GB1582860A - Device - Google Patents

Description

(54) DEVICE ql) We, IMPERIAL CHEMICAL INDUS TRIES LIMITED, a British Company, of Imperial Chemical House, Millbank, London SW1P 3JF, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment-: --; This invention relates to articles having a surface pattern and to the preparation and use of such articles.

Many articles having a surface pattern are prepared by depositing upon the surface of a substrate a thin layer of a suitable resist material, removal of at least part of the deposited material and treatment of the substrate underlying the area from which the deposited material has been removed.

We have found that it is advantageous, particularly in the preparation of patterned articles in which the pattern is finely defined, if the resist material is laid down in the form of what conveniently may be described as a 'Langmuir film'. Langmuir films are laid down by a process, described below, whereby the molecules of the film are regularly aligned and it is conjectured that the relative lack of molecular irregularity in a carefully deposited Langmuir film contributes to the preparation of an accurate pattern.

Thus, the present invention provides a method of preparing an article having a surface pattern, which comprises the steps of depositing a resist material in the form of a Langmuir film upon a substrate, removing at least part of the Langmuir film from the substrate and treating at least part of the substrate underlying the area from which the resist has been removed.

The method of forming Langmuir films is well knows and involves the passage of a substrate through a thin, preferably monomolecular, layer of film forming material carried on the surface of a supporting liquid, and it is possible to obtain thereby films of material which do not require the evaporation of a large volume of solvent for their formation; disruption of the film as a result of solvent removal is thereby obviated.

Our technique, described herein for con venience as our Langinuir technique is sub stantially as follows. The supporting liquid or sub-phase is preferably one which is inert to the film forming material, that is, it does not react chemically with the material of the layer or of the substrate under the con ditions employed in the process of the invention (although on occasion it may be desirable to include materials, possibly in solution, which do not act disadvantage ously with the film yet may react with it for example by altering the ionic composi tion), nor will it usually be a solvent for the film forming material, although some solubility is not necessarily detrimental so long as the layer of film forming material is not thereby destroyed or its formation prevented. The liquid may be organic or inorganic; usually we prefer to use an aqueous liquid conveniently water; the presence of certain inorganic ions in an aqueous supporting liquid has sometimes been found to improve the stability of the film-forming layer and may be taken up by the film as it is deposited so affecting its composition in a desirable manner.

Thus, we have found it desirable on occa sion to include in a highly purified aqueous supporting liquid metallic, conveniently cadmium, ions in sufficient concentration to link hydrophilic terminations of adjacent mole cules of a fatty acid film-forming material.

Other ions may be similarly employed where they have the requisite properties, and we have found it advantageous to adjust the pM of the supporting liquid to slightly acid (of the order of 5.3) to control ionisation of the metallic salt to the optimum level.

Formation of the thin layer of the material on the surface of the supporting liquid is most conveniently effected by- applying to the surface of the liquid an appropriate volume of a solution of the film-forming material in a volatile solvent which is immiscible with the supporting liquid under the process con ditions employed and which evaporates after application of the solution to the liquid surface. The concentration of the film-forming material in the solution is selected such that the evaporation of the solvent leaves behind a layer of the desired thickness (usually monomolecular) on the surface of the supporting liquid. The preferred filmforming materials employed according to the invention, and used in association with an aqueous supporting liquid, are those having a molecule comprising both hydrophilic and hydrophobic components, the hydrophilic component tending to immerse below the surface while the hydrophobic component tends to remain unwetted and to project from the surface.

Transfer of the film-forming material to the substrate is effected by dipping the substrate into the supporting liquid and withdrawing it, so that a coherent layer of the film-forming material (conveniently referred to herein as the Langmuir film) adheres to the surface of the substrate. Provision of means for maintaining the integrity of the layer on the liquid is necessary and this means can comprise a sweep or paddle, preferably responsive to a microbalance which constantly measures the pressure of the layer upon the supporting liquid and responds thereto to sweep together the supported molecules so that the layer does not rupture. This feature of maintaining pressure upon the layer is important for the production of an aligned continuous film upon the substrate.

The rate of passage of the substrate through the layer is preferably low, of the order of 3 mm/min for the first 2 or 3 films, although higher rates, up to say 30 mm/min may be acceptable for subsequent layers.

The temperature of the aqueous support liquid is preferably 21 + -210C although obviously any appropriate temperature may be employed.

The film-forming material may be any suitable material which is capable of acting as a resist or precursor thereof to the particular reagents employed in the preparation of the patterned articles if the invention and which is capable of being laid down as a Langmuir film as described above. Particular examples of materials that may be employed as resist materials according to the invention include organic materials e.g.

selected from fatty acids, particularly straight chain fatty acids containing 12 to 20 carbon atoms, for example stearic acid and arachidic acid and salts thereof, anthracene C4 to C12 carboxylic acids and compounds mentioned in our UK Patent Application No.

34264/75 (Serial No. 1,572,182).

The fact that our Langmuir films consist of regularly and precisely aligned molecules or molecular structures makes them particularly useful as photoresist materials, in which, by virtue of the sharp edge effect combined with the photographic process, particularly sharp and precise boundaries may be attained. The very low density of defects that may be attained enables a thin Langmuir film resist to have a similar resist effect to a much thicker resist film laid down by conventional commercial thickness techniques using solvent deposition. This is a property which makes such films useful not only in the preparation of waveguides (as described in our British Patent Appl. Nos.

8072 (Serial No. 1,580,180) and 26150 of 1976 but in many other fields where relatively thin, high definition photoresist potential is required. Thus, we contemplate that a Langmuir film of an appropriate radiation polymerisable material may be deposited upon a support and exposed to imagewise radiation which polymerises exposed portions of the film (Figure 2).

Removal of unexposed portions of the film by any appropriate means e.g. dissolution by a suitable solvent leaves a residual photoresist having high definition potential (Figure 3). It will be apparent to the skilled man that such techniques therefore may find application for example in the preparation of printing plates and related filds where resist techniques are already employed, e.g.

in litho graphic replication processes, for defining micro-electronics device patterns where sub-micron resolution is required, in diffraction gratings etc. Usually we prefer to employ for such processes as the above materials which polymerise with only a small area change, or in such a manner that dimensional change is predictable and can be allowed for.

In order to obtain the highest quality Langmuir films i.e. films having maximum molecular alignment, we have found that control of certain aspects of the deposition process are critical, although for less demanding applications perhaps less stringent control of the film formation may be acceptable.

(a) a contamination free sub-phase and purified film material; (b) the pressure-relative molecular area relationship (c) the pH of the sub-phase (d) the temperature of the sub-phase (e) deposition rate and pressure variation in the suspended monolayer during deposition.

(a) Any alien molecules found in the subphase as contaminants can become incorporated into the film structure. Their presence in the structure may modify the nature of the polar/non-polar orientation of the successive layers and may therefore result in wetted areas being occluded into the multilayer film. The occluded water may cause splitting of the film, or pin-holes or the alien molecules may act as absorption or scattering centers. It is therefore important that the tank should be scrupulously clean, the film material extremely pure and the sub-phase water distilled several times, e.g.. in a quartz apparatus to exclude contaminants (typically acid and alkaline potassium permanganate distillation stages are included with neutral distillation to remove organic contaminants).

(b) At low pressure the film molecules are loosely packed and occupy a large area, i.e.

they are -separated by an exposed water surface. If the molecules remain loosely packed the film deposited upon the substrate will contain molecular vacancies often filled with water. On the other hand, if the molecules are too tightly packed the film on the subphase could collapse, resulting in agglomer sated regions which are several layers too thick. The monolayer on the subphase should therefore be compressed to the point at which the molecular area remains effectively constant for a large change in pressure, thus ensuring that small changes in pressure caused by the immersion and withdrawal of the substrate do not cause the film to break up or fold.

(c) The degree of ionization of a fatty acid in the sub-phase depends upon the pH of the solution. A strongly basic sub-phase will cause enhanced ionization and hence increase the fatty acid to salt conversion. In addition a relatively high pH will improve the film/substrate adhesion. We have found adhesion satisfactory for films deposited at pH between 5 and 7. These conditions produced adequate film deposition on most metal oxide substrates (e.g. Awl203, MgO).

(d) The condensation of the molecular layers to form a monolayer on the sub-phase surface is made easier by lowering the temperature. The lower temperature may, however, increase the viscosity of the monolayer making it brittle. On the other hand temperatures can cause expansion and ultimately may give thermal forces strong enough to overcome the rather weak Van der Waal's forces which bind the molecules together. This may enhance molecular solubility and make the layer unstable. This may enhance molecular solubility and make the layer unstable. The preferred temperature range for cadmium stearate film deposition for example is 19-220C but the skilled man will have no difficulty in determining optimum temperatures for other materials.

(e) It is important, where a plurality of layers is deposited, that the initial layers laid down should be well formed, as faults in these layers may be replicated in later layers.

To ensure good adhesion the initial layers are preferably laid down at low immersion and withdrawal rates. After a few layers have been laid down the rates may be increased somewhat. Low rates are conveniently of the order of 3 mm/min, higher rates being, say, 24--30 mm/min.

The film must be maintained in the condensed phase during deposition so that the molecules are not too loosely nor too tightly packed. In our Patent Application No.

8072/76 (Serial No. 1,580,180) we show the pressure changes during immersion and withdrawal of the substrate. The operating point (ca 50 mg/cm) is chosen so that the pressure variation is maintained between 25 and 35 mg/cm. The immersion and withdrawal rate affects the amplitude of the perturbations and it is this factor which sets the maximum substrate velocity.

It will usually be necessary to provide a resist of thickness greater than that of one molecule and the advantage of the process of the invention is enabling this to be done while still allowing control of the film thickness will be apparent.

Thus, the thickness of the -Langmuir film may be increased by repeated treatment of the substrate by the process previously described whereby additional layers are deposited upon the substrate and we have found it possible by careful control of the processing conditions to deposit several hundred layers (300 or more) with few defects so enabling accurate resist layers of 0.7 to 1.5 microns or more to be fabricated.

Details of the apparatus (shown diagrammatically in Figure 1) and coating process which may typically be employed in the performance of the invention are as follows: the sub-phase e.g. water, was contained in a rectangular glass tank 1 containing sub-phase 2 to a depth of 6 cm. The resist, e.g. cadmium arachidate 3 was dispersed on the water surface to fill the region between the two glass plates 4 and the gravity operated barrier 5. The compression force on the molecules was measured by monitoring the film surface tension by a Wilhemy plate balance. The tank was covered by a dust cover 6 to exclude ambient dust. The whole system was placed on an antivibration table 7 to prevent film collapse due to vibration.

The motion of the substrate 8 (which was as constant in velocity as possible) was controlled by a linear motion drive operated through a reduction gear box, the direction of motion being controlled by reversing microswitches on the gantry. The features of constant velocity immersion and withdrawal and freedom from vibration are very important for the production of regular films.

The technique of the invention is illustrated by the following examples.

Exaniple 1 A flat surface of a soda-lime glass plate was lapped using successively finer grades of carborundum powder, then polished using successively finer grades of diamond particles suspended in oil on a solder faced plate on an optical flat polishing machine and finally polished using an expanded polyurethane lap and a chemical etch abrasive polishing compound ("Syton") until the surface was optically flat to approximately half a wavelength and smooth to approximately 50 A.

The surface was then thoroughly cleaned.

A Langmuir film was next deposited upon the substrate by dipping the plate, repeatedly, vertically into water carrying on its surface a monomolecular layer of cadmium arachidate until a resist layer 200 molecules thick had built up.

Example 2 Example 1 was repeated using cadmium stearate as the resist material. By varying the number of dippings films of different thicknesses, e.g. 1750, 2300, 2500, 3100, 2580 and 5400 A were prepared.

WHAT WE CLAIM IS:- 1. A process for preparing an article having a surface pattern which comprises the steps of depositing a resist material in the form of a Langmuir film upon a substrate, removing at least part of the Langmuir film from the substrate and treating at least part of the substrate underlying the area from which the resist has been removed.

2. A process according to claim 1 in which prior to removal of part of the Langmuir film the film is subjected to imagewise modification.

3. A process according to claim 2 in which modification is by polymerisation of part of the film.

4. A process according to claim 3 in which polymerisation is effected by irradiation of the film.

5. A process according to claim 3 or 4 in which unpolymerised parts of the film are subsequently removed.

6. A process according to any one of the preceding claims in which after removal of part of the film from the substrate the substrate surface underlying the area from which the film has been removed is treated to impart to it a surface pattern.

7. A process according to claim 6 in which treatment is by etching.

8. A process according to any one of the preceding claims in which the film is a fatty acid.

9. A process according to either one of claims 3 or 4 in which polymerisation occurs with only small area change.

10. A process according to any one of claims 3, 4, 5 or 8 in which the film comprises a photoresist material which undergoes polymerisation upon exposure to radiation.

11. A process according to claim 1 and substantially as hereinbefore described.

12. A product obtained by the process of any one of the preceding claims.

13. A microcircuit according to claim 12.

14. A lithographic plate according to

Claims (1)

1. claim 12.

   15. A diffraction grating according to claim 12.

   16. A product according to claim 11 and substantially as hereinbefore described.