GB2263981A - Process for developing and etching compound layers of photoresist and polyimidesimultaneously - Google Patents

Description

1 2263981 Process for developing and etching compound layers of

photoresist and polyimide simultaneously

The invention covers a process based on the description in claim 1.

Highly heat stable, dielectric relief patterns of polyimides are used in the manufacture of electrical and electronic parts. These layers are formed either by patterning a polyimide layer directly or by preparing a relief pattern on a possibly already partially imidized polyimide precursor (polyamide acid), which is then processed by heating. The term "Polyimide" in the following document shall include especially also polyamide acids and partially imidized polyimide precursors, which are transformed into polyimides, using a thermal treatment, only after the formation of the patterned structure.

Photosensitive polyimides can be used to prepare the relief pattern. Typically however is a piocess, in which a nonphotosensitive polyimide is overcoated with a photoresist. The photoresist is then exposed through a photomask with the desired pattern and processed. The relief pattern can then be formed in the polyimide layer by dissolving the polyimide with an etchant in those areas 2 where the photoresist has been removed. The photoresist layer is finally totally removed. To simplify the process it is desirable to combine the development of the photoresist and the etching of the polyimide precursor in one step using only one processing solution.

Such a process is known from GB 22 34 365A. A polyimide layer is formed as the protective enclosure for the metallic pattern of a wire strain gage. This layer has holes for the contact areas. A layer of polyimide and a positive working photoresist layer are applied consecutively to the metallic pattern to achieve this. After the patterned exposure, the compound layer is processed with a developer. Since all developers for positive photoresists also dissolve polyimide precursors, L-he polyimide layer is etched away at the same time as the photoresist pattern is developed.

A similar process is described in the E. 1. du Pont de Nemours and Company, Wilmington, Delaware, USA (February 1990) data sheet "PyralinO Polyimide Coatings".

In this case, a layer of partially imidized polyamide acid, coated on a silicon wafer, is overcoated with a typical, commercially available positive working photoresist a nd exposed through a photomask. Then, in one step, the photoresist is developed and the polyimide precursor is etched. Typical, commercially available developers for positive resists or aqueous solutions of a base, such as sodium- or potassium hydroxide or tetra alkylammonium hydroxide are used as processing fluids. The thickness of the polyimide layer can be from 1 micron up to 2.5 micron.

If one uses the known process to prepare polyimide structures with dimensions of the order of magnitude of 10-50 microns in a layer with a 3 thickness of 10 microns, representing an aspect ratio near 1, one observes that it is practically impossible to etch the polyimide layer at the same time the photoresist is developed, in a way that reproduces sufficiently exact the pattern of the mask. An acceptable result can be achieved by optimizing the exposure and development conditions. However the processing window is so small that even small variations of the exposure, developer temperature or activity or even the development time result in the polyimide layer being etched too much or not being completely removed in areas, that should be open.

The objective of this invention is a process of simultaneous developing and etching a compound layer of a polyimide precursor and a positive working photoresist, that independent of the normally occurring variations in processing conditions, will deliver small, high quality polyimide structures which correspond closely to the dimensions of a photomask.

This problem is solved by a process for preparing a patterned structure in the layer of a polyimide or polyimide precursor, that is coated on a substrate, by using a temporary overcoat of a positive working photoresist, whereby the development of the photoresist and the etching of the polyimide layer is done in one step using the same processing solution. This processing solution is characterized in that it consists of a developer for the positive working photoresist, which contains an additive. The thickness of the two layers, as well as the type and amount of the additive are adjusted, so that the dissolving time for the polyimide layer is 100 to 400% of the dissolving time for the exposed photoresist layer.

4 In a preferred embodiment of this invention, the processing conditions are chosen so that the dissolving time of the polyimide is 100 to 300% of the dissolving time of the photoresist.

The dissolving time of the polyimides is understood to be the minimum time required for the processing liquid to completely dissolve a single polyimide layer of the prescribed thickness coated on the particular substrate using the prescribed processing conditions. The dissolving time of the photoresist layer is the optimum developing time for an optimally exposed single layer coating, optimized to give an exact dimensional reproduction of the mask.

The invention is based on the recognition that a sufficient processing latitude and an acceptable reproductibility of the result can be achieved in spite of the unavoidable variations in exposure and developer temperature and -activity of the processing solution, only if the time to develop the photoresist layer has a certain relationship to the time required to dissolve the polyimides.

Using the teachings of the invention, it is easy to determine by simple pretrials the processing conditions required for the optimum processing latitude.

The skilled practitioner has different possibilities to adapt his/her process to the teachings of this invention. The dissolving time of the polyimide layer depends especially on the degree of imidization, the layer thickness and the type and composition of the processing solution. In contrast, the development time of the photoresist layer depends mainly on its type and thickness and also on the type and composition of the processing solution.

The thickness of the polyimide layer is determined by the application in most cases and therefore not freely variable. Polyimide layers up to 10 microns and thicker can be patterned with the process of this invention. It is normal to partially imidize the polyimide precursor layer by heating, before the photoresist is applied, in order to improve the adhesion of the polyimide to the substrate. Preferably under the invention, the degree of imidization should not exceed 20% during this process step. The degree of imidization is here defined as that fraction of the carboxyl groups of the polyamide acid, capable of imidization, that have already reacted to form the imide. It can be determined based on publications of the manufacturer, such as the data sheet quoted above, by choosing the time and temperature of the heating process or also by using IR-spectroscopy on the partially imidized samples.

The thickness of the photoresist layer on the other hand can be selected freely. For economical reasons however, one would work with the thinnest possible lavers. Such layers, after exposure, will dissolve relatively quickly in typical developers for positive working resists. It is L.hen not possible to adjust the other processing conditions, s o that the relationship of the dissolving times falls in the region covered by the invention, especially if thicker polyimide patterns are to be prepared.

Therefore the invention suggests adding an additive to the developer, which increases the dissolution of the polyimide and/or retards the dissolution of the photoresist. The amount of additive should be between 1 and 20 volume % to avoid a noticeable reduction during use without affecting the 6 photoresist development properties of the processing solution. A preferred concentration range is from 2 to 8 volume %.

_Isopropanol is the preferred additive.

The developers covered by this invention are aqueous alkali solutions. They can contain alkali hydroxide or ammonium as the alkali component and if necessary be buffered with salts. Preferred are aqueous solutions of tetraalkylammonium hydroxides, in which the alkyl groups have 1 to 4 carbon atoms, equal or different and can also be substituted with hydroxyl groups. Especially preferred is tetramethyl ammonium hydroxide.

In a preferred application of the process covered by this invention, the properties of the layers, the composition of the processing solution and the processing conditions are adjusted, so that the dissolution time of the photoresist layer is 10 to 20 seconds and for the polyimide layer 15 to 60 seconds.

A further part of the invention is a preferred processing solution used in the process covered by this invention. It contains tetramethyl ammonium hydroxide in an aqueous solution with a normality from 0.2 to 0.4 and 2 to 8 volume % isopropanol.

The polyamide acids used to prepare the patterned polyimide layers are synthesized in known methods from dianhydride components and primary diamines.

7 The following compounds can for example be used as dianhydride components:

pyromellitic acid dianhydride benzophenone tetracarbonic acid dianhydride, 2,2-bis-(3,4- dicarboxyphenyl)propane dianhydride 2,2-bis-(3,4dicarboxyphenyl)hexafluoropropane dianhydride 3,31,4,41-biphenyl tetracarbonic acid dianhydride 1,2,5,6-naphthaline tetracarbonic acid dianhydride.

In addition to others, the following can be used as primary diamine:

4,41-diaminediphenylether m-phenylenediamine p-phenylenediamine m- or p-xylenediamine 4,41- diaminobenzophenone, 4,4-diaminediphenylmethane, 1,4-bis-(p-aminophenoxy)benzol.

The polyamide acids are used as solutions in a solvent such as Nmethylpyrrolidone, with the concentration range of preferably 10 to 30 weight %. The solution is applied to the substrate with a typical coating process that permits the selection of a specific layer thickness such as spin coating. The layer is subsequently heated to dry it and if necessary to partially imidize it. This heating process can for example take 10 to 60 min at 80 to 160'C.

8 The photoresist layer can be formed from typical, commercially available positive working photoresists. These normally contain a binder component consisting of an alkali soluble polymer, for instance a phenolic polymer or 5 a novolac, and a photosensitive component, for instance a chinondiazide compound or a acid forming compound. The resists can additionally have further additives to influence the spectral sensitivity, to improve the wettability and the adhesion to the substrate, as plasticizer and to increase the light sensitivity, such as trihydroxybenzophenone for example which is described in U.S. Patents 4, 650,745 and 4,626,492.

The resists are applied as solutions, where the solvents can aliphatic ketones, ethers and esters as well as aromatics. This includes for instance alkyleneglycolmonoalkylether such as.ethylcellosolve or butylglycol, cyclohexanone, butanone, butylacetate, toluol, xylol and their mixtures. The selection of the solvents and their mixtures is determined mainly by the type of binder and the photosensitive component.

Relief patterns, which exactly reproduce the dimensions of the mask, can be prepared by the process of this invention. it is however also possible to reproducibly prepare openings, that are smaller or larger than the dimensions of the mask. In this way one can possibly work with fewer of these expensive photomasks.

The development time is generally shorter, when instead of a simple developer, a processing solution according to the invention is used. This is not only a cost reduction, but also presents a quality advantage, since the adhesion of the polyimide layer to the substrate is impacted less.

9 The processing solution covered by this invention can be utilized longer, meaning that more parts can be processed with a certain amount, because the result depends-less on the activity of the processing solution. The additive described in the invention can further reduce the formation of sludge from residues of the dissolved layers.

The process by the invention can be used to prepare patterned polyimide layers, that are constituents of integrated circuits, printed circuits on ceramic substrates or other electronic assemblies.

Example:

An adhesion promoting layer and then a solution of a polyamide acid formed from 4,41-diaminodiphenylether and pyromellitic acid anhydride (PyralinO Type PI 2540 of the E. I. du Pont de Nemours and Company, Wilmington, Delaware, USA) in N-methylpryyolidone was spincoated on a silica wafer.. A 9 micron thick layer of partially imidized polyimide was obtained after 30 minutes drying at 120'C. A 2.5 micron thick layer of a photoresist according to example 1 of U.S. Patent 4,626,492 was applied to it. The compound layer was exposed through a photomask using a mercury pressure lamp with a total energy density of 120 mJ/sqcm and processed at 200C with a liquid having a composition listed in Table 1. The photoresist layer was finally stripped by rinsing with nbutylacetate.

The photomask consisted of opaque lines 20 microns wide with a spacing of 20 microns. Some areas of the mask were covered with neutral density filters, having densities increasing from 1% to 60%, so that the exposure under the open places of the mask ranged from 1.2 to 72 mi/sqcm.

_The dissolution times were determined as described on single polyimide and photoresist layers applied to the substrate.

Tahl t- 1 Sam-ole A B Processing 0.27 n 19 parts by volume Solution tetramethylammonium of 0.27 n hydroxide in water tetramethylammonium hydroxide in water + 1 part by volume of isopro-panol Total Processing 120 60 time (see) Dissolution time 20 20 -ohotoresist (sec) Dissolution Time 100 40 Poolvimide (sec) 1 The width of the open spaces obtained on the polyimide line patterns were measured with the microscope to determine the processing latitude, here represented by the exposure latitude. The deviation of the open space width on the polyimide from that of the mask is given as a function of the transmission of the neutral density filters, that means the exposure in the Figure 1.

11 Only those exposure steps with cleanly formed patterns were included, that had spaces open to the substrate as well as lines that were not attacked. The results show, that with the current technique (sample A), only spaces with negative deviations from the dimensions of the mask can be prepared; with higher exposures, the lines are also etched away. The curve is also relatively steep throughout, meaning that small variations in the exposure result in relatively large changes in the space width.

The process covered by the invention (sample B) shows in addition to this characteristic a preferred working area between 25 and 60% transmission, in which the space width depends only slightly on the exposure on the exposure and the mask dimension can be reproduced without deviation.

Similar results are achieved, when a typical commercially available material (AZ 1350 J, Hoechst AG, Frankfurt) is used as photoresist and a typical commercially available developer (AZ 312 MIF + water; 1+1) without or according to the invention with an addition of isopropanol 1+ 19 is used as developer.

Similarly comparably results are achieved by using a polyamide acid from benzophenonetetracarbonic acid dianhydride and 4,41-diaminodiphenylether with a fraction of m-phenylenediamine (PyralinO PI Type 2550 from E. I. du Pont de Nemours and Company, Wilmington, Delaware, USA) as polyimide precursor.

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Claims (10)

1. Process for preparing a patterned structure in a layer of polyimide or polyimide precursor on a substrate, by using a temporary covering layer of a positive working photoresist, whereby the developing of the photoresist and the etching of the polyimide layer occurs during the same process step using the same processing solution, characterized in that the processing solution is a developer for the photoresist, which contains an additive, and that the thickness of the two layers and the type and amount of the additive are selected such that the dissolving time for the polyimide layer is 100 to 400% of the dissolving time for the exposed photoresist layer.

2. Process according to claim 1, wherein the dissolving is time of the polyimide layer is 100 to 300% of the dissolving time for the exposed photoresist layer.

3. Process according to claim 1 or claim 2, wherein the developer for the positive working photoresist is an aqueous solution of a tetraalkylammonium hydroxide in which the alkyl groups have 1 to 4 carbon atoms and may be substituted with hydroxyl groups.

4. Process according to claim 3, wherein said tetraalkylammonium hydroxide is tetramethylammonium hydroxide.

5. Process according to any one of claims 1 to 4, wherein the amount of additive in the processing solution ranges from 1 to 20 vol.%.

6. Process according to claim 5, wherein the amount of additive in the processing solution is from 2 to 8 vol.%.

7. Process according to any one of claims 1 to 6, wherein the additive is isopropanol.

8. Process according to any one of claims 1 to 7, wherein the degree of imidization of the polyimide layer is no higher than 20%.

9. Process according to any one of claims 1 to 8, wherein the dissolving time of the photoresist is 10 to 20 seconds, and for the polyimide is 15 to 60 seconds.

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10. Processing solution f or use in a process according to any one of claims 1 to 9, consisting of an aqueous solution of tetramethylammonium hydroxide with a normality of 0.2 to 0.4 and an isopropanol content of 2 to 8 volume %.