Post Exposure Bake Research Articles

Physical Vapor Deposition of Molecular Glass Photoresists: A New Route to Chemically Amplified Patterning

AbstractA negative tone photoresist film, consisting of a molecular glass, a photoacid generator, and an acid labile crosslinker, was prepared by physical vapor deposition, a solvent‐free process. Subsequent to deposition, the coevaporated monomers were exposed using 365 nm radiation, subjected to a post exposure bake step, and developed in aqueous base to produce sub‐micron patterns. Combinatorial techniques were used to aid optimization of the photoresist by systematic variations in composition and exposure dose. Development factors such as concentration and time were also optimized.

Delamination Analysis of Low-Temperature Processed SU-8 Photoresist for MEMS Device Fabrication

Negative SU-8 photoresist processed at several levels of lower temperature than conventional approach was investigated, and its low-temperature processing has been characterized in terms of delamination. According to two phases of statistical design of experiment (DOE), initially statistically significant variables of SU-8 photoresist processing were selected, and the selected variables were further investigated to find their effects on delamination of SU-8 microstructure. A three leveled factorial designed experiments were performed followed by a 2(6-1) fractional factorial designed as a screening experiment. Characterizing low-temperature SU-8 photoresist process, 27 samples were fabricated and the degree of delamination was measured. In addition, nine additional samples were fabricated for the purpose of verification. Employing a neural network modeling technique, a process model is established, and response surfaces are generated to investigate the degree of delamination associated with three process parameters: post exposure bake (PEB) temperature, PEB time, and exposure energy. From the response surfaces generated, two significant parameters associated with delamination are identified, and their effects on delamination were analyzed. The higher the post exposure bake (PEB) temperature at a fixed PEB time and the more delamination occurred. In addition, the higher the dose of exposure energy, the lower the temperature at which the delamination begins and the larger the degree of delamination. The results identified acceptable ranges of the three process variables to avoid the delamination of SU-8 film, which in turn might lead to potential defects in MEMS device fabrication.

Synthesis and Characterization of Some Acrylate Copolymers and their Evaluation as Aqueous Developable Photoresists

In the recent past, it has been found that the use of onium salts as acid-generating species has improved the performance of photoresists, particularly for microelectronic applications. However, little research has been dedicated to the use of these photo-initiators in aqueous-developable resists. This work describes the synthesis and characterization of some water-soluble polymers and investigates the feasibility of employing one of the onium salts in the formulation of a resist polymer for use in aqueous conditions. The ability of this onium salt to photo-initiate the cationic polymerization of epoxies residues was exploited using water-soluble copolymers, containing glycidyl methacrylate (GMA). The comonomers included methacrylic acid. Various copolymers were prepared by radical copolymerization typically in methyl ethyl ketone. It was found that the copolymer containing 85 mol.% of GMA unit in feed was soluble in aqueous base and crosslinked in the presence of photogenerated acid caused by acid-initiated ring opening polymerization of the pendant epoxide groups. A good pattern with reasonable resolution was attained by exposure to a UV lamp with λ = 365 nm light followed by a post exposure bake at 80° C for 1 min.

Study of stress and adhesion strength in SU-8 resist layers on silicon substrate with different seed layers

We present the details of our study on the internal stresses and adhesion strengths of SU-8 structures to different substrate seed layers. The effect of adhesion promoter-methacryloxy [propl] trimethoxysilane (MPTS), and OmniCoat-and different seed layer combinations (Ti/Cu/Ti, Ti/Cu, Cr/Au, and Cr/Au/Cr) was examined for internal stress and adhesion strength in 650-µm-thick SU-8 films. Increased stress and poor adhesion have led to the delamination of SU-8 in some cases. Adhesion and stress have proven to be the function of process parameters such as soft bake (time and temperature), exposure dose, post-exposure bake (time and temperature), and development time. We have found that a 100 silicon wafer containing a titanium-copper-titanium (Ti/Cu/Ti) seed layer with MPTS as the adhesion promoter yielded a thick SU-8 film with a lower value of stress and consequently better adhesion for processing in deep x-ray lithography (DXRL). A detailed correlation of the effects of adhesion and internal stress on the SU-8 film is discussed. An analysis of the possible chemical bonding interactions occurring between SU-8, and its adhesion promoter and/or substrate is also presented.

Control of the out-of-plane curvature in SU-8 compliant microstructures by exposure dose and baking times

The effect of processing conditions on the curvature of SU-8 cantilevers up to 6400 µm long is determined by experimental methods. Our observations suggest that a zero curvature condition can be achieved by controlling the exposure dose and post-exposure baking time for any given SU-8 thickness. The curvature of SU-8 cantilevers processed with different exposure doses and post-exposure baking times has been measured and reveals that the gradient of crosslinking density throughout the SU-8 film is the cause of out-of-plane stress. A general model for the SU-8 cantilever curvature is developed which explains the behavior of SU-8 structures that have been processed with different conditions and can be used to predict the general effect on the SU-8 cantilever curvature for different SU-8 thicknesses and process parameters.

Simulation of mask proximity printing

The paper presents a simulation approach for mask proximity printing. The simulation steps include image formation in air and in photoresist, post-exposure bake, and chemical development and analysis of resist profiles. The intensity distribution in air and in resist in proximity distance from the mask is described by a fast frequency domain method that is based on scalar diffraction analysis. The accuracy and the performance of the method are compared with rigorous electromagnetic field computations that take mask topography effects and the finite conductivity of absorber materials into account. The computed intensity distributions inside the resist are coupled to an established standard simulation flow, which is also used in the simulation of projection printing. The resulting resist profiles can be evaluated in terms of linewidth, sidewalls, and other parameters. Finally, an application of the simulation procedure for the simulation of process windows and optimization of linewidth biasing is shown.

Challenges of the mask manufacturing approaching physical limits

Over the past 25 years, following the International Technology Roadmap for Semiconductors 〈 http://www.itrs.net/Links/2006Update/FinalToPost/08_Lithography2006Update.pdf〉, 2006 [1], the main feature size of integrated circuits has decreased from approximately 3 μm to 70 nm. With feature sizes well below the exposure wavelength of the stepper, resolution enhancement features such as serifs, scatter-bars, and hammer heads are added to the mask design. Given a 4:1 reduction from mask to wafer, the resolution enhancement features, such as scatter bars, are roughly the same size as main features on the wafer. Recently, with the reduction of mask feature size, mask-manufacturing technology faces several problems in satisfying customer needs for resolution, CD uniformity, and CD linearity. The problems result, in part, as the legacy of material and process choices made in the early days of mask making. For example, the use of chrome as an absorber was suitable material choice for wet etch binary mask processes, but this material is now seen as problematic current dry etch process. Another general source of problems for the mask industry is its small size relative to wafer manufacturing. As a result, vendors focus material and equipment development efforts on wafer, and then make adaptations to fit mask-making requirements. Nowadays the patterns of high-end lithographic masks are written by variable shaped beam 50 kV e-beam writers with minimum beam size of as little as few nm. However, the latent pattern after writing differs significantly from the final pattern on the mask due to interactions during post exposure bake, resist development, and etch processes so the final pattern is a convolution of these effects. The parameters of interest are resolution, critical dimension uniformity (CDU), pattern fidelity, CD linearity, iso-dense as well as clear-dark bias, transmission of the transparent substrate and absorber, and birefringence. Besides these requirements, there are implicit, not specified, expectations that the mask has to fulfill. To this group of parameters belongs, for example, reflectivity of the surface, chemical and environmental stability of the mask, and mechanical properties. This paper will present some issues which must be solved in order to satisfy future needs for lithographic masks. The focus of the presentation is to examine shortcomings of materials used in today's technology and provides a description of the properties needed for future materials.

Photosensitive Poly(benzoxazole) Based on Poly(o-hydroxy amide), Dissolution Inhibitor, Thermoacid Generator, and Photoacid Generator

A positive-type photosensitive polybenzoxazole (PSPBO) based on a poly(o-hydroxy amide) (PHA), a dissolution inhibitor (DI) 9,9-bis(4-tert-butoxycarbonyloxyphenyl)fluorene (t-BocBHF), a thermoacid generator (TAG) isopropyl p-toluenesufonate (ITS), and a photoacid generator (PAG) (5-propylsulfonyloxyimino-5H-thiophene-2-ylidene)-(2-methylphenyl)acetonitrile (PTMA) has been developed. ITS was easily prepared by reaction of p-toluenesulfonyl chloride with isopropyl alcohol. The dissolution behavior of the PSPBO system was studied in relation to PTMA loadings, and post exposure baking (PEB) temperature and time. The PSPBO consisting of PHA (73 wt %), t-BocBHF (18 wt %), ITS (7.5 wt %), and PTMA (1.5 wt %) exhibited a sensitivity of 33 mJ/cm2 and a contrast of 5.1 when exposed to 365 nm light (i-line) and developed with an aqueous alkaline developer, 2.38 wt % tetramethylammonium hydroxide solution (TMAHaq)/2.5 wt % iso-propanol (i-PrOH). A clear positive image with 3 μm features was produced by contact-printing and converted into a poly(benzoxazole) (PBO) pattern upon heating at 250 °C for 20 min. Thus, the ITS is effective as a TAG for improving the sensitivity and low temperature cyclization of PHA into the PBO as well to meet practical requirement in the industry.

Numerical study on photoresist etching processes based on a cellular automata model

For the three-dimensional (3-D) numerical study of photoresist etching processes, the 2-D dynamic cellular automata (CA) model has been successfully extended to a 3-D dynamic CA model. Only the boundary cells will be processed in the 3-D dynamic CA model and the structure of “if-else” description in the simulation program is avoided to speed up the simulation. The 3-D dynamic CA model has found to be stable, fast and accurate for the numerical study of photoresist etching processes. The exposure simulation, post-exposure bake (PEB) simulation and etching simulation are integrated together to further investigate the performances of the CA model. Simulation results have been compared with the available experimental results and the simulations show good agreement with the available experiments.

Thermal treatment for optical proximity correction

For 32-nm technology node, thermal treatment is one of the process extension techniques with 193-nm ArF lithography equipment and chemically-amplified resist (CAR). However, it is difficult to use these techniques in the manufacture process because the optical proximity effect of thermal effects is quite severe. In this paper, thermal processes, such as softbake, post-exposure bake and thermal reflow, are described and modeled to investigate the property changes of a positive type, 193-nm CAR. The simulated results agree well with the experimental results. For the optical proximity correction (OPC) of the thermal effects, an orthogonal functional method is introduced. Due to contact hole (C/H) pattern, the underbake of post-exposure bake (PEB) and thermal reflow are performed for the 45-nm critical dimension (CD) by using the OPC simulated images, to demonstrate the possibility of controlling these thermal processes for the formation of 32-nm CD.

Photosensitive polybenzoxazole based on a poly(o‐hydroxy amide), a dissolution inhibitor, and a photoacid generator

AbstractA positive‐type photosensitive polybenzoxazole (PSPBO), based on a poly(o‐hydroxy amide) (PHA), the dissolution inhibitor (DI) 9,9‐bis(4‐tert‐butoxycarbonyloxyphenyl)fluorene (t‐Boc BHF), and the photoacid generator (5‐propylsulfonyloxyimino‐5H‐thiophene‐2‐ylidene)‐(2‐methylphenyl)acetonitrile (PTMA), was developed. Several new tert‐butoxycarbonylated compounds as DIs for PSPBOs were prepared from phenolic compounds having a cardolike structure with di‐tert‐butyl dicarbonate in the presence of 4‐dimethylaminopyridine. Among them, t‐Boc BHF and 5,5′,6,6′‐tetra(tert‐butoxycarbonyl)‐3,3,3′,3′‐tetramethyl‐1,1′‐spirobiindane acted as excellent DIs, giving a large dissolution contrast between the exposed and unexposed areas in a 2.38 wt % tetramethylammonium hydroxide solution (TMAHaq)/5 wt % iso‐propanol (i‐PrOH). The dissolution behavior of this PSPBO system was studied in relation to the PTMA and t‐Boc BHF loadings and postexposure baking temperature. A PSPBO consisting of PHA (77 wt %), t‐Boc BHF (20 wt %), and PTMA (3 wt %) exhibited a sensitivity of 34 mJ/cm2 and a contrast of 5.8 when exposed to 365‐nm light (i‐line) and developed with an aqueous alkaline developer, 2.38 wt % TMAHaq/5 wt % i‐PrOH. A clear, positive image with 6‐μm features and a 10‐μm‐thick pattern with high sensitivity and contrast was produced by contact printing and converted into polybenzoxazole patterns upon heating at 350 °C for 1 h. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 661–668, 2007

Acid Diffusion Length Corresponding to Post Exposure Bake Time and Temperature

The post exposure bake (PEB) step in lithography is important for fabricating good patterns when a chemically amplified resist is used. Hydrogen ions or acid is generated by a photoacid generator through light exposure. The generated acid diffuses and acts as a catalyst for chemical amplification during the PEB step. Acid diffusion length (ADL) affects the deprotection of a resist polymer, such that linewidth is affected by ADL. The common parameter that determines ADL is the acid diffusion coefficient D; thus, we must determine D accurately in order to obtain the actual linewidth. However, D cannot be unambiguously determined for the actual PEB temperature and time. ADL has become a critical factor for 100 nm patterns and below. Thus, the accurate ADL determination becomes an important issue for better linewidth prediction by simulation. To match ADL and PEB time and temperature, we attempted to determine the relationship between the PEB parameters and ADL. As a result, we obtained a reasonable ADL.

Analysis of Cross-linking Reactions for High Sensitivity and Non-antimonite Permanent Photoresist for MEMS

The permanent photoresist is a versatile material to fabricate three-dimensional structures for the Micro Electro Mechanical Systems (MEMS) field. The replacement to non-antimonite initiators is ongoing toward the mass manufacturing. We investigate on the reaction mechanism of cross-linking formulating chemically amplified negative-tone photoresists by adopting an infrared spectra measurement system. The consumption of epoxy group is monitored during post exposure bake (PEB). The patterning profile of 40 μm lines with 30 μm thickness indicates no significant difference on the photoresist formulation. However, the FT-IR measurement results and the simulation models describe the difference in cross-linking behavior in tested photoresists. The activation energy of cross-linking reaction is also calculated. The impact on cross-linking reaction from the residual solvent in photoresist film is also discussed utilizing a Thermal Desorption Spectrometer (TDS).

Adhesion Promoting Photo-Acid Generator APPAG - A New Class of Lithographic Material

ABSTRACTPhoto-acid generator (PAG) is the key component responsible for the increased sensitivity of chemically amplified resists in use today for microelectronics production. Concentration variation of PAG through the thickness of the photoresist film adversely affects materialís performance. To offset reduced acid concentration at the bottom of the resist, we have developed adhesion promoting photo acid generators, called as a class “APPAG” that enhance acid concentration at interface between the resist and the substrate. An overview on the preparation and characterization of two siloxane based APPAG materials along with a performance comparison of commercial DUV and EUV resists on APPAG is provided. Longer diffusion length photo-acid generator (APPAG 9) retained square profile of critical dimension (CD) at off-focus values and was found to give nearly a 50% improvement in depth of focus for 250nm node DUV lithography. For EUV lithography, both shorter diffusion length APPAG 6 and APPAG 9 were shown to substantially improve performance envelope for 100nm dense lines and spaces at reduced post exposure bake (PEB).

Study on the Material Design for LWR Improvement

It is generally believed that the chemically amplified reaction between photo-generated acid catalyst and acid labile group of polymer during post-exposure bake (PEB) process plays a critical role for the reduction of line width roughness (LWR) in ArF lithography. In this work, we revealed experimentally how large the chemically amplified reaction affects LWR, and developed a new resist system with low LWR. Aerial image contrast dependence on LWR revealed that the innate LWR in a conventional ArF photoresist, which is independent of the aerial image contrast, was 5 nm. Surface roughness of a non-patterned resist film at half-exposed area, which was well correlated with LWR, was measured by AFM. The surface roughness increased from 1.7 nm to 10.8 nm during PEB process. The half-exposed area was baked and again dissolved into organic solution, and spin-coated on Si wafer, and then developed with 2.38 % TMAH solution. The recoated half-exposed resist film caused a 60 % reduction on the surface roughness. It revealed that uniform distribution of deblocked polymer was important factor for roughness reduction. HPLC analysis indicated that distribution of acidic group formulation in the polymer was gradually extended with increasing exposure dose. A Resist system that suppresses the chemically amplified reaction successfully reduced LWR from 6.5 nm to 4.8 nm.

Effect of process parameters and packing density on dimensional errors for densely packed high-aspect-ratio SU-8 microstructures in x-ray lithography

X-ray lithographic conditions for high-aspect-ratio SU-8 resist structures are characterized for potential application in x-ray optics and bioMEMS. The effects of the main process parameters, such as exposure dose, postexposure bake, development time, and the packing density of the microfabricated features, on the increase in feature size at the top portion of the resist (as compared to that in masks) are investigated. We find that lower postexposure bake and exposure dose leads toward minimizing dimensional errors. Further improvements in reducing the dimensional errors can be achieved by overdeveloping the structures. Using overdevelopment, we demonstrate an improvement in dimensional error for a given structure from 5 to 3.3%.

Positive Pattern Formation and its Mechanism from Ionic bonded Negative Photosensitive Polyimide

We found positive pattern formation by ionic bonded photo-sensitive polyimide (PSPI). The PSPI is composed of poly(amic acid), photo-reactive tertiary amine (such as N.N-dimethylaminoethylacrylate), and photo-initiator. The PSPI basically has negative photo-sensitivity. However, we found that positive pattern formation from the PSPI when adding a post-exposure bake (PEB) about 120-140 °C. By adding PEB, imidization rate of unexposure area of PSPI is higher than that of exposed area. High imidizaiton of unexposed area of PSPI makes hard to solubule to alkaline solution. The imidization rate difference attributes to glass transition temperature difference between exposed area and unexposed area.

A Novel 3-D Dynamic Cellular Automata Model for Photoresist-Etching Process Simulation

A novel three-dimensional (3-D) dynamic cellular automata (CA) model is presented for a photoresist-etching process simulation (photoresist-dissolution simulation and development simulation). In the 3-D dynamic CA model, the Moore neighborhood is adopted, and the boundary cells are only processed by using a boundary cell array, a corresponding linked list of pointers to the boundary cells, and a state flag to indicate the relations between the cells and the etching boundary. A time-compensation method is also introduced to speed up the photoresist-etching simulation. Therefore, the simulation speed is greatly increased compared with that of the static 3-D CA model, and the preferential etch in different directions reported in cell-removal models is significantly reduced. The 3-D dynamic CA model was successfully tested using some well-known etch-rate distribution test functions and has been shown to be stable, accurate, and fast. Exposure simulation, post-exposure bake simulation, and photoresist-etching simulation have been successfully integrated together to further study the effectiveness of the 3-D dynamic CA model. Simulation results show an agreement with available experimental results

Process dependence of the thermal conductivity of image reversal photoresist layers

Thermal transport in polymer layers is an important consideration in various lithography techniques, including immersion lithography and thermolithography. The in-plane thermal conductivity of commercially available photoresist AZ 5214E is determined at each stage of lithography processes using ultrathin (<100nm) freestanding membrane devices. The authors find that UV exposure does not lead to any appreciable change in thermal conductivity whereas cross-linking induced by postexposure bake results in a slight increase (∼5%). The thermal boundary resistance across interfaces between the resist layers and metal films/substrates is also found to be significant. The experimental techniques and data presented here will facilitate a systematic evaluation of thermal phenomena during lithography processes.

Surface modification of SU8 photoresist for shrinkage improvement in a monolithic MEMS microstructure

The effect of O2 plasma treatment on the surface property of exposed and unexposed SU8 photoresist has been investigated for the fabrication of a monolithic MEMS microstructure. It can solve the non-uniformity problem of second resist coating on the SU8 with high intrinsic shrinkage after exposure and post-exposure baking (PEB) in the fabrication of the stacked polymer–metal or polymer–polymer structure, which was used in the application of microfluid, bio and chemistry. The thickness difference of untreated SU8 before PEB between the exposed and unexposed SU8 was about 0.3% while that after PEB increased to about 6%. It could result in large non-uniformity of about 18 µm thickness difference for the following second resist coating on the hydrophobic surface without plasma treatment. The surface property of SU8 in terms of the contact angle and surface energy can be adjusted by O2 plasma treatment for enhancing the coating uniformity of the following resist. The measured contact angles of the exposed and unexposed SU8 decrease with O2 plasma time, corresponding to the increased surface energy determined by the Lifshitz–van der Waals/Lewis acid–base approach. It displayed that the similar hydrophilic surface property can minimize the thickness difference of second resist coating on the first shrunken SU8. A monolithic nozzle plate with a physical resolution of 600 dpi in a single column was demonstrated for an inkjet application based on the improved uniformity.