GB2117669A

GB2117669A - Polymeric films

Info

Publication number: GB2117669A
Application number: GB08206563A
Authority: GB
Inventors: Christopher Simon Winter; Richard Harfield Tredgold
Original assignee: NAT RES DEV; National Research Development Corp UK
Current assignee: NAT RES DEV; National Research Development Corp UK
Priority date: 1982-03-05
Filing date: 1982-03-05
Publication date: 1983-10-19
Also published as: EP0114851A1; WO1983003165A1; GB8306026D0; JPS59500339A; GB2121315B; GB2121315A

Abstract

A process for the preparation of an ordered polymeric film on a substrate, which process comprises: (i) providing a reservoir of the amphiphilic preformed polymer; (ii) advancing the polymeric film receiving substrate into or onto the reservoir at least once; and (iii) recovering the substrate coated with the polymeric film.

Description

SPECIFICATION Polymeric films This invention relates to polymericfilms; more particularly, this invention relates to very thin, highly ordered polymeric films on a substrate; to their preparation, for example, by the Langmuir-Blodgett (L-B) technique; and to semiconductor and superconductor devices utilising them.

It is known that successive monomolecular layers of amphiphilic organic molecules, for example soaps such as neutral or acid calcium stearate, may be deposited on a substrate, for example glass, by the L-B technique (J. Am. Chem. Soc. 56495 (1934) and 57,1007(1935)). Polymericfilms may be formed in situ by the L-B technique by utilising, as amphiphilic organic molecule, an unsaturated ester of a fatty acid such as vinyl stearate which is subsequently radiation polymerised, for example by exposure to a y-source such as 60Co. (J. Polym. Sci. Al 10, 2061 (1972)). More recently, this technique has been used with qualified success in the laboratory to provide gate insulators in field effect transistors and to increase the efficiency of photodiodes in solar cells.

However, the mechanical and thermal stability of the films hitherto produced has been poor and has prevented their practical commercialisation.

This invention seeks to provide thin ordered polymeric films of improved mechanical and thermal stability.

According, therefore, to one aspect of this invention there is provided a process which comprises: i) providing a reservoir of the preformed polymer; ii) advancing the polymeric film receiving substrate into the reservoir at least once; and iii) recovering the substrate coated with the polymeric film.

In accordance with one embodiment of the invention, the reservoir comprises a monomolecular layer of the preformed polymer formed at a fluid phase interface. Preferably, the monomolecular layer is maintained at a constant surface pressure.

The fluid phase interface is suitably one between a liquid and a gas or vapour. For convenience the liquid is preferably an aqueous medium and the gas is air.

In accordance with a further embodiment of the invention, the reservoir comprises a solution of the preformed polymer. Suitably, the solution comprises an organic solvent, for example chloroform.

The preformed polymer is suitably an organic polymer, desirably one which is amphiphilic. Preferably it is a vinyl polymer which is rendered amphiphilic by the presence of both hydrophobic and hydrophilic pendant groups.

Suitable hydrophobic groups include unsubstituted or mono- or poly- halo- or hydrocarbyloxysubstituted hydrocarbyl(oxy) groups. By "hydrocarbyl(oxy)" is meant herein hydrocarbyl or hydrocarbyloxy. Examples include aryl and aralkyl groups such as phenyl or benzyl and alkyl(oxy), groups such as C40 to C4, preferably C20 to Clo alkyl(oxy), groups such as n-octadecyloxy and n-hexadecyl. It is an important feature of this invention that comparatively short hydrophobic groups may be used, for example phenyl.

Suitable hydrophilic groups include hydroxyl; poly(ethyleneoxy); N-pyridyl; N-pyrrolidyl; carboxyl, and precursors which are hydrolysable thereto, for example cyano-, amido, imido, acid anhydride and acyl chloride.

Preferred preformed polymers are copolymers of an unsaturated acid anhydride, such as maleic anhydride, with styrene; a C12to C22alk-1-ene; ora C10 to C20 vinyl ether.

Mixtures of hydrophobic and of hydrophilic groups may be incorporated into the polymer but, for greater ordering, it is preferred that there is one hydrophobic and one hydrophilic group. It is also preferred that the copolymer is an alternating copolymer. Examples include poly(n-octadecyl vinyl ether/maleic anhydride); poly(styrene/maleic anhydride) and poly(maleic anhydride/octadecene-1).

In accordance with the first embodiment of the invention, the preformed polymer is suitably incorporated as a monomolecular layer at the fluid phase interface by dissolving it in a volatile organic solvent, such as chloroform, and adding the solution in an amount calculated in known manner (essentially by determining the effective area per molecule from the absorption isotherm and then determining the quantity of solution required to give a monomolecular layer over a known area) to leave, on evaporation, a monomolecular layer.It is preferred that the monomolecular layer is equilibrated for 1 to 3h at 30 to 40 C, typically 35 C. it is then subjected, prior to advancing the substrate through it, to a constant surface pressure, typically of 30 to 45 mNm-, by means of an adjustable boom of polyethylene tetrafluoride tape which confines the monomolecular layer.

The process of the present invention is applicable to a wide variety of substrates, including glasses such as aluminosilicate glasses, metals such as aluminium, chromium, nickel, brass, steel, cast iron, silver, platinum or gold, metal oxide layers on aluminium ortin, plastics such as polystyrene, poly(ethylene terephthalate), cellulose acetate or polypropylene plastics and, in accordance with one particularly preferred aspect of this invention, semiconducting materials, for example silicon single crystals; amorphous silicon; Ill-V compounds such as BN, BP, AISb, GaN, GaP, GaSb, GaAs, InP, InAs, preferably GaP, GaAs and InP; Il-VI compounds such as CdS, Cd, Se, CdTe, ZnO and ZnS, preferably CdS and CdTe; and IV-VI compounds such as PbS and PbTe.In accordance with another particularly preferred aspect of this invention the substrate may be a superconducting material, for example a superconducting metal, or a superconducting alloy thereof, of Groups IIIA, IVA, VA, VIVA, VIII, IIB, IIIB or IVB of the Periodic Table, such as Nd, Ti, Zn, Hf, Th, V, Nb, Ta, Ro, Ru, Os, Zn, Cd, Hg, Al, Ga, In, TI, Sn, Pb, preferably Nd, Nb and Sn including the compound Nb3Sn.

The substrate may need preparation in known manner prior to coating; for example, silicon may need to be etched and it may need to have a thin coating of oxide formed thereon.

The substrate is advanced through the reservoir of preformed polymer in known manner, for example by being coupled to a simple variable speed motor, typically at a speed of 0.5 to 50, preferably 1 to 10, mm.min#'. The substrate may, depending (it is believed) on whether it is wetted on only one or both the advancing and recovering operations accrete one (X-mode) or two (Y-mode) ordered polymeric films. The advancing and recovering operations may be repeated, if desired, to build up thicker ordered films.

When the final recovering operation has been effected it is desirable to dry the coated substrate, suitably overnight, in helium or nitrogen.

An electrode, for example 50A to 1000W of Au can then be evaporated at ambient temperature to -1000C.

It is then desirable to give the dried coated substrate onto which an electrode has been evaporated an annealing treatment in which it is maintained, typically for 2 to 4 days, at a temperature from ambient to 2000C, preferably above 50 C typi- cally from 100#to 1800C, such as 150"C which is believed to enhance the ordering of the polymeric film.

The monomolecular layers coated onto substrates in accordance with the present invention are more heat stable and resistive and, when annealed, more ordered than those prepared in accordance with prior art methods. The layers can act as gate insulators in field effect transistors and efficiency enhancers in photodiodes.

Semiconductor and superconductor devices which comprise a preformed polymeric coating formed in accordance with the method of this invention include photoresists; Josephson junctions of a stable variety capable of use in ultra high speed computer memories; gas detectors in field effect transistors of suitable structure; and electroluminescent devices.

The following Examples illustrate the invention.

EXAMPLE 1 A glass Langmuirtrough having dimensions 50 cmx 16 cm x 6 cm was filled with 2,500 ml of 2.5 x 10-4M aqueous Cd Cl2 at pH=5.6. The trough was then supported in a water bath maintained at 35OC which was itself seated on massive foundations to minimise vibration.

500 ijI of a solution of 0.1 mg ml- of poly(n-octadecyl vinyl ether/maleic anhydride) in chloroform were next spread on the aqueous surface in the trough to form a monolayer. The film was left for 2h to hydrolyse the anhydride groups and equilibriate.

It was then compressed by an adjustable boom of poly(ethylene tetrafluoride) at a surface pressure of 30 mNm-' and was maintained under constant pressure for 20 minutes by the boom which was adjustable to provide constant pressure by a differential feedback mechanism.

A single crystal S-doped n±Gap slice, polished on both sides, was next lowered at a speed of 3mm mien~1 through the monolayer and, after immersion, was removed at the same speed. The semiconductor, now coated on both sides with a monomolecular layer of the polymer, was then dried overnight in helium. Finally, the coated semiconductor was stored in dry nitrogen for 3d and then a Au electrode 50A thick was evaporated onto a coated surface at room temperature at 10-5 Torr.

EXAMPLES 2 and 3 In these Examples the procedure of Example 1 was repeated utilising, respectively, a chloroform solution of poly(styrene/maleic anhydride) and of poly(maleic anhydrideloctadecene-1).

Claims

1. A process for, the preparation of the ordered polymeric film on a substrate, which process comprises: i) providing a reservoir of the preformed polymer; ii) advancing the polymeric film receiving substrate into the reservoir at least once; and iii) recovering the substrate coated with the polymeric film.

2. A process according to Claim 1 wherein the reservoir comprises a monomolecular layer of the preformed polymer formed at a fluid phase interface.

3. A process according to Claim 2 wherein the monomolecular layer is maintained at a constant surface pressure.

4. A process according to Claim 2 or 3 wherein the fluid phase interface is an airlaqueous medium interface.

5. A process according to Claim 1 wherein the reservoir comprises a solution of the preformed polymer.

6. A process according to Claim 5 wherein the solution comprises an organic solvent.

7. A process according to any preceding claim wherein the polymer is an amphiphilic organic polymer.

8. A process according to Claim 7 wherein the polymer is a vinyl polymer.

9. A process according to Claim 8 wherein the vinyl polymer is rendered amphiphilic by the presence of both hydrophobic and hydrophilic pendant groups.

10. A process according to Claim 9 wherein the hydrophobic group comprises phenyl or C,0 to C20 alkyl(oxy) groups.

11. A process according to Claim 9 or 10 wherein the hydrophilic groups comprise carboxyl groups or precursors which are hydrolysable thereto.

12. A process according to any of Claims 9, 10 or 11 wherein the preformed vinyl polymer is a copolymer of an unsaturated acid anhydride with styrene a C12 to C22 alk-1 -ene or a C10 to C20 vinyl ether.

13. A process according to any of Claims 9 to 12 wherein the preformed vinyl polymer is an alternating copolymer.

14. A process according to any preceding Claim wherein the substrate comprises a semiconducting material.

15. A process according to Claim 10 wherein the semi-conducting material comprises silicon, a Ill-V compound or a Il-VI compound.

16. A process according to any preceding claim wherein the substrate comprises a superconducting material.

17. A process according to Claim 16 wherein the superconducting material comprises Nd, Nb, or Sn, or a compound thereof.

18. A process according to any preceding claim wherein the substrate is advanced through the reservoir of preformed polymer at a speed of 0.5 to 50 mm min-1.

19. A process according to any preceding claim wherein conditions are such that the substrate is coated in the X-mode.

20. A process according to any of Claims 1 to 18 wherein conditions are such that the substrate is coated-in theY-mode.

21. A process according to any preceding claim wherein the advancing and recovering operation are repeated.

22. A process according to any preceding claim wherein, after the final recovery operation, the coated substrate is dried.

23. A substrate coated with a preformed polymeric film whenever prepared by a process according to any of the preceding claims.

24. A substrate according to Claim 23 which comprises a semiconducting ora superconducting material.

25. A substrate according to Claim 24 onto the polymeric surface of which an electrode has been evaporated.

26. A substrate according to Claim 25 which is annealed.

27. A substrate according to Claim 26 wherein the annealing is effected at a temperature above 50 C.

28. A substrate according to any of Claims 23 to 27 wherein the thickness of the coating is as greater than 20A.