Post Exposure Bake Research Articles

Resist pattern peeling assessment in DUV chemically amplified resist

As device size shrinks resist line peeling becomes more challenging. In this paper we studied the resist pattern peeling based on resist processing parameters and type of bottom antireflective coating (BARC), for patterning trench structures with different duty ratios, in copper and low k dual damascene process. To minimize resist poisoning in dual damascene process, acetal-based resist was used. Significant improvement in via poisoning was observed with this chemistry as compared to environmentally stable chemically amplified resist chemistry but at the cost of pattern peeling. In order to solve pattern peeling problem we tried to analyze key factors such as compatibility with BARC, post-exposure bake, BARC curing, adhesion and their effects. Pitch dependency on pattern peeling margin is observed.

Acid–base reactions in a positive tone chemically amplified photoresist and their effect on imaging

The inclusion of base in chemically amplified resist formulations is known to improve imaging, yet the mechanism by which this occurs, and the effect of different chemical types of base is poorly understood. Previously we showed that tetrabutylammonium hydroxide, an ionic base, added to p-tert-butyloxycarbonyloxystyrene (PTBOCST)-t-butyl iodonium perfluorobutane sulfonate (TBI-PFBS) positive tone chemically amplified resist affects local acid concentrations in exposed photoresist through a proportional neutralization mechanism. We have now extended these studies to include a neutral base, 7-diethylamino-4-methylcoumarin, and show that it operates by a qualitatively different process, competitive neutralization. Using the rate coefficients obtained for both bases, we investigate the impact of the details of the neutralization kinetics on image formation in PTBOCST/TBI-PFBS resist as a function of pitch, dose, base concentration, and temperature. The results show that base dynamically reduces local acid concentrations in competition with acid-catalyzed deprotection, and does not significantly block acid diffusion as a primary process. Thus, the selection of base can be expected to play an important and complex role in the evolution of the acid and deprotected resist images during postexposure bake.

A polymeric photobase generator containing oxime–urethane groups: Crosslinking reaction and application to negative photoresist

AbstractA polymeric photobase generator containing oxime–urethane groups was prepared by copolymerization of methyl methacrylate and methacryloxyethyl benzophenoneoxime urethane, and its photo and thermal crosslinking reaction after irradiation was examined from the measurement of UV and IR absorption spectral changes, insoluble fraction, and molecular weight changes. The photo‐crosslinking reaction of the copolymer film was more efficient when irradiations were carried out with 310 nm UV light in the presence of benzophenone than with 254 nm UV light without the addition of benzophenone. The crosslinking reaction increased after postexposure baking (PEB), and this thermal crosslinking reaction mechanism was studied from the identification of the photolysis products of a model compound, benzophenoneoxime phenylurethane, by a high‐performance liquid chromatography. The results indicate that the thermal crosslinking reaction of the copolymer after PEB is due to the formation of urea‐type chemical bonds. Resist properties of the copolymer were examined from the measurement of normalized thickness and micropattern development. A negative tone image with a resolution of 2 μm was obtained with this copolymer, having a sensitivity (D) of 1200 mJ/cm2 and contrast (γn) of 1.31, when irradiation was carried out with 310 nm UV light in the presence of benzophenone following chloroform development. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 975–984, 2004

Crosslinking of Oligomers Utilizing Sensitized Photoreaction at 366 nm and Thermal Decomposition of Pendant Carbamoyloxyimino Groups

Crosslinking using pendant basic groups generated on irradiation at 366 nm and thermally generated isocyanato groups from oligomers bearing carbamoyloxyimino (COI) groups was demonstrated. Acetophenone O-methacryloyloxyethylcarbamoyloxime (AMCO) and 2-butanone O-methacryloyloxyethylcarbamoyloxime (BMCO) were used as monomers bearing COI groups. Benzophenone and anthracene showed little effect on the photolysis of COI groups in AMCO and BMCO in oligomers, and isopropylthioxanthone (iPTX) sensitized for COI groups in AMCO efficiently. On irradiation of an oligomer of AMCO, BMCO, and methyl methacrylate (MMA) at 366 nm in the presence of iPTX, only COI groups in AMCO were photolyzed. These oligomer films became insoluble on irradiation, and the insolubilization was much enhanced by post-exposure bake (PEB) at 150 °C due to crosslinking between photo-generated basic groups and thermally generated isocyanato groups. Furthermore, we succeeded in the preparation of an oligomer consisting of AMCO, BMCO, and 2-(acryloyloxy)thioxanthone, which showed similar photoreactivity and solubility behavior to AMCO-BMCO-MMA containing iPTX.

KMPR Photoresist Process Optimization Using Factorial Experimental Design

MicroChem Corp. and Nippon Kayaku Co., Ltd. have jointly developed a new, photoimageable coating composition for use in fabricating a wide variety of micro-electromechanical systems (MEMS) and optical MEMS. This chemically amplified thick-film photoresist, known as KMPR, has many of the same high-aspect ratio photo imaging properties as solvent developable SU-8, but can be developed in a conventional aqueous alkaline developer (TMAH) and readily stripped from the wafer.Single-factor experimental methods rarely find the optimum performance of a photoimageable coating efficiently and often fail to detect the interactions so common in complicated photochemical systems. By combining the settings of several factors simultaneously in an array (experimental design), it is possible to separate out the main effects and their interactions.1This paper presents the optimization of KMPR for contact alignment and near diffraction-limited, thick film reduction stepper applications. A Sequence of Taguchi style, half-factorial experimental designs was used to evaluate the process parameters for a 5 and 50 Mm thick film exposed on a contact aligner followed by a third half-factorial design for a 10 Mm thick film of KMPR photoresist exposed on a 365 nm, 0.40 N.A. stepper. Independent factors, such as: soft bake time, focus, exposure dose, initiator activation time (post-exposure bake time), developer time and delay time were evaluated

Characterization of photoresists for ArF‐excimer laser lithography using monoenergetic positron beams

AbstractThe volume and depth distributions of open spaces in photoresist films designed for ArF‐excimer laser light exposure were studied with monoenergetic positron beams. We measured the Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons for acrylate and cyclic olefin‐co‐maleic anhydride (COMA) polymers spin‐coated onto Si wafers with methyl amyl ketone, cyclohexanone, and propylene glycol methyl ether acetate solvents. The volume of open spaces in both prebaked acrylate and COMA films was estimated to be 0.12 nm3. The volume of open spaces in acrylate films decreased up to 20% by postexposure baking, but no large change was observed for COMA films. The decrease in the open volume was attributed to the removal of large molecules (adamantyl) from flexible main chains after the chemically amplified reaction of acrylate. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 341–346, 2004

Statistical limitations of printing 50 and 80 nm contact holes by EUV lithography

Nanoscale photolithography requires accurate formation of very small resist images using high energy photons and a high sensitivity resist. Historically it has been presumed that the primary technical challenges for design of a photoresist that will image with high accuracy under these conditions are shot noise effects, i.e., statistical variation of overall photon intensity from place to place on the wafer, and line-edge roughness. While these issues have been examined to various degrees, other aspects of the resist response have not received comparable attention. These include the effect of the statistics of the chemical reactions during post-expose bake and dissolution on resist images, and the effect of the fluctuations of the spatial distribution of photons within a printed feature. In order to examine the impact of all of these we have carried out a series of simulations of resist image formation as a function of dose for arrays of 50 and 80 nm contact holes printed with EUV (13.4 nm) radiation using an experimentally validated reaction-diffusion model that mimics ESCAP-class photoresists. The results show that intensity and spatial fluctuations of the light as well as simple variations in the local chemistry all affect contact size. The simulations indicate that the degree of precision contemplated for printed nanoscale features will require an extraordinary level of control over the resist chemistry and processing.

Approach to critical dimension error budget analysis and specification estimation by the Monte Carlo method

A systematic approach to determine the specifications for process latitude in a 80 nm node device is suggested based on critical dimension (CD) error budget analysis using the Monte Carlo method. Using an aerial image simulation with the Gaussian convolution model, CD error budget analysis is performed on line/space (L/S), island, and contact patterns in a 90 nm node device. The portions contributed by mask uniformity are 60%, 66%, and 50% for the L/S, island, and contact patterns, respectively, and they occupy a dominant part in comparison with ones from other factors. The contact pattern occupies 12% for focus variation, which shows the larger portion than the analysis result for L/S and island patterns. The contribution of the 1 nm grid size in the mask layout is approximately 10% of the in-field uniformity. The residual terms which include variations from aberration, inaccurate measurement, postexposure baking, development, line-edge roughness, etc. occupy 21% for L/S and contact patterns, and 16% for island patterns. Based on CD error budget analysis, specifications in a 80 nm node device are calculated. The portion contributed by each process variation in the 80 nm node device is assumed to be approximately equal to those in the 90 nm node device. The specifications for mask uniformity are 6.8 and 5.8 nm for L/S and island patterns, respectively, while they are 2.9 nm for the contact pattern. The small margin for the contact pattern is due to a large mask error enhancement factor. The contact pattern shows a drastic decrease in flare, focus margin, illumination uniformity, and mask transmittance. In the 80 nm node device, the grid size of the mask layout to generate the patterns should be reduced to less than 0.3 nm.

Synthesis and characterization of novel copolymers with acid‐labile ketal moieties derived from camphor

AbstractNovel diastereomeric acrylic ketal monomers derived from (+)‐camphor and (±)‐camphor were synthesized. To investigate the applications of the camphor derivatives on positive‐tone photoresists, the acrylic ketal monomers were copolymerized with methyl methacrylate, methacrylic acid, and n‐butyl methacrylate. The optical activities of the chiral monomers and polymers were all evaluated. After UV irradiation and postexposure baking, the optical activity of the polymers decreased because of the decomposition of the acid‐labile pendant chiral groups. The existence of alicyclic camphyl groups increased the etching resistance of the photoresists. The thermogravimetric properties of the copolymers, the exposure curves, the lithographic evaluation of the positive‐tone photoresists, and the effects of alicyclic groups on the plasma etching resistance of the copolymers were all investigated. A resolution of a line‐and‐space pattern of 0.3 μm was achieved. Acid‐catalyzed dehydration crosslinking was also found in this system. Sufficient UV irradiation and heat treatment could cause the acid‐catalyzed dehydration crosslinking of pendant carboxyl groups and thereby increase the efficiency of the thermal resistance of the polymers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2969–2978, 2003

Fabrication of optical structures using SU-8 photoresist and chemically assisted ion beam etching

We develop a method for the fabrication of optical structures in GaAs substrates using UV holographic lithography in SU-8 resist, pro- cessed to fabricate a mask, followed by chemically assisted ion-beam etching (CAIBE). The technique is based on simple processing steps without procedures of mask transfer, enabling easy fabrication of optical structures. A predevelopment relief behavior is investigated to optimize the processing parameter to form an etching mask in SU-8. By adjusting both exposure dose and time in the postexposure bake (PEB), an SU-8 mask with a flexible duty cycle and high profile quality can be easily produced. Furthermore, an optical structure with a rectangular shaped profile and a 1-mm period in a GaAs substrate is produced by optimizing the processing parameters during the CAIBE process. © 2003 Society of

An in-situ temperature measurement system for DUV lithography

Spatial uniformity of temperature across a silicon wafer is an important requirement during the post-exposure bake step of the deep-ultra-violet lithography process. Closed-loop temperature control provides a means by which the stringent temperature specifications can be achieved, provided that accurate feedback signal is available. As each new wafer is loaded for processing, its level of thermal contact with the temperature sensor may vary, leading to erroneous measurement of the wafer temperature. Such variation in thermal contact manifests itself as changes in the time constant of the sensor. This paper describes a method for in-situ estimation of the sensor parameters, and proposes an algorithm for post-processing the sensor output to improve measurement accuracy. Experimental results are presented to demonstrate the effectiveness of the algorithm in improving the accuracy of the feedback signal, thereby reducing the undesirable influence of poor thermal contact on the performance of the closed-loop temperature control system.

Effects of treatment parameters in electric-field-enhanced postexposure bake

This article demonstrates the improved lithographic performance of Shipley UVIIHS with an electric-field treatment during postexposure bake (PEB) for KrF optical exposures. Linewidth trends with treatment are given and explained through simulation of the acid motion. An upward electric field of approximately 10 V/μm was applied during the first 7 s of PEB to crack the surface inhibition, and thereafter the field polarity was altered every 7 s to enhance the acid-catalyzed reactions. Compared with standard PEB, the electric-field treatment resulted in deeper trenches, wider openings and significant compensation for underexposure. In some cases, electric-field-enhanced PEB led to more profile tapering, indicating that a larger downward direct electric field is needed to enhance the deprotection reactions in the lower half of the resist.

FT-IR study of a chemically amplified resist for X-ray lithography.

Microelectronic devices for future applications demand lithographic performance that falls within the 0.10 microm region and below. Chemically amplified resists (CARs), such as the positive tone commercial UVIII resist, offer a substantial gain in sensitivity, resolution, and process efficiency in deep ultraviolet, e-beam, and X-ray lithographies. In this work, the UVIII resist is characterized for X-ray lithographic applications by studying the "deprotection" or acid generation-diffusion process of the resist under different conditions of post-exposure bake (PEB) temperature and time, and of X-ray exposure dose. The X-ray irradiation from a copper anode at a wavelength of 1.33 nm was at an intensity of 30 microW/cm2 on the resist surface. The deprotection process of the resist during PEB was accurately monitored by using Fourier transform infrared (FT-IR) spectroscopy. The infrared absorption peaks at 1151, 1369, and 2977 cm(-1) in the spectrum of the UVIII resist were found to be useful indicators for the completion of deprotection. Results of the experiments showed that the performance of UVIII could be optimized at the PEB temperature of 140 degrees C, a time of 2 min, and X-ray exposure dose of 12 mJ/cm2. The change in resist thickness after PEB was also measured. The results were confirmed by scanning electron microscopy (SEM) in which a test structure as small as 0.12 microm was obtained in a 1-microm-thick UVIII resist layer.

Retrospective of work at Bell Laboratories on the effect of fluorine substitution on the properties of photoacid generators

An overview is given on the work done at Bell Laboratories in which fluorine substitution was employed to tune the structure property relationships of chemically amplified resists. In particular, this paper will detail how structural changes in 2-nitrobenzyl photoacid generator (PAG) affect molecular properties such as quantum yield, thermal stability, and in turn also influence the lithographic characteristics of photoresist formulations such as sensitivity and post-exposure bake (PEB) and post-exposure delay (PED) latitude.

Sub-150 nm, high-aspect-ratio features using near-field phase-shifting contact lithography

A simple technique, near-field phase shifting contact lithography, is described which can extend the resolution of contact lithography to less than 200 nm. Sub-150 nm, high-aspect-ratio features have been produced using chromeless phase-shifting masks, a Karl Suss MA-6 contact aligner and off-the-shelf SPR 510 photoresist. The technique is neither purely near field nor purely phase shifting considering subwavelength features have been patterned in resists whose thickness is twice the longest exposure wavelength. Conventional 5 in. chrome-on-quartz masks were reactive ion etched to generate precisely varied phase steps from 375 nm to 512 nm using standard CF4 etch chemistries and then stripped of chrome. These masks were then used to expose SPR 510 spun on bare silicon wafers to broadband and filtered UV irradiation. After development in an aqueous base developer, features were examined as a function of exposure wavelengths, shifter depth, resist thickness, resist type, exposure dose, and postexposure bake temperature and time. Features as small as 128 nm with aspect ratios of >5:1 have been printed with this technique with minimal sidewall roughness.

Applications of a low contact force polyimide shank bolometer probe for chemical and biological diagnostics

This paper reports on the detection of nano-scale chemical variations in photosensitive polymers and biological variations in cancerous tumor cells that have been accomplished for the first time using scanning thermal probes that we have developed. It also reports on changes that have been made over older versions of this probe to help achieve these capabilities. The probe is fabricated by a 6-mask surface micromachining process using polyimide as the structural material. A unique assembly sequence that involves flipping over the probe accommodates the future integration of circuitry on the same substrate. The probe has measured spring constant 0.082 N/m for a 250μm×50μm×3 μm probe. It offers a tip diameter of 50 nm. Probes are used to study exposed but undeveloped photoresist latent images in features of 70 nm, the acid diffusion in chemically amplified photoresist during post exposure bake, and HeLa cells. Lateral spatial resolution of <50 nm, topographic resolution of <1 nm and thermal resolution of <1.2 mK are demonstrated. Wet scanning capability, which widens the possibility of biochemical applications, is also demonstrated for the first time. The structure, fabrication, and assembly of the probe and the interface circuit used for these experiments are described.

50 GB Read Only Memory Disc with Dual Layer Structure

We developed a 50 GB dual layer read only memory (ROM) disc made by electron beam recorder (EBR) recording. The practical recording velocity was realized by control of the recording pulse and the adjustment of the post exposure bake (PEB) condition of the chemically amplified resist. An applicable replication process of dual layer ROM disc to production was realized. The recording condition was optimized for each layer. A sufficient jitter value was obtained from each layer.

Influence of ArF Resist Components and Process Conditions on the PEB Sensitivity

Control of the critical dimension (CD) during photo lithographic process is becoming more and more critical as the CD size decreases and wafer size increases. Among other factors, PEB (post-exposure bake) sensitivity [ΔCD(nm) /ΔTemp(°C)] of photoresists plays an important role in achieving uniform CD. ArF resists are the preferred materials for the critical layers in the 90 and 65 nm nodes where in the CDs are less than 100 nm. The influence of resist components such as type of PAG, solvent, and blocking ratio as well as the processing conditions were investigated in the ArF resist formulations. It was observed that almost all the resist components influence the PEB sensitivity. Use of bulky and long chain sulfonic acids such as perfluorooctane sulfonic acid, low blocking ratio and low boiling solvents reduce the PEB sensitivity but at the expense of other resist properties such as resolution and line-edge roughness (LER). As expected the cation type of the photoacid generators (PAGs) did not show any effect on the PEB sensitivity. On the processing side, lowering PEB temperature and increasing soft bake (SB) temperature help improve the PEB sensitivity but the LER becomes worse. Results of these studies along with the performance of some of the optimized ArF formulations are presented.

Synthesis and Lithographic Characterization of Poly(5-vinyluracil) Derivative.

Enol ether derivative of poly(5-vinyluracil) released completely the tert-butyl groups above 195 °C and gave poly(5-vinyluracil). The resist film as spin coated from chloroform solution containing photo-acid generator released completely the tert-butyl groups after UV light (250 nm) irradiation followed by post exposure baking. Solubility of the obtained polymer containing uracil unit was significantly different from the original polymer containing alkoxy units in both polar (TMAH aq for negative) and nonpolar (anisole for positive) solvents as developer. The results were compared with values of the poly(6-vinyluracil) derivative, and the difference were attributable to the difference of the polymer structures.

A New Positive-working Alkaline developable Photoresist Based on Partially O-tert-butoxycarbonylmethylated-tetra-C-methylcalix[4]resorcinarene and a Photoacid Generator.

A new positive-working low-molecular-weight photoresist has been developed. The photoresist consisted of the matrix, tetra-C-methylcalix[4]resorcinarene (p-t-BM-C4-R) in which the OH groups were protected with tert-butoxycarbonylmethyl groups (protecting ratio: 27 ∼ 60%), and a photoacid generator (PAG), 5-(propylsulfonyloxyimino-5H-thiophen-2-ylidene)-2-methylphenyl-acetonitrile (PTMA). The p-t-BM-C4-R (protecting ratio: 40%) containing PTMA (2 wt%) showed a high sensitivity (10 mJ/cm2) and a contrast 11 after the irradiation with g-line, post-exposure baking at 120 °C at 60 sec, and developing with 2.38 wt% tetramethylammonium hydroxide aqueous solution (TMAHaq) at 20 °C for 10 sec.