times may be significantly reduced.

The exposure/development steps may be performed either sequentially or simultaneously and without any new resist being added to the original resist before the exposure/development steps are repeated. Where the process is performed sequentially, the process may further include the step of ashing the surface of the radiation sensitive resist after exposure of the resist to the radiation and before development of the exposed resist.

Preferably, the radiation sensitive resist is exposed to radiation having a wavelength centred at 1nm generated by means of a laser plasma source. Alternatively, the radiation may be synchrotron radiation, other wavelengths of electromagnetic radiation, or radiation in the form of an electron or ion beam.

The exposed resist may be developed using either a wet or dry etchant. In the case of a dry etchant ions from an electron cyclotron resonance ion source may be used or alternatively a plasma or reactive ions. The dry etch process is particularly suited to the simultaneous exposure and development of the resist.

In a second aspect the present invention provides a process of lithographically forming a microstructure comprising selectively exposing a radiation sensitive resist to radiation, thereafter ashing the surface of the exposed resist and developing the exposed resist. Preferably, the process includes repeating the steps of exposure, ashing and development to new resist revealed by the development of said exposed resist.

In a third aspect the present invention provides a process of lithographically forming a microstructure comprising selectively exposing a radiation sensitive resist to radiation through a first mask and through a second different mask embedded below the surface of said resist, developing the exposed resist and exposing and developing new resist revealed by the development of said exposed resist. Preferably, said first mask is deposited on the surface of said radiation sensitive resist.

The present invention also provides microstructures formed by the above described processes including a mask for use in lithographic etching, a microwave waveguide suitable for use with GHz to THz frequencies and electro-optic devices such as semiconductor laser components including a wavelength selective diffraction grating, and microelectromechanical systems (MEMS) such as minature pumps and motors, Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a diagram illustrating a first lithographic process in accordance with the present invention;

Figure 2 is a diagram illustrating an alternative lithographic process in accordance with the present invention:

Figure 3 is a picture of a microstructure formed using a double exposure/development process in accordance with the present invention;

Figures 4a and 4b illustrate a workpiece employing two separate embedded masks for fabricating a waveguide former; and

Figures 5a and 5b show a waveguide structure fabricated using the waveguide former of Figure 4b.

To assist in an understanding of the present invention the following is an analysis of the radiation dose or fluence necessary for

lithographic etching of a resist to a thickness D _n for the conventional LIGA process and for the process in accordance with the present invention.

Using Beer's law for calculating the irradiation l _D1 needed for a selected etching depth D,: l _D1 = l, x e ^D1rtβ where t _e =1/μ (where μ is the absorption coefficient of the resist) and l _s is the surface radiation dose or fluence required to saturate the resist at a thickness « .

The irradiation l _Dn needed for a selected etching depth D _n =nD, is:

On >« * ^β

Whereas the irradiation l _nD1 needed for a selected etching depth D ₁ repeated n times with the exposed resist removed between each irradiation step is: l _nD1 *nl _D1 =n x I, x e ^D1Λ *

For example, for an irradiation wavelength of 1nm, where D^lδμm, n=6 and D _n =90μm and the resist is AZ 114PF for which l _s =20mJcm ² and t _e =5μm: l _Dn =1.3x10 ⁶ Jcnτ ² but l _nD1 =2.4Jcnv ² . Hence, in mathematical terms there is a factor of approximately 5x10 ⁵ between the two processes. Moreover, generally this factor also applies to the total exposure time required for the desired etching depth. Thus, whereas the conventional LIGA process could involve an exposure time of up to 4 hours for an etch of 100μm, with the present invention the total exposure time using the same radiation is reduced to a matter of minutes.

Figure 1 illustrates a first arrangement for performing the lithographic process in accordance with the present invention. A workpiece 1 consisting of a substrate 2 mounted on a backing plate 3 and having a layer of a resist 4 on its front surface of around 10Oμm is shown with a mask 5 deposited thereon of about 3μm thickness. The workpiece 1 is

positioned within an etching chamber 6 which has a beryllium window 7, around 10μm in thickness, which acts as a filter only allowing through radiation with a wavelength of approximately 1nm. Helium is provided within the etching chamber 6 at around atmospheric pressure to displace the air which would otherwise absorb the soft x-ray radiation.

The resist 4 is a chemically amplified resist (CAR) such as AZ PF514 or AZ PN114 or may consist of PMMA or an equivalent resist material suitable for lithographic etching. The mask 5 is preferably of gold with a thickness ideally in the range 0.1-5.0μm depending on the total depth of exposure required. Where necessary masks having a thickness greater than 5.0μm may be used where for example hard x-rays are used as the incident radiation. Alternative mask materials having good x-ray and handling properties may also be employed, for example Hafnium or tantalum. The mask 5 is deposited on the resist using any conventional metallisation techniques such as sputtering, evaporation or electroplating and the mask 5 may be patterned again using any suitable conventional technique. As a result of the thickness of the mask shown in Figure 1, the mask was patterned using a wet etching technique. In an alternative arrangement, the mask 5 may be separately supported within the etching chamber 6 between the resist 4 and the beryllium window 7. With this arrangement the mask 5 is free standing and the exposure is performed using a projection lithographic technique.

X-rays 8 having a wavelength centred at 1nm (1KeV) are directed through the beryllium window 7 at the mask 5 which is approximately 2cm from the x-ray source and expose the resist 4 at the regions where the mask material is absent. The period of exposure of the resist to the x-rays and the number of repeated irradiation is determined in dependence on the desired etching depth.

In Figure 1 the x-rays 8 are shown generated by a laser plasma x-ray source details of which may be found in WO94/26080, the

contents of which is incorporated herein by reference. The laser plasma x- ray source consists of a target tape 9, preferably of copper, at which a KrF excimer laser beam 10 is focused to a 10μm point. The beam 10 preferably consists of successive trains of short pulses of light with a pulse duration in the range 1-1 Ops with individual pulses focused at adjacent but different points on the target tape 9. The pulses of laser light incident on the target tape 9 generate a plasma 11 from which ions 12 of the target material and the x-rays 8 emerge. The ions 12 are stopped by the helium within the chamber. However, droplets of the target material 13, which might otherwise contaminate the etching process, are ejected from the rear of the tape 9 thereby ensuring a clean plasma source. It will of course be apparent that alternative radiation sources such as a synchrotron may be employed instead of the laser plasma source described above.

Following a first period of irradiation the exposed regions of the resist are baked if necessary and developed using any convenient agent such as a wet etchant in the form of a conventional chemical developer into which the workpiece 1 is dipped. Thereafter the workpiece 1 is returned to the chamber and the resist 6 is reirradiated for a further period of time to irradiate the new regions of the resist revealed by removal of the regions of the resist exposed in the first irradiation. The new regions are subsequently developed in the same manner as described above. This procedure is repeated as many times as necessary to achieve the depth of etch desired after which electroforming and any other finishing processes such as polishing are performed. It has been found that where the resist is exposed to soft x- rays to larger depths such as 15μm per exposure, the surface of the resist can be over-exposed resulting in tone reversal where the resist changes from negative resist to positive resist and vice versa to a depth of around 0.5μm (L7Ϊ0). In the case of chemically amplified resists reversal occurs for a fluence twenty times the saturation dose I, or more. Where reversal

occurs, following irradiation of the workpiece a thin layer of the surface of the resist is removed by ashing or an alternative suitable process before development to remove the upper reversed layer (for chemically amplified resists, after post-exposure baking). With the arrangement illustrated in Figure 1 the steps of exposure and development are performed sequentially. In an alternative arrangement illustrated in Figure 2 the exposure and development of the resist may be performed simultaneously. The workpiece 1 in Figure 2 is the same as that of Figure 1 but is mounted within a reaction chamber 15 which is coupled to a microwave source 16 via a waveguide 17. Preferably the reaction chamber 15 is evacuated. A magnetic field coil 18 surrounds the reaction chamber 15. The arrangement constitutes an electron cyclotron resonance (ECR) ion source which subjects the workpiece 1 to a parallel beam of ions 19. The waveguide 17 includes a beryllium window 20 which is transparent to x-rays.

X-rays 21 generated by a synchrotron for example pass through the window 20 to expose the unmasked regions of the resist 6. Simultaneously the parallel beam of ions 19 generated by the ECR ion source etch the exposed regions of the resist revealing new resist for exposure and etching, ideally the exposure rate and the etching rate are substantially equal and are adjusted to maintain this balance. The backing plate 3 may be heated either continuously or intermittently to post- exposure bake the rear of the resist depending upon the resist material used. Finally, the etched resist is electroformed and any other finishing steps performed as necessary.

Although described in terms of a simultaneous process, the arrangement of Figure 2 may also perform sequential exposure and etching as described with reference to Figure 1. Also, it will of course be understood that the laser plasma source of Figure 1 may alternatively be used as the source of x-rays in Figure 2. AN ECR ion source is used as a

dry etchant in Figure 2, alternative dry etchants which may be employed include reactive ion etchants, an ion beam use in the milling mode or a plasma. In the case of an ion beam in milling mode the mask may be required to be much thicker than the thicknesses given above and/or of an alternative material. Further alternative irradiation sources include other wavelengths in the electromagnetic spectrum, for example deep ultra-violet (e.g. λ=10-100nm), ultra-violet (e.g. λ=0.25μm) and electron and ion beams preferably having an energy of around 100 KeV. An appropriate resist material is chosen for the source of radiation used. In Figure 3 an example of a two step etch performed using the process described above with reference to Figure 1 is shown. The Figure shows a thick chemically amplified resist which was exposed twice using soft x-rays and a free standing mask in close proximity to the surface of the resist which was moved between exposures. Between exposures the resist was baked, ashing of the surface performed and the exposed resist developed using a wet etchant. The two etched structures with the second shifted with respect to the first may be clearly seen in the Figure. The process described above may be employed with both positive and negative resists and in the case of negative resists there is an additional benefit that the resist allows lift off due to its ability to act as an inbuilt separation layer. This is particularly useful where the resist forms the final material for the end product in a micromechanical structure for example a micromotor.

In addition, where soft x-rays are the exposing radiation thereby enabling thin masks to be used, additional masks may be embedded within the resist to enable the formation of complex structures. An example of a former for the fabrication of such a complex structure is shown in Figures 4a and 4b. Figure 4a illustrates the workpiece for fabricating the waveguide structure shown in Figures 5a and 5b. The workpiece comprises a sacrificial silicon wafer base 30 on top of which is a

lower resist layer 31. A lower mask 32 is sandwiched between the lower resist layer 31 and an upper resist layer 33. Finally an upper mask 34 is deposited on the surface of the upper resist layer 33. This workpiece when lithographically etched using the process described above produces the waveguide former shown in Figure 4b. The resist former in Figure 4b was fabricated using four exposure steps of 15μm each. The first two steps etched the resist using the upper mask 34 and the usual baking, ashing and development employing a wet etchant were performed between exposures. After the second exposure and development the lower embedded mask 32 was revealed and the subsequent two etching/developing steps were performed using a combination of the upper and lower masks. Each of the masks were approximately 3-4μm thick and the individual exposures took approximately 5 minutes each. The effect of pinhole defects in the masks can be clearly seen in Figure 4b. In Figures 5a and 5b the electroformed waveguide structure fabricated with the former of Figure 4b is shown with Figure 5b being an enlargement of Figure 5a. The waveguide structure has a 2.5THz waveguide cavity of 100μm in length, 25μm in width and 48μm in depth. The structure was fabricated by sputtering gold on the former. With this process waveguides for GHz and THz frequencies can be fabricated.

Examples of alternative microstructures which can be fabricated using the process described include masks for use in the conventional LIGA process and integrated semiconductor laser devices including diffraction gratings.