WO2003049873A1

WO2003049873A1 - Plastic substrates with polysiloxane coating for tft fabrication

Info

Publication number: WO2003049873A1
Application number: PCT/US2002/039307
Authority: WO
Inventors: Sunder Ram
Original assignee: Flexics, Inc.
Priority date: 2001-12-06
Filing date: 2002-12-06
Publication date: 2003-06-19
Also published as: AU2002351323A1; US20030108749A1

Abstract

A structure and method for protecting a plastic substrate from heat damage during fabrication of thin film transistors on the substrate. A polymer coating is applied to the plastic substrate that can act as a thermal barrier and withstand the silicon crystallization temperature provided by laser annealing of amorphous silicon. A combination of both inorganic and organic polymer material, and specifically a polysiloxane coating, is found to prevent damage to the plastic substrate during the crystallization process. A thin layer of polysiloxane liquid resin can be applied on the substrate by spin, dip or other similar techniques. In order to enhance the cross linking density of the polymer network, the coating is subjected to a short pre-cure at one temperature followed by a longer postcure at a higher temperature for several hours. A thin layer of oxide can be deposited over the polymer coating prior to the deposition of an a-Si film if desired, or, alternatively, the a-Si film may also be applied directly over the polymer coating.

Description

Plastic Substrates with Polysiloxane Coating for TFT Fabrication

Background Of The Invention

Field of the Invention The present invention relates to the fabrication of thin-film transistors on inexpensive, low-temperature plastic substrates. More specifically, the invention relates to a novel way of coating the plastic substrates to protect them from the rigors of the fabrication process.

Related Art

A recent development in the manufacture of display panels for such applications as

computers, cellular telephones, and personal data assistants ("PDAs"), is an interest in

manufacturing the backplanes for such displays on plastic substrates rather than on standard

glass, quartz or silicon wafer-based substrates. It is believed that the use of plastic substrates will result in displays that are 1) lighter in weight than present displays, 2) flexible, which

will help to prevent damage from some mishandling such as impact or dropping of the device containing the display, and 3) lower in cost.

The physico-mechanical properties of the plastic substrate are very important for

making flexible panel displays. In addition to requiring excellent dimensional stability of the

film, characteristics such as surface and thickness uniformity, light transmission, surface

scratch resistance, adhesion, chemical resistance and, permeability of moisture and gas play

key roles in the development of liquid crystal display ("LCD") and organic light emitting

diode ("OLED") displays.

The types of plastic for which these properties are suitable for use in displays are incapable of withstanding the processing temperatures used in conventional thin film transistor fabrication techniques, which typically may reach 600° C or more. Thus, various

techniques have been developed for reducing the temperatures required.

One such technique is to use a short laser pulse to crystallize silicon. The pulse

generates a sufficiently high temperature to crystallize the silicon locally, without subjecting the entire substrate to the same high temperature. Thus, in a thin-film transistor ("TFT") fabrication process such as that shown in Carey et al, U.S. Patent No. 5,817,550, a plastic

substrate is coated with an oxide such as silicon dioxide (SiO₂). An amorphous silicon ("a- Si") film is deposited on the oxide-coated plastic substrate, and is then subjected to a pulse

from a short-pulse ultra-violet laser, such as an XeCl excimer laser having a wavelength of

308 nm, for a time of less than 100 ns, to form a polycrystalline silicon ("poly-Si") film.

Plastic substrates may tolerate localized temperatures above their melting point for

extremely short periods, since the substrate itself may act as a heat sink and carry heat away

from the point of high temperature. However, a high enough temperature will exceed this capacity and cause damage to the substrate. Tests suggest that even the localized high

temperature generated during the short pulsed-laser crystallization process may cause local

damage to the plastic substrate if the thickness of the SiO₂ coating layer is less than 2 μm. (A

layer of a different oxide may need a different thickness.) Since the process time to deposit

an oxide layer with a thickness of 2 μm is around 20 minutes, it is obvious that requiring a

layer of this thickness will significantly reduce manufacturing throughput. Another problem

is that the oxide is somewhat brittle, and a layer this thick may crack and render the device

unusable.

Summary of the Invention In order to reduce the time needed to deposit the SiO₂ layer and thus shorten the

process while still protecting the plastic substrate, the present invention utilizes a polymer

coating applied on the plastic substrate that can act as a thermal barrier and withstand the

silicon crystallization temperature provided by the laser. It is advantageous if the polymer coating also has low moisture permeability and can thus act as a moisture barrier as well,

although this is not a necessary part of the present invention.

A polymer coating which is a combination of both inorganic and organic polymet

material, and specifically a polysiloxane coating, is found to prevent damage to the plastic

substrate during the crystallization process. A thin layer of polysiloxane liquid resin, when combined with a proper mixture of

solvents, can be applied on the substrate by spin, dip or other similar techniques in less than

30 seconds. In order to enhance the cross linking density of the polymer network, the coating

is subjected to a short pre-cure at one temperature followed by a longer postcure at a higher

temperature for several hours. This curing can be carried out in a batch process, and thus

does not affect the throughput. A thin layer of oxide can be deposited over the polymer

coating prior to the deposition of an a-Si film if desired, or, alternatively, the a-Si film may also be applied directly over the polymer coating.

Other objects and advantages of the present invention will become apparent from the

following description and accompanying drawings.

Description of the Drawings

The accompanying drawings, which are incorporated into and form a part of the

disclosure, illustrate an embodiment of the invention and its method of use, and, together with

the description, serve to explain the principles of the invention. Figure 1 is a cross-sectional view of a plastic substrate after bottom oxide and

amorphous silicon depositions, and illustrating pulsed laser irradiation, according to the prior

art.

Figure 2 is a cross-sectional view of a plastic substrate after polymer coating, bottom

oxide and amorphous silicon deposition, according to one embodiment of the present

invention.

Figure 3 is a cross-sectional view of a plastic substrate after polymer coating and

amorphous silicon deposition, according to another embodiment of the present invention.

Detailed Description of the Preferred Embodiment

Figure 1 illustrates the prior art as shown in Carey et al, U.S. Patent No. 5,817,550. A plastic substrate 10, after cleaning and annealing if necessary, is coated with a first layer 1 1 of a thermally insulating dielectric material like SiO₂. The layer 11 may be applied by

sputtering, physical vapor deposition (PVD), plasma enhanced chemical vapor deposition

(PECVD), or any other manner not requiring high temperatures.

The plastic may be one of a variety of types having characteristics that make it

acceptable for use as a substrate in a display device. Most tests to date have utilized

polyethylene terephthalate (PET) as the substrate material, which cannot withstand

temperatures greater than about 120°C. However, other materials having suitable

characteristics are believed to include polyethylene naphthalate (PEN), polycarbonate (PC),

polyarylate (PAR), polyetherimide (PEI), polyethersulphone (PES), polyimide (PI), Teflon polyperfluoro-alboxy fluoropolymer (PFA), polyether ether ketone) (PEEK), polyether ketone

(PEK), polyethylene tetrafluoroethylenefluoropolymer (PETFE), and polymethyl methacrylate

and various acrylate/methacrylate copolymers (PMMA). Certain of these plastic substrates can withstand higher processing temperatures of up to at least about 200°C, and some to 300-

350°C without damage.

After deposition of the insulating layer 11, an amorphous silicon film 12 having a thickness of 10 to 500 nm (most commonly in the range of 50 to 100 run) is deposited on the

insulating layer 11 by PECVD at a temperature of approximately 100°C. The a-Si film 12 is

then crystallized to form a poly-Si film by irradiating it with one or more laser pulses, as indicated at 13 in Figure 1. Again, an excimer laser is typically used, such as an XeCl

excimer laser having a 308 nm wavelength.

As above, in the case of PET, tests suggest that the thickness of the insulating layer

11 , if made of SiO₂, must be at least approximately 2 μm in order to prevent damage to the

plastic during the laser annealing process. The use of other plastics as substrate material, or

other dielectric materials as insulating layers, may require a different thickness.

It is known that certain inorganic polymers have characteristics of resistance to

temperature, ultraviolet light and hydrolysis. Thus, the application of an inorganic polymer as a film between the plastic substrate and the oxide layer or silicon layer has the potential to protect the plastic substrate from thermal damage during the laser irradiation 13. However,

inorganic polymers generally require high temperatures to achieve cross-linking, which is not

a practical proposition for temperature-sensitive plastic substrates. Also, inorganic polymers

tend to be brittle, and get more brittle as the thickness increases, and in this respect may not

offer an advantage over an oxide layer.

Certain organic polymers like polyurethane or epoxies may also provide heat

resistance and are quite flexible. However, organic polymers may absorb water and thus are not acceptable for display applications. What is needed is a polymer that combines the benefits of both organic and inorganic

polymers while minimizing the defects of each. One area of chemistry that has been regarded

as an alternative for ambient film forming and cross-linking has been a hybrid of inorganic/organic materials, generally known as polysiloxanes. Polysiloxanes have been used as abrasion resistant coatings on such items as contact lenses and airplane windows, made from polycarbonates and acrylates, but do not appear to have been used as thermal or moisture barriers, or on plastics such as polyesters like PET and PEN.

The typical polysiloxane reactions involving hydrolytic silanol condensation are,

Si — OR + H₂O «^■ Si — OH + ROH

Si —OH + HO - Si → Si — O - Si + H₂O

Si —OH + RO-Si → Si — O - Si + ROH

where R may be one of hundreds of organic groups. In general, aromatics, which contain

benzene, have a tendency to turn yellow and thus do not meet the requirement of good light

transmission. Aliphatics, which contain carbon chains, on the other hand, usually stay clear.

By combining organic and inorganic polymers, an acceptable compromise may be

found in which the film properties, such as adhesion, flexibility, chemical resistance and

durability, are all within acceptable limits. An ideal combination of organic and inorganic

moieties is clearly not always easy to attain. A polymer with too low a level of organic

component tends to produce films with too high a polysiloxane characteristic, i.e. glass-like

films, but with poor qualities in other areas. Systems with too high a level of organic

component, on the other hand, may detract from the polysiloxane properties, as well as being

more difficult to prepare in a stable dispersion.

Polysiloxane based coatings give quite different properties than conventional epoxies

and polyurethanes. A well-formulated polysiloxane system can impart excellent adhesion, flexibility, scratch resistance and chemical resistance. The glass transition (T_g) temperature

of polysiloxanes after ageing is typically over 100°C, while epoxies and polyurethanes with

similar solids content have glass transition temperatures on the order of 60°C and thus will

riot protect the substrate. (The T_g of PEN is about 120°C.)

Another potential benefit is that a layer of polysiloxane or other similar material

having a thickness of at least several microns may create a composite that has greatly improved thermo-mechanical properties (i.e. has a lower coefficient of thermal expansion,

resulting in less dimensional change between process steps) than the plastic substrate alone. Moreover, the film can potentially act as planarization layer, creating a surface that is smoother than the substrate surface.

The internal stress of the polysiloxane film is also very low when compared to high

solids epoxies, for example. The polysiloxanes exhibit a higher level of hydrophobic

characteristics in relation to conventional coating materials. The combination of high

hydrophobicity coupled with a high Tg allows polysiloxanes to be considered as a potential

for moisture barrier applications, as well as a thermal barrier.

Figure 2 illustrates one embodiment of the present invention. As in Figure 1, there is

a plastic substrate 10. Now, however, before the insulating layer 11 is added, a thin layer 14

of polymer material, such as polysiloxane, is deposited on the substrate by any method

suitable to its particular composition. For example, the polymer may be applied by dipping

the substrate in it, or spinning it on in the same fashion as many photoresist materials. The

insulating layer 11 and silicon layer 12 are then added as before, although the insulating layer 11 may be significantly thinner than in Figure 1. (The layer is added for reasons discussed

below, since it is no longer the means for insulating the plastic substrate 10 from heat.) Figure 3 illustrates an alternative embodiment of the present invention. As in Figure 2, plastic substrate 10 is first covered with a layer 14 of polymer material. However, now no

insulating layer is present and silicon layer 12 is deposited directly on polymer layer 14.

One polysiloxane coating resin that was evaluated for this application is CrystalCoat™ MP-101, which is manufactured by SDC Coatings Inc., Anaheim, CA. A similar material, TS-56HF, made by Tokuyama Corporation of Japan is also being

investigated. Spinning the MP-101 on to a plastic substrate for 20 seconds produced a layer

in the range of 1.5 to 2 μm. The MP-101 was then pre-cured at 92°C for 15 minutes,

followed by a postcure at 115°C for 3 hours.

The PEN substrate coated with MP-101 showed no visual damage when subjected to chemical compatibility tests using acetone, methanol and various acids including hydrofluoric

acid. The evaluation of the moisture barrier properties of polysiloxanes in general, and MP-

101 in particular, is being pursued, but initial tests show no significant moisture absorption.

It is believed that a thicker polymer layer will be more heat and moisture resistant, and

that if the polymer layer is thick enough and smooth enough, and defect free, then no oxide is

necessary and the amorphous silicon may be deposited directly on the polymer layer as shown

in Figure 3. However, attempts to increase the thickness of a layer of MP-101 to more than 3

μm are believed to result in the surface becoming less uniform than a thinner layer due to the

presence of streaks and lines, and unacceptable for further processing. One approach that

appears to avoid this problem is to spin on a coat of the material, cure it, and then add another

coat to achieve the desired thickness.

Another concern is that there may be small defects in the polymer layer. An approach

being investigated is to spin on a coat of polymer such as MP-101, cure it, and then add a thin

layer of oxide, for example a layer of SiO₂ that is 0.5 μm thick or less, to cover these defects and smooth the surface if necessary. This will still result in a reduction in the processing time

needed to grow the oxide layer of approximately 80% or more.

It is known in the industry that the handling of a bare flexible plastic sheet is an area of concern, due to scratches that may be left in the surface at various process steps. Application of the polysiloxane coating on both sides of the plastic wafer at the initial stage

of the process would also serve to create an abrasion resistant layer for the ensuing steps.

Many alternative embodiments are possible but have not yet been tested. For

example, dual or multiple layers of polysiloxane and an inorganic coating might also be

considered, and their heat and moisture permeation barrier characteristics tested. Another

layer of polysiloxane might be added on top of the oxide, or even multiple alternating layers

of polymer and oxide might be used.

As an alternative to thermal cure systems, development of polysiloxane barrier films using radiation cure chemistry including ultra violet (UV) and electron beam (EB) technology

will also be reviewed and conducted. This should provide an instant film without the requirement of a long post-curing step.

In the foregoing specification, the invention has been described with reference to

specific embodiments thereof. It will, however, be evident that various modifications and

changes can be made thereto without departing from the broader spirit and scope of the

invention as set forth in the appended claims. The specification and drawings are,

accordingly, to be regarded in an illustrative rather than a restrictive sense. Therefore, the

scope of the invention should be limited only by the appended claims.

Claims

ClaimsWhat is claimed is:

1. A composite material for use in fabricating semiconductor devices, comprising:

a plastic substrate; a substantially transparent dielectric layer; and

a polymer layer between the plastic substrate and the dielectric layer that

protects the plastic substrate from heat damage during processing of the semiconductor devices.

2. The composite material of claim 1, wherein the plastic substrate is a material selected

from the group consisting of PET, PEN, PC, PAR, PEL, PES, PI, Teflon PFA, PEEK, PEK,

PETFE and PMMA.

3. The material of claim 1 , wherein the polymer material in the thermal barrier is a

combination of one or more organic polymers and one or more inorganic polymers.

4. The material of claim 1, wherein the polymer material in the thermal barrier is a

polysiloxane.

5. The material of claim 1, wherein the dielectric layer is comprised of SiO₂, SiN, Al O₃

or polyamide.

6. The material of claim 1, further comprising a layer of silicon.

7. The material of claim 6, wherein the silicon is amorphous silicon.

8. The material of claim 6, wherein the silicon is polycrystalline silicon.

9. The material of claim 6, wherein the silicon is crystalline silicon.

10. A method of producing a composite material for use in fabricating semiconductor devices, comprising:

providing a plastic substrate;

applying a layer of polymer material over the plastic substrate that protects the

plastic substrate from heat damage during processing of the semiconductor devices;

and

applying a substantially transparent dielectric layer over the thermal barrier.

11. The method of claim 10, wherein step of providing a plastic substrate further

comprises providing substrate composed of a material selected from the group consisting of PET, PEN, PC, PAR, PEL, PES, PI, Teflon PFA, PEEK, PEK, PETFE and PMMA.

12. The method of claim 10, wherein step of applying a layer of polymer material further

comprises applying a material which is a combination of one or more organic polymers and

one or more inorganic polymers.

13. The method of claim 10, wherein the step of applying a layer of polymer material

further comprises applying a material which is a polysiloxane.

14. The method of claim 10, wherein the step of applying a transparent dielectric layer

further comprises applying a layer of SiO₂, SiN, Al₂O or polyamide.

15. The method of claim 1 , further comprising the step of applying a layer of silicon over

the dielectric layer.

16. The method of claim 15, wherein the step of applying a silicon layer further comprises applying a layer of amorphous silicon.

17. The method of claim 15, wherein the step of applying a silicon layer further comprises

applying a layer of polycrystalline silicon.

18. The method of claim 15, wherein step of applying a silicon layer further

comprises applying a layer of crystalline silicon.