GB2148769A - Topographic feature formation by ion beam milling of a substrate - Google Patents

Abstract

Material is selectively eroded with high precision from a substrate using a mask of material which is ion beam resistant. In the milling of steps or channels in a lithium niobate (LiNbO3) substrate to form an integrated optic device e.g. waveguide mirror, a suitable masking material is polyimide. The pattern is first produced in a resist layer deposited on the polyimide layer and subsequently transferred to the polyimide.

Description

SPECIFICATION Topographic Feature Formation This invention relates to topographic feature formation and is concerned with processes whereby topographic features can be formed with high precision by ion erosion of material from a substrate.

In the field of integrated optics it is required that three dimensional features be formed with faithful reproduction of an idealised geometry, and on a microscopic scale. In integrated optics the edge roughness of features must be small compared to an optical wavelength over relatively large areas.

According to one aspect of the present invention there is provided a process for the formation of topographic features at a surface of a substrate comprising the steps of providing a masking pattern on the substrate surface and ion beam milling the substrate surface through the masking pattern, the masking pattern being comprised of a mask material which is ion beam resistant.

According to another aspect of the present invention there is provided a process for the formation of topographic features at a surface of a substrate comprising the steps of providing a corresponding polyimide masking pattern on the substrate surface and ion beam milling the substrate surface through the masking pattern.

The expression "ion beam resistant" is to be understood to mean a mask material which will survive, and maintain precise edge definition during, ion beam milling of the substrate, particularly lithium niobate (LiNbO3) substrates.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows the form of a typical polyimide polymer repeat structure; Figs. 2 to 12 illustrate the process steps employed for the fabrication of an optical waveguide mirror, and Fig. 13 illustrates "photo doping" of Ag2Se/GeSe resists.

Ion beam milling is known to offer a means of eroding material from areas of a substrate which are defined by a pattern existing in an overlay of a masking material with suitable properties. The technique may be used for patterning metals, insulators, or semiconductors, including materials which are chemically inert. A vital aspect of the ion beam milling technique is the choice of masking material, which has to satisfy the requirements that (i) the masking material should have a low erosion rate for bombardment by the ions to be used to mill the substrate, (ii) the masking material should be suitable for deposition at the required thickness without decomposition, loss of adhesion to the substrate, or geometrical distortion, and (iii) no degradation of the masking layer should occur whilst a pattern is being formed in it.

Examples of the application of ion beam milling to integrated optics include ion beam milling steps in lithium niobate (LiNbO3) substrates which can act as optical waveguide mirrors and ion beam milling channels in lithium niobate in order to accurately locate optical fibres for coupling to an optical integrated circuit.

We have now found that a suitable masking material for ion milling of lithium niobate is polyimide.

Polyimides are derived from condensation of diamines with polycarboxylic acids or their anhydrides. Polyamic acid is formed by thermal condensation-polymerisation of an acid dianhydride with a difunctional base. The reaction of pyromellitic acid dianhydride (PDMA) with 4.4 diaminodiphenyl ether (DADPE) in an appropriate solvent produces the polyamic acid resin. The polyamic acid converts to polyimide at temperatures high enough to remove the solvent and to initiate the imide ring closure. Typical solvents are 1-methyl pyrrolidinone with an aromatic hydrocarbon.

Polyimide cure studies have shown that final crosslinking is a function of temperature, and optimum film properties are developed when the film is totally crosslinked. Decomposition takes place at temperatures above 500 C. Atypical polymer repeat structure form is shown in Fig. 1.

The repeat structure includes molecular water, and two molecules of water are generated per polymer repeat unit. This gives rise to 6 to 8% water per polymer repeat unit, dependent on the polymer average weight. During curing the water loss represents a significant loss of solvent from the film.

It should be noted that part cured films are hygroscopic and for optimum results atmospheric water must be excluded from a curing film. Without such water exclusion high temperature decomposition will occur before final crosslinking.

The properties of polyimide which make it suitable for use an an ion beam milling masking material for use on lithium niobate include the following, namely (i) polyimide is eroded more slowly than most materials by argon ions, argon ions being used to remove lithium niobate; (ii) polyimide may be deposited to a thickness of at least 10 lim by spinning, drying and curing; a featureless layer being formed which has a flat surface even in the presence of substrate irregularities; (iii) cured polyimide is chemically inert and possesses elasticity which can absorb the stress of differential thermal expansion; thereby being suited to survive a range of physical and chemical processes, and (iv) polyimide is quickly eroded by the impact of oxygen ions; thus allowing ease of patterning orfinal removal.

An example of the use of polyimide as the masking material for ion beam milling will now be described with respect to the process steps illustrated in Figs. 2 to 12 which indicate the fabrication of an optical waveguide mirror.

In a first process step (Fig. 2) a polyimide layer 1 is applied to a substrate 2 comprised by a planar waveguide formed in lithium niobate. The polyimide layer 1 is typically 10 pm thick and applied by spinning, drying and curing. The polyimide is coated with a thin charge spreading layer 3 typically of gold and 200 A thick (Fig. 3). The charge spreading layer 3 is coated with an electron beam resist 4, for example PMMA (polymethyl methacrylate) 1 um thick (Fig. 4). The resist is baked and then exposed to an electron beam (direct electron beam writing) and developed to produce the undercut profiled portion 5 (Fig. 5). Titanium 6 is evaporated onto the thus processed substrate (Fig.

6). A solvent, for example MlBK, is then employed to lift off the resist 4 and the titanium evaporated thereon (Fig. 7). The processing so far described results in the formation of a high quality edge in the titanium film 6 on the polyimide. A replica of the pattern thus produced in the titanium film 6 is then formed in the polyimide layer (patterning of the polyimide layer) by oxygen ion bombardment through the titanium film 6 which comprises a titanium mask (Fig. 8). The exposed charge spreading layer 3 is removed during this oxygen ion bombardment (oxygen reactive ion etching (RIE)).

The lithium niobate thus exposed through the pattern etched in the polyimide (polyimide mask) is ion beam milled (Fig. 9) to the required depth. This may be accomplished by inert argon ions, or, alternatively, by the use of reactive ions, for example, derived from fiuorinated or chlorinated hydrocarbons (reactive ion etching). The thus processed substrate is metallised (Fig. 10) for example by deposition of aluminium 7 to cover the walls of the topographical feature (well 8) to a thickness of approximately 1000 A for example. Ion etching normal to the basic substrate surface is employed to remove the aluminium from all horizontal surfaces thereof whilst leaving it on the vertical walls (Fig. 11). Subsequently the polyimide 1 1 is removed from the lithium niobate by use of oxygen plasma stripping (Fig. 12).During the stages illustrated in Figs. 10 to 12 the polyimide performs a secondary role in protecting selected areas of the lithium niobate during metallisation of the exposed ion beam milled edges. The chief role of the polyimide in the processing is, however, to transfer the precise geometry provided by the electron beam definition to the substrate without degradation of the fine detail thereof.

Whereas the above processing employs electron beam definition during the patterning of the polyimide other definition systems may alternatively be employed. UV radiation and a suitable organic photoresist, for example AZ1 350H is one possibility. The processing involved is substantially similar to that described above with respect to Figs. 2to 12. However, a charge spreading layer is not required and the baking of the photoresist includes a first baking, followed by soaking in chlorobenzene and a subsequent second baking. Inorganic resists may be used. The use of GeSe or Ag2Se/GeSe films as inorganic resists give very high resolution compared with standard organic materials and, for a precision patterning process, these resists are very attractive.

Ag2Se/GeSe resists are amorphous chalcogenide films which when exposed to UV or electron irradiation undergo a "photo doping" effect wherein silver diffuses into GeSe matrix. It has been proposed that the irradiation produces electron hole pairs in the GeSe and the holes migrate to the Ag2Se layer where they produce silver ions and elemental selenium, see Fig. 13. The electrons are trapped at the defect sites in the lattice and charge neutrality is achieved by the migration of silver ions to the trapped electrons. Typical sensitivities to electron beam irradiation (10 kV) are in the range 10-5 to 10-4 C/cm2. Higher sensitivities are achieved at lower acceleration voltage due to increased possibility of interaction (stopping probability) between thin films and low energy electrons.This combination also reduces the effects of proximity distortion, which is a limitation in standard thick organic resists at higher exposive energies.

The inorganic resists are undergoing much development and have been shown to be nonswelling, non-deforming, high-contrast, dry process compatible and capable of resolving to their intrinsic limit of 100A. The GeSe or GeSe/Ag2Se inorganic resists are resistant to oxygen plasma and thus a high resolution feature in a thin inorganic resist layer can be transferred faithfully into an underlying thicker polyimide layer by oxygen RIE, producing high aspect ratio vertical features.

The sensitivity of inorganic resists to UV, electron beam and X-ray radiation allow many forms of exposure to be used. However in the case of electron beam exposure the charge spreading layer 3 required over the polyimide for organic resists is not necessary for inorganic resists and this coupled with the ease of oxygen RIE makes this processing advantageous. Patterning of a polyimide layer by means of a GeSe inorganic resist merely comprises the steps of coating the polyimide with GeSe by an evaporation technique, the GeSe is then exposed through an appropriate photomask, plasma developed, and the unwanted polyimide removed by oxygen RIE. Thus, in comparison with the organic resists processes described above, at least three process stages, baking, evaporate titanium and solvent lift off, are omitted.

Since the processes described above employ lithographic reproduction of desired shapes, they are, therefore, versatile with respect to location and orientation upon the substrate surface. The use of polyimide provides superior edge quality in comparison with conventional techniques.

Claims (18)

1. A process for the formation of topographic features at a surface of a substrate comprising the steps of providing a masking pattern on the substrate surface and ion beam milling the substrate surface through the masking pattern, the masking pattern being comprised of a mask material which is ion beam resistant.

2. A process for the formation oftopographic features at a surface of a substrate comprising the steps of providing a corresponding polyimide masking pattern on the substrate surface and ion beam milling the substrate surface through the masking pattern.

3. A process as claimed in claim 2 wherein the substrate is of lithium niobate (LiNbO3).

4. A process as claimed in claim 2 or claim 3, wherein the polyimide masking pattern is provided by depositing a layer of polyimide on the substrate surface, coating the polyimide layer with a resist layer, defining said pattern in the first resist layer and transferring said pattern to the polyimide layer.

5. A process as claimed in claim 4 wherein the resist layer is comprised of an organic resist material.

6. A process as claimed in claim 4 as appendantto claim 2 including the step of employing the patterned organic resist layer to define said pattern in a titanium layer and produce a titanium mask via which the polyimide is oxygen reactive ion etched.

7. A process as claimed in claim 5 or claim 6 wherein the organic resist material comprises polymethyl methacrylate, which organic resist material is exposed by electron beam irradiation.

8. A process as claimed in claim 5 or claim 6, wherein the organic resist material comprises photoresist, which organic resist material is exposed by UV radiation.

9. A process as claimed in claim 4 wherein the resist layer is comprised of an inorganic resist material.

10. A process as claimed in claim 8, wherein the inorganic resist material comprises GeSe or GeSe/ Ag2Se.

11. A process as claimed in claim 9 or claim 10, wherein the pattern is defined in said inorganic resist material by exposure to electron beam, UV or X-ray radiation and plasma developing.

12. A process as claimed in claim 11 and including oxygen reactive ion etching of the polyimide to transfer said pattern from the developed inorganic resist material to the polyimide layer.

13. A process as claimed in any one of the preceding claims wherein the ion beam milling of the substrate comprises argon ion bombardment.

14. A process as claimed in any one of claims 1 to 12 wherein the ion beam milling of the substrate comprises bombardment by reactive ions.

15. A process as claimed in any one of claims 2 to 14 wherein subsequently to the ion beam milling the polyimide masking pattern is removed with oxygen plasma.

16. A process as claimed in any one of claims 2 to 13 wherein subsequently to the ion beam milling at least one face of the topographical features formed in the substrate is metallised whereby to form a mirror.

17. An article including topographical features formed in a substrate by a process according to any one of the preceding claims.

18. An integrated optic device including topographical features formed in substrate by a process according to any one of claims 1 to 16.