WO2019008602A1 - Bi-layer resist approach of photolithographic patterning over pmma based polymer dielectrics - Google Patents

Abstract

The present invention discloses a method of performing conventional UV/Deep-UV lithography over pristine Poly(methyl methacrylate) (PMMA) or PMMA based hybrid/blended and multilayered polymer dielectric systems by employing bilayer photoresist stack. The method does not involve any complicated process such as chemical modification of pristine PMMA layer using cross-linking and photo-crosslinking agents, using additional protective/capping layer to protect PMMA layer, employing chemical wet etching or high- temperature processing. The bi-layer photoresist stack comprises a relatively thin top layer of a positive photoresist like Shipley's S1813, and a relatively thick, planarizing bottom resist layer of PMGI. Isopropyl alcohol (dimethyl carbinol) is used as photoresist stripper for easy stripping of the top resist layer during metal lift-off without dissolving or damaging pristine PMMA or PMMA based hybrid/blended and multi-layered polymer dielectric systems. The method is more cost efficient because of the low-temperature processing steps and use of conventional photoresists/developers to perform photolithography

Description

BI-LAYER RESIST APPROACH OF PHOTOLITHOGRAPHIC PATTERNING OVER

PMMA BASED POLYMER DIELECTRICS

FIELD OF THE INVENTION:

The present invention generally relates to a method of forming a fine pattern using conventional UV/Deep-UV lithography (hereafter, simply referred to as photolithography) for fabricating organic/inorganic/perovskite-based electronic devices (including microelectronic devices) and more particularly relates to a method of performing photolithography over pristine poly(methyl methacrylate) (PMMA) or PMMA based hybrid/blended and multi-layered polymer dielectric systems (hereinafter, simply referred to as PMMA based polymer dielectrics) without any degradation of their physical or electrical properties.

BACKGROUND OF THE INVENTION:

PMMA has been one of the most widely studied polymer dielectrics for the last few decades. PMMA is a versatile material having a unique combination of excellent properties like high transparency from the near UV to the near IR wavelengths, high chemical resistance, good mechanical flexibility, thermal stability, excellent insulating electrical behavior, film formability, mouldability, easy shaping, and biocompatibility. Because of its excellent chemo-physical and thermal properties, PMMA finds its application in various technological fields and has been used extensively for optical devices [Yeniay A, Gao R, Takayama K, Gao R and Garito A F 2004 IEEE J. Lightwave Technol. 22 154], polymer- MEMS devices [Chung J, Huang Y and Hsu W 2008 Fabrication of polymer-based vertical comb drive using a double-side multiple partial exposure method Proc. MEMS 2008 pp 475-8],[ Zhao Y and Cui T 2003 Fabrication of high-aspect-ratio polymer based electrostatic comb drives using the hot embossing technique J. Micromech. Microeng. 13

430-5], Bio-MEMS devices [Kin Fong Lei, Chih-Hsuan Chang, and Ming-Jie Chen, ACS

Applied Materials & Interfaces 2017 9 (15), 13092-13101] , Nano-electronics [Chang, E. Y.

l et al. Submicron T-shaped gate HEMT fabrication using deep-UV lithography. IEEE Electron Device Lett. 15, 277-279 (1994)], and organic (or flexible) electronics.

In recent years, organic electronics has emerged as a potential candidate for low cost and flexible thin film electronic industry. For the fabrication of truly low cost and flexible electronics, organic polymer gate dielectrics are the most suited candidates. The key features of the polymer dielectrics/semiconductors such as solution processability, mechanical flexibility, and low-temperature processing are the main contributor to the manufacturing of low cost and flexible electronic devices.

Among the very popular polymer gate dielectrics, Poly(4-vinyl phenol) (PVP) has been employed most frequently for the applications such as organic integrated circuits, organic imagers because of the possibility to perform photolithography over crosslinked PVP using different approaches. However, the presence of the hydroxyl groups (-OH groups) in PVP dielectric film easily attracts H₂0 to cause slow polarization, severe hysteresis and substantial threshold voltage shift (AVT) during gate bias stress.

Contrary to this, PMMA performs outstandingly when it comes to the role of the polymer gate dielectric layer as it results in a smooth and clean interface between PMMA and most of the organic semiconductors. It also has been used as a buffer layer to reduce the hysteresis by introducing a thin layer of PMMA in between dielectric gate layer and the organic semiconductor layer. For example, Huang et al., ATP Advances 3, 052122 (2013), has shown reduced hysteresis and enhanced mobility in the polyvinyl alcohol (PVA) dielectric based pentacene organic thin film transistors (OTFTs) by introducing thin buffer layer of PMMA between PVA gate dielectric and pentacene semiconductor interface. Because of its excellent chemo-physical properties, thermal properties and low dielectric constant, PMMA is one of the promising host matrices to realize composite or hybrid dielectrics based on polymer matrices with embedded inorganic components. J. Li et al. , J. Mater. Chem., 2012, 22, 15998-16004, demonstrated high-performance, low voltage OTFTs employing PMMA/high- k Vinylidene fluoride-trifluoroethylene-chlorofloroethylene terpolymer), P(VDF-TrFE-CFE), bi-layer as a gate dielectric layer. In the context of fabrication of OTFTs, a considerable amount of effort has been invested in improving the performance of solution processed OTFTs to make it compatible for various applications such as low-cost integrated circuits, active pixel image sensors and active matrix pixel drivers for a display application, etc. For the improved performance of OTFTs channel length scaling, the patterned gate electrode and isolation between the devices by using photolithography is a promising approach for large-scale manufacturing.

However, solution-based processing of organic or polymer materials severely restrict the realization of smaller feature size devices with high resolution and controlled process accuracy due to the incompatibility of the organic polymers with the commonly used photoresists and photoresist developers. Conventional photoresist processing causes severe degradation of the organic polymers resulting in poor device performance. Low organic solvent resistance and difficulty to perform photolithography over polymer gate dielectrics or semiconductors impose severe restriction in the commercialization of this future technology.

Photolithography is a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light- sensitive chemical photoresist on the substrate. A series of chemical treatments then enable deposition of new material in the desired pattern upon the material underneath the photoresist. It is the standard method of the printed circuit board (PCB) and microprocessor fabrication. Difficulty in using lithographic patterning is one of the critical challenges in the area of organic electronics, and it restricts the use of organic polymer dielectrics and semiconductors for applications such as integrated circuits, LED driving circuits, image sensors, RFID's, displays, etc.

Generally, different approaches are available to perform photolithography on top of organic polymers such as (1) usage of a protective layer like cytop, followed by 0₂ plasma etching, (2) employing crosslinking molecules or perfluorinated polymers or (3) using water-soluble resist. However, none of these processes allow a truly low cost, low temperature and easy fabrication which are the key metrics of organic electronics. In this direction, there has not been much progress in developing a solution to facilitate photolithography over pristine PMMA or PMMA based polymer dielectrics. For instance, in an attempt to demonstrate the fabrication of patterned and PMMA gate dielectric based

100 OTFTs using photolithography, I. Mejia et al. showed the possibility of fabricating photolithographically patterned top gate bottom contact (TGBC) OTFT structure by etching the deposited metal layer for gate electrode over PMMA. The method is compatible only with the TGBC OTFT fabrication since in this approach the gate dielectric layer is protected by the deposited metal layer over which lithography is performed. The etching process is not a

105 preferred method for the bottom gate bottom contact (BGBC), bottom gate top contact (BGTC) and top gate top contact (TGTC) device configuration because it can damage or contaminate the dielectric- semiconductor or the metal- semiconductor interface. A better option is to use lift-off processing along with photolithography to pattern metal or dielectric layer, employing a positive or a negative photomask in the lithographic process.

110

The critical issue in realizing micropatterns of a thin film over PMMA using photolithography is the incompatibility of the PMMA with the commonly used photoresists, photoresist developers, and photoresist strippers. Meanwhile, several research groups have suggested different means of crosslinking PMMA to enhance its solvent resistance and

115 improved dielectric performance such as (1) cross-linking poly (methyl methacrylate) by ion- beam irradiation, (2) methods of photo-crosslinking the polymer by chemically modifying the base polymer, (3) bi-functionalized PMMA for effective cross-linking by TAAC reaction at a relatively low temperature (100°C) and (4) cross-linking based on blending an insulating base polymer such as poly(methyl methacrylate) with organosilane crosslinking agent (1, 6-bis

120 (trichlorosilyl) hexane).

All the methods mentioned above have some disadvantages and do not facilitate easy and low-temperature processing as well as large scale-low cost fabrication. Some of the techniques are not compatible for large area applications. Some methods require controlled 125 environment processing, for example, in the process of crosslinking PMMA reported by Noh et. al. Organic Electronics: physics, materials, applications, vol. 10, no. 1, pp. 174-180, the crosslinking requires the presence of moisture which imposes constraint over the choice of organic semiconductors, because most of the organic semiconductors degrade once they interact with the ambient. Most of the other reported methods are limited to labs and require 130 rigorous chemical processing and understanding which results in complex processing and poor adaptability. Meanwhile, there is no prior art which specifies a method to perform lithography over the reported modified PMMA or PMMA based polymer dielectrics.

Among the methods available for producing metal or oxide microstructures for 135 semiconductors, a bi-layer lift-off is one common method employing a bi-layer photoresists stack to pattern metal or dielectric films in the micrometer or sub-micrometer range. The liftoff refers to the process of exposing a pattern into photoresist, depositing a thin film such as a metal or dielectric over the entire area, then washing away the photoresist to leave behind the film only in the patterned area. By using two different types of resist on top of each other, one 140 can precisely pattern the top resist and then undercut the bottom resist to form a very nice liftoff profile. MicroChem's polymethylglutarimide (PMGI), LOR, or Shipley's LOL series resist works well as under layers for i-line and broad-band lithography. The top layer can be positive or negative tone photoresist and it can lift-off up to -2/3 of the bottom layer thickness.

145

Many methods employing bi-layer resist stack for photolithography are known in the existing art. For example, United States Patent No. 7960097 to Frank Hin Fai Chau et al., entitled "Methods of minimizing etch undercut and providing clean metal liftoff deals with fabrication of semiconductor devices by performing photolithographic technique over silicon 150 nitride dielectric layer where a bi-layer resist mask is employed, which comprises of bottom resist layer and top resist layer. The bottom resist layer includes PMGI/PMMA, and top imaging photoresist layer includes positive photoresist such as AZ 9245, Shipley Company's S I 800 series or Shipley SCI 827.

155 United States Patent No. 5122387 to Hiroshi Takenaka et al., entitled "Developing solution and pattern forming method using the same" deals with a method for forming patterns on a semiconductor (GaAs) substrate by employing poly-(dimethylglutarimide) as lower resist layer and PMMA as upper resist layer and uses tetramethyl ammonium hydroxide as developer.

160 United States Patent No. 9684234 to Seth B. Darling et ah, entitled "Sequential infiltration synthesis for enhancing multiple-patterning lithography" deals with multiple-patterning lithography methods in which PMMA and PMGI are used as resist layers, isopropyl alcohol, and methyl isobutyl ketone are used as a developer, and tetramethylammonium hydroxide is 165 used as a stripper.

PCT application No. 1990003598 to James E. Lamb et al., entitled "Multifunctional photolithographic compositions" deals with a process of photolithography over microelectronic substrates. The process does not employ any crosslinking of the polymer and uses 170 multifunctional sublayer consisting essentially of poly-(vinyl pyridine) and alkaline developer solution. The above procedure does not allow any decomposition of PMMA.

A reference titled "Electronic properties of organic thin film transistors with nanoscale tapered electrodes" to Jeongwon Park relates to a bi-layer photoresist photolithography 175 process where heavily doped Si substrate and the S1O2 served as a common gate and gate dielectric, respectively. PMGI acts as a bottom resist layer, and Microposit® S I 805 photoresist is used as a top resist layer. Microposit® MF319 of Shipley Corporation is used as a developer solution.

180 Though the existing prior art discloses bi-layer resist approach (using bi-layer resist stack) for performing photolithography over different dielectric layers, none of the prior art employs bi- layer resist approach of photolithographic patterning over pristine PMMA or PMMA based polymer dielectrics. Also, none of the references use isopropyl alcohol (dimethyl carbinol) as a stripper for easy stripping of the top resist layer during metal lift-off process.

185

Hence, there is a need for developing a method of photolithographic patterning which facilitates the photolithography over pristine PMMA or PMMA based polymer dielectrics without using high-temperature processing, chemical modification using cross-linking and photo-crosslinking agents, chemical wet etching, and protective/capping layer which results 190 in high temperature and high fabrication cost. Thus, the present invention also opens up a new avenue to implement widely accepted photolithography technology in many exciting areas like organic electronics successfully along with tremendous flexibility in choosing PMMA and different PMMA based polymer 195 dielectrics

SUMMARY OF THE INVENTION:

An object of the present invention is to provide a process flow to perform photolithography 200 over pristine PMMA or PMMA basedpolymer dielectrics employed in various applications such as a polymer gate dielectric layer in the fabrication of OTFTs.

Another object of the present invention is to provide optimized photolithography process steps which result in no physical or electrical degradation of pristine PMMA or PMMA based 205 polymer dielectrics.

Another object of the present invention is to provide a method to perform photolithography and realizing the patterned micron size features over pristine PMMA or PMMA based polymer dielectrics without employing any chemical modification like crosslinking, photo- 210 crosslinking, chemical wet etching or using capping layer.

Another object of the present invention is to provide optimized low-temperature process steps of photolithography over pristine PMMA or PMMA based polymer dielectrics to facilitate the use of flexible substrates in various applications such as a polymer gate dielectric layer in 215 the fabrication of flexible OTFTs.

Yet another object of the present invention is to use methacrylic polymers based resists especially polyglutarimide or poly-(dimethylglutarimide) based resists obtained through imidization of PMMA such as MicroChem' s Polymethylglutarimide (PMGI) as a bottom 220 resist layer coated directly over pristine PMMA or PMMA based polymer dielectrics. Still another object of the present invention is to use isopropyl alcohol as a stripper for resist layers like Shipley's S 1813 positive photoresist, which allows easy stripping of the top resist layer during metal lift-off process without dissolving or damaging pristine PMMA or PMMA 225 based polymer dielectrics.

According to the present invention, a method for performing photolithography over pristine PMMA or PMMA based polymer dielectrics is provided to realize three-dimensional micron size features employing chemically resistant bi-layer resist stack.

230

The method involves the formation of bi-layer resist stack of methacrylic polymers based resists especially polyglutarimide or poly-(dimethylglutarimide) based resists such as MicroChem' s Polymethylglutarimide (PMGI) as a bottom resist layer (which acts as the first layer of bi-layer resist stack) coated directly over pristine PMMA or PMMA based polymer 235 dielectric layer followed by the formation of the top resist layer (that acts as the second layer of bi-layer resist stack) of a typical g-line, i-line, broadband, deep UV, and 193nm (positive or negative tone) resist by spin coating method. The method is a low-temperature process and facilitates the easy lift-off of deposited metal layers.

240 Also, in the present invention, isopropyl alcohol is used as resist stripper during lift-off process. For developing the exposed bi-layer resist stack mentioned above, tetramethyl ammonium hydroxide (TMAH) based developers such as Shipley's MF319 are used without any physical or electrical degradation of pristine PMMA or PMMA based polymer dielectrics.

245

These objectives and advantages of the invention will become more evident from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

250

The objective of the present invention will now be described in more detail concerning accompanying drawings, in which: FIG. 1 represents a schematic of pristine PMMA or PMMA based polymer dielectric film on 255 a substrate;

FIG. 2 represents a schematic of formation of methacrylic polymers based resist especially polyglutarimide or poly-(dimethylglutarimide) based bottom resist layer over pristine PMMA or PMMA based polymer dielectric film on a substrate;

260

FIG. 3 represents a schematic of formation of the top resist layer over the bottom resist layer;

FIG. 4 represents a schematic of exposing the bi-layer resist stack using suitable radiation such as UW Deep-UV radiation through a photomask;

265

FIG. 5 represents a schematic of pattern formation after developing the exposed bi-layer resist stack employing tetramethylammonium hydroxide based developer;

FIG. 6 represents a schematic of lift-off of the deposited metal over the patterned structures 270 using isopropyl alcohol which acts as a top resist layer stripper;

FIG. 7 represents a schematic of photolithographically patterned metal layer over pristine PMMA after removal of the bottom resist layer;

275 FIG. 8 represents a schematic of fabricated PMMA based MIM devices;

FIG. 9 represents a graph showing capacitance as a function of applied electric field for the two device sets fabricated using a shadow mask and lithographic patterning process of the present invention, respectively;

280

FIG. 10 represents a graph showing dielectric constant as a function of applied electric field for two device sets fabricated using a shadow mask and lithographic patterning process of the present invention, respectively; 285 FIG. 11 represents a graph showing leakage current density as a function of applied electric field for two device sets fabricated using a shadow mask and lithographic patterning process of the present invention, respectively; and

FIGS. 12(a) and (b) show typical topographical AFM images of pristine PMMA layer before 290 and after processing the layer through the lithographic patterning process of the present invention respectively.

DETAILED DESCRIPTION OF THE INVENTION:

295 The present invention provides a method to perform photolithography over pristine PMMA or PMMA based polymer dielectrics to realize three dimensional micron size features employing chemically resistant bi-layer resist approach without using any complicated chemical modification such as crosslinking, photo-crosslinking, etc. of pristine PMMA or using additional protective/capping layer to protect PMMA layer or employing chemical wet

300 etching or high-temperature processing.

The method comprises the optimized process steps which result in no physical and electronic degradation of the PMMA film during the whole photolithography process and facilitates easy implementation of widely used photolithography over pristine PMMA and different 305 PMMA based polymer dielectrics in various applications such as polymer-MEMS devices, nano-electronic devices, and organic electronics successfully. Using the bi-layer resist approach in accordance with the present invention also reduces the complexity of the process as well as makes it cost efficient because of the low-temperature processing steps and use of conventional photoresists/developers to perform photolithography.

310

In the bi-layer resist approach, the present method involves the formation of bi-layer resist stack of a relatively thick methacrylic polymers based resist especially polyglutarimide or poly-(dimethylglutarimide) based resist, more specifically MicroChem's PMGI, as a bottom resist layer (also the first layer of bi-layer resist stack) coated directly over pristine PMMA or 315 PMMA based polymer dielectric film followed by the formation of the relatively thin top resist layer (also the second layer of bi-layer resist stack) of a typical g-line, i-line, broadband, deep UV, 193nm (positive or negative tone) photoresist, more specifically Shipley's S 1813 photoresist, on top of the bottom resist layer by spin coating method.

320 The bottom resist layer is chemically compatible with the PMMA or PMMA based polymer dielectrics, that is, the bottom resist layer does not attack chemically on the polymer chain, with resultant reduction in physical properties, including oxidation; reaction of functional groups in or on the chain, and depolymerization; physical change, including absorption of solvents, resulting in softening and swelling of the PMMA or PMMA based polymer

325 dielectrics; permeation of solvent through the PMMA or PMMA based polymer dielectrics and dissolution in a solvent.

The Poly-(dimethylglutarimide) based resists such as MicroChem's PMGI are compatible with conventional g-line, i-line, broadband, deep UV, 193nm, and e-beam photoresists, for

330 example, Shipley's S 1813 positive photoresist and PMMA (e-beam photoresist). PMGI is used more often as a lift-off resist in a bi-layer resist systems. PMGI is resistant to general solvents, less susceptible to intermixing with other polymers or resists when stacked. Poly- (dimethylglutarimide) based photoresists are also sensitive to Deep-UV (240-290nm) exposure which facilitates the possibility of realizing sub-micron scale patterns. The physical

335 and electrical characterization shows that PMGI is compatible with the PMMA and does not interact with PMMA layer or does not create any damage to the PMMA layer.

For developing the exposed bi-layer resist stack mentioned above, tetramethylammonium hydroxide (TMAH) based developers such as Shipley's MF319 are used without causing any 340 physical or electrical degradation of the PMMA layer. The developer is used after exposure for developing the top and bottom resist layers.

On the other hand, PMMA is very sensitive to most of the organic solvents including Acetone which makes the stripping of the developed photoresist difficult. However, isopropyl alcohol 345 interacts with PMMA with an extremely slow reaction rate. Isopropyl alcohol is more precisely a very weak solvent for the low molecular weight PMMA and known as a non- solvent for high molecular weight PMMA such as 950 K or 495 K PMMA. Hence, the present invention uses isopropyl alcohol as a photoresist stripper for easy stripping of the top resist layer during metal lift-off process without dissolving or damaging PMMA layer. 350 During the lift-off process, the bottom resist layer is not stripped and thus protects the pristine PMMA below it.

The photolithography method of the present invention comprises the following steps:

355 (a) Formation of pristine PMMA or PMMA based polymer dielectric film on a rigid/flexible substrate made of a material such as glass, Si/Si02, aluminum foil or a plastic substrate (PET, PEN substrates), etc. as shown in FIG. 1.

(b) Formation of a bi-layer resist stack on the substrate prepared in step (a) comprising:

360

I. A relatively thick methacrylic polymers based resist especially polyglutarimide or poly-(dimethylglutarimide) based bottom resist layer, more specifically MicroChem's PMGI as a bottom resist layer (also the first layer of bi-layer resist stack) formed by spin coating followed by annealing of the formed film at 365 temperature in the range of 70°C to 150°C, more specifically, annealing

MicroChem's PMGI bottom resist layer at 115°C temperature, as shown in FIG. 2.

II. A relatively thin top resist layer (also the second layer of bi-layer resist stack) of Shipley's S 1813 or a conventional g-line, i-line, broadband, deep UV, 193nm 370 (positive or negative tone) photoresist coated on top of the bottom resist layer prepared in step (I), as shown in FIG. 3

(c) Exposing the bi-layer resist stack to the activating radiation, more specifically to UV/Deep-UV radiation suitable for the top resist layer, as shown in FIG. 4

375

(d) Developing the bi-layer resist stack using tetramethylammonium hydroxide based developers such as Shipley's MF-319, as shown in FIG. 5 (e) Metal lift-off is done by employing isopropyl alcohol as the top photoresist layer stripper.

380 During the metal lift-off, isopropyl alcohol does not interact with the bottom resist layer.

Bottom resist layer acts as a barrier between the pristine PMMA layer and isopropyl alcohol, as shown in FIG. 6. In the process of metal lift-off using isopropyl alcohol, the temperature of isopropyl alcohol is maintained at sufficient temperature, more preferably at a temperature in the range of 35°C to 55°C, to enhance the reactivity of isopropyl

385 alcohol in order to enable the stripping of the top photoresist layer. Temperature is the most crucial parameter in the present invention to achieve perfect lift-off of the metal layer using isopropyl alcohol without damaging the pristine PMMA layer.

(f) Bottom resist layer is removed by using the tetramethylammonium hydroxide (TMAH) 390 based developer solutions such as Shipley's MF319 which results in a patterned metal layer over pristine PMMA without dissolving or damaging pristine PMMA layer, as shown in FIG. 7.

To analyze the physical and electrical degradation, if any, of the PMMA layer due to the bi- 395 layer resist approach of photolithography in accordance with the present invention, two sets of metal-insulator-metal (MIM) devices employing pristine PMMA as an insulator, as shown in FIG. 8, has been fabricated and detailed physical and electrical characterization of the PMMA films has been performed.

400 Fabrication steps:

1. Deposition of the aluminum anode electrode on the cleaned glass substrate by thermal evaporation of 60 nm thick Al contact layer.

405 2. Spin coating a 450 nm thick blanket layer of pristine PMMA from 6 wt.% solution of

PMMA in anisole solvent on top of the formed aluminum anode electrode. The film is annealed at 120°C temperature for 30 minutes.

3. Formation of Aluminum cathode electrode (top electrode): Two sets of MIM devices

410 are prepared; in one set of the devices the top contact of the MIM structure (A1/PMMA/A1) is patterned by using shadow mask, and in the second set of devices the top contact is patterned by using bi-layer resist approach of photolithography in accordance with the present invention.

415 Physical damage is analyzed using the experimental methods explained below:

1. Thickness measurement: Thickness of the pristine PMMA layer is measured by using optical ellipsometry and optical profilometer before and after performing photolithography steps of the present invention. The results indicate that there was no

420 change in the thickness of the layer before and after processing the layer through the lithography process of the present invention.

2. Surface morphology: Surface morphology of the polymer layer can have a significant influence in many applications such as morphology of gate dielectric layer influences

425 the performance of OTFTs. Surface morphology of pristine PMMA layer before and after processing the layer through the lithography process of the present invention are investigated by AFM imaging. Fig. 12(a) and (b) show typical topographical AFM images of pristine PMMA layer before and after processing the layer through the lithography process of the present invention, respectively. The surface root mean

430 square (rms) roughness of the pristine PMMA layer over a 1 μιη x 1 μιη area was found to be 0.2 nm and 0.6 nm before and after processing the layer through the lithography process of the present invention, respectively. Hence, there was no significant change observed in the surface roughness of the PMMA film before and after processing the film through the present method of photolithography. The pristine

435 PMMA layer had a very smooth surface even after processing the layer through the lithography process of the present invention.

To understand the effect of the bi-layer resist approach of photolithography in accordance with the present invention on the electrical and dielectric properties of the pristine PMMA 440 layer, electrical characterization of fabricated metal-insulator-metal (MIM) devices is done as shown in FIG. 9, FIG. 10 and FIG. 11. The dielectric constant (ε) and the leakage current density plots as a function of the applied electric field are shown in FIG. 10 and FIG. 11. There is a small increase in the dielectric constant (2.63 to 2.92) of the layer exposed to the lithographic patterning process of the present invention.

445

The leakage current density in the device fabricated using bi-layer resist approach of photolithography in accordance with the present invention is reduced almost by two orders of magnitude compared to the device fabricated using a shadow mask. The measurements of MEVI structure concludes that there is no damage to the dielectric behavior of pristine PMMA 450 layer after performing the photolithography steps of the present invention and results in improved insulating behavior in terms of reduced leakage current density.

Thus, the results are auspicious and convincing regarding the ease and the adoption of the process employing all the conventional elements frequently used in conventional UV/Deep- 455 UV lithography processing.

While the preceding written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the 460 specific embodiment, method, and examples herein. The invention should therefore not be limited by the above-described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention as claimed.

Claims

Claims: 465

1. A bi-layer resist stack for performing conventional UV/Deep-UV lithography, comprising: a. a thick bottom resist layer deposited directly over PMMA or PMMA based 470 polymer dielectrics, and

b. a thin upper resist layer deposited on top of said bottom resist layer.

2. The bi-layer resist stack as claimed in claim 1, wherein said bottom resist layer is chemically compatible with said PMMA or PMMA based polymer dielectrics.

475

3. The bi-layer resist stack as claimed in claim 1, wherein said upper resist layer is a g-line, i-line, broadband, deep UV, or 193nm (positive or negative tone) photoresist.

A method for performing conventional UV/Deep-UV lithography using a bi-layer resist stack, comprising the steps of:

a. forming pristine PMMA or PMMA based polymer dielectrics on a rigid or flexible substrate, wherein said substrate is selected from glass, Si/Si02, flexible metal sheet preferably stainless steel, aluminum or flexible plastic substrate specifically a PET or, a PEN substrate;

b. forming a bi-layer resist stack on said substrate prepared in step (a), wherein the said bi-layer resist stack comprising: (i) a thick bottom resist layer deposited directly over said PMMA or PMMA based polymer dielectrics by spin coating followed by annealing of said formed layer at a temperature in the range of 70°C to 150°C and (ii) a thin upper resist layer deposited on top of said bottom resist layer by spin coating;

c. exposing the said bi-layer photoresist stack to the suitable UV or Deep-UV radiation through a photomask;

d. developing said exposed bi-layer photoresist stack using developer to form patterned structures;

e. depositing a metal over said patterned structures;

f. performing lift-off of said top resist layer using photoresist stripper; and g. removing said bottom resist layer using said developer.

5. The method as claimed in claim4, wherein said thin upper resist layer is a g-line, i-line, 500 broadband, deep UV, or 193nm (positive or negative tone) photoresist.

6. The method as claimed in claim4, wherein said thin upper resist layer is most preferably Shipley's S 1813 positive photoresist.

505 7. The method as claimed in claim 4, wherein said thick bottom resist layer is methacrylic polymers based resist preferably polyglutarimide or poly-(dimethylglutarimide) based resist obtained through imidization of PMMA.

8. The method as claimed in claim 4, wherein said thick bottom resist layer is MicroChem's 510 polymethylglutarimide (PMGI).

9. The method as claimed in claim 4, wherein said thick bottom resist layer is annealed most preferably at 115°C temperature.

515 10. The method as claimed in claim 4, wherein said developer is based on tetramethylammonium hydroxide (TMAH).

11. The method as claimed in claim 4, wherein said developer is more preferably Shipley's MF-319 or MF-321.

520

12. The method as claimed in claim 4, wherein said photoresist stripper is isopropyl alcohol.

13. The method as claimed in claim 4, wherein said photoresist stripper is maintained at a temperature during the metal lift-off, more preferably in the temperature range of 35°C to 55°C.

530