Optical lithography

This is the first in a series of papers describing a theoretical process model for positive photoresist. This model, based upon a set of measurable parameters, can be used to calculate the response of photoresist to exposure and development in terms of image surface profiles. The model and its parameters are useful in many ways, from measuring quantitative differences between different resist materials to establishment of process sensitivities and optimization of the resist process within a manufacturing system. In this paper, the concepts of photoresist modeling are introduced by following the exposure and development of a photoresist film on silicon exposed by a uniform monochromatic light flux. This very simple example provides insight into the complex nature of the photoresist process for reflective substrates. The accompanying paper, "Characterization of Positive Photoresists," gives detail about measurement of the new photoresist parameters. It is supported by "In-Situ Measurement of Dielectric Thickness During Etching or Developing Processes" which discusses automated experimental techniques needed to establish photoresist development rates. These resist parameters provide a complete quantitative specification of the exposure and development properties of the resist. They also allow quantitative comparisons: lot to lot, material to material, and processing condition to processing condition. The fourth paper, "Modeling Projection Printing of Positive Photoresists," applies the process model to one technique of photoresist exposure. This paper contains the detailed mathematics of the model. The model is then used to calculate line-edge profiles For developed resist images.     

Submicron resolution photolithography by spectral shaping

A novel exposure technique based on spectral shaping over a narrow exposure band (305-335 nm) has been used to achieve resolution comparable to the exposing wavelength in positive photoresist. Two hundred arrays of comb meander patterns with linewidths as small as 0.20 µm, and 3 cm in length were vacuum contact printed in AZ2415 resist on 50 mm diameter oxidized silicon substrates. The average standard deviation of linewidth variations is only 300 Å and resist pattern cross-sections show vertical edge profiles with aspect ratios of 2:1 or better.     

Characterization of a photoresist with wavelength selected tone

The line edge profile simulation of a tone-switching resist system, obtained from the dissolution model of dual-sensitized, novolak-based resist in aqueous developer, is described. The model incorporates the actinic response of both a positive and a negative sensitizer in a two-pattern lithographic process that simultaneously exposes the same resist film. These response data are combined with dissolution rate measurements to establish a model for the resist and carry out SAMPLE simulation of resist line edge profiles for contact and projection printing. The model predictions are compared with SEM micrographs of exposed resist features     

Optimization of the composition of a new MRS-type negative resist

Sensitizer concentration is optimized for a new negative photoresist, MRL (Micro Resist for Longer wavelengths) with the assistance of computer simulation. The resist, which has photosensitivity in the ordinary UV region, resembles a deep UV resist MRS in terms of light absorption characteristic. It is found that a photosensitizer concentration of 20 wt% (based on the resin) is suitable for a reduction projection exposure system that utilizes UV light at 365 nm. A steep profile resist image of 0.7-µm lines and 0.7-µm spaces in a 1.0-µm thick resist layer is obtained using the MRL of optimized composition and the exposure system.     

Binary and phase shifting mask design for optical lithography

The authors propose a number of pre-distorted mask design techniques for binary and phase-shifting masks. Their approach is based on modeling the imaging mechanism of a stepper by the Hopkins equations and taking advantage of the contrast-enhancement characteristics of photoresist. Optimization techniques such as the branch and bound algorithm and simulated annealing algorithm are used to systematically design pre-distorted masks under incoherent and partially coherent illumination. Computer simulations are used to show that the intensity contour shapes and developed resist shapes of their designed mask patterns are sharper than those of conventional masks. The designed phase-shifting masks are shown to result in higher contrast as well as sharper contours than binary masks. An example of phase conflicting masks designed with the algorithm is shown to outperform a simple intuitive design. This example indicates that a fairly general design procedure consisting of alternating phase shifts and their optimized phase-shift masks is a viable candidate for future phase-shifting mask design     

Fabrication of integrated circuits using the electron image projection system (ELIPS)

ELIPS is based on the use of an electron image tube principle to project large area (5-cm diameter) ultrahigh resolution (1 micron) electron images from a patterned photocathode onto an electron-sensitive resist layer, thereby replacing the conventional photoresist optical procedures in integrated circuit fabrication [1]. Integrated circuits of minimum linewidths of 0.5 mil have been fabricated to show the practicality of the technique through the full set of mask exposures necessary for a DZTL quad dual input NAND gate. The necessary alignments between successive masks were achieved by an optical dead reckoning technique which allowed the electron image to be adjusted in position and orientation from a known "zero" position to the separately determined location of the pre-existing pattern on the sample. Accuracy of such alignments was ± 2 micron. The much more desirable alignment technique, in which simple devices in the form of alignment marks on the target wafer detect separate electron beams emanating from the cathode, allows rapid alignment to submicron registration and the potential for the complete automation of alignment and exposure. The system provides for the rapid registration and exposure of wafers at high resolution over large areas with great depth of focus under vacuum clean, contactless conditions using an electroresist insensitive to ambient light. It makes practical the fabrication of very high-density planar devices allowing extremely complex large area circuits to be formed. Patterned photocathodes were fabricated by conventional photoresist techniques for the DZTL circuit discussed. For high-resolution (1 micron) patterns, cathodes can be defined in the electron micro-plotter (computer-controlled scanning electron microscope).     

Linewidth variations in photoresist patterns on profiled surfaces

Photoresist thickness nonuniformities in the vicinity of profile steps on substrate surfaces lead to linewidth variations of AZ 1350 photoresist geometries. The effect increases with increasing reflectivity of the substrate, decreasing photoresist layer thickness, and decreasing contrast transfer of the exposure system. It is shown that the photoresist linewidth is maximum when the resist thickness is a multiple of half the exposure wavelength in the resist.     

3-D computer simulations of resolution effects of multidetector point focusing SPECT imaging

The authors present an efficient algorithm and the results of its application in simulating the three-dimensional (3-D) projection data resulting from a 3-D distribution of radioactivity. The algorithm was applied to a series of geometrical mathematical phantoms and to a realistic mathematical brain phantom. The authors simulated the projection data from a multidetector single-photon emission computed tomography (SPECT) system with point focusing collimators. The simulated projection data were then reconstructed using the manufacturer's software. The objects simulated included simple geometrical solids such as spheres and sheets, as well as the distribution of muscarinic cholinergic receptors in a realistic brain slice. Spheres were chosen as a model for brain structures such as caudate nucleus, thalamus, and cerebellum; sheets were selected as representing lateral cortical gray matter regions. The results of these simulations indicate the existence of significant qualitative and quantitative artifacts in reconstructed human brain images.     

Shorter-wavelength lithography utilizing MRS-type negative resists

New negative photoresists, called MRS-type resists, are successfully applied to deep-UV 1:1 projection printing and 365 nm 10:1 reduction projection printing. The MRS-type resists are characterized by intense absorption of exposure light and absence of swelling in aqueous developer solutions. They resolve steep profile submicron images in a 1.0 μ thick film. They are not adversely affected by reflected light from water surfaces. In order to use MRS-type resists with broader development latitude, optimizing the extent of light absorption is important because the resist profiles strongly depend on development conditions due to increasing solubility towards the resist bottom.     

Modeling of Novolak-based positive photoresist exposed to KrFexcimer laser UV radiation at 248 nm

A model is presented for calculating the local exposure and the development rate for a Novolak-resin naphtoquinone-diazide sensitized photoresist exposed to a KrF excimer laser UV radiation at 248 nm. The measured transmission of the pulsed UV radiation through the resist is presented and compared to the simulated one using the model. Classical bleaching characteristics (i.e. the resist transmittance increases with the dose) are observed at low dose exposure with low energy per pulse. When the dose is increased, the photoresist transmittance reaches a maximum and starts to decrease. This behavior is assumed to be due to UV radiation effect on the resin. The model describes a general photoresist with absorbing components, each with two distinctive initial and postexposure states. The model is applied to the case of a Novolak-based photoresist where the two components are the photoactive compound and the resin     

SU-8胶及其在MEMS中的应用

SU 8胶是一种负性、环氧树脂型、近紫外线光刻胶。它适于制超厚、高深宽比的MEMS微结构。SU 8胶在近紫外光范围内光吸收度低 ,故整个光刻胶层所获得的曝光量均匀一致 ,可得到具有垂直侧壁和高深宽比的厚膜图形 ;它还具有良好的力学性能、抗化学腐蚀性和热稳定性 ;SU 8胶不导电 ,在电镀时可以直接作为绝缘体使用。由于它具有较多优点 ,被逐渐应用于MEMS的多个研究领域。本文主要分析SU 8胶的特点 ,介绍其在MEMS的一些主要应用 ,总结了我们研究的经验 ,以及面临的一些问题 ,并对厚胶技术在我国的应用提出建议和意见     

Control of photoresist properties: a Kalman filter based approach

The photolithography step in the semiconductor manufacturing process becomes increasingly critical as linewidths decrease for the next generation of integrated electronics. It therefore becomes necessary to reduce variations in photoresist parameters such as resist film thickness and photoactive compound concentration during manufacture. In this paper, we present a simple feedback scheme for accurately regulating these parameters. Our approach involves obtaining a simple, static process model the coefficients of which are recursively adjusted based on previous wafer measurements. We use this adaptive model to determine appropriate input setpoints for the next wafer. The effectiveness of this scheme in reducing process drift is exhibited by experimental data. Our results supports the widespread contention that modern feedback control offers the promise of improving the semiconductor manufacturing processes, often with relative ease and minimal capital cost     

Properties of a high resolution negative photoresist

The recent advantages in designing large and very large integrated circuits have imposed stringent requirements on the processes used in the production of semiconductor devices. The requirements on photoresists have been increased not only with regard to improved exposure tools but also in view of other process steps such as plasma etching, ion implantation, etc. A new developed negative photoresist, Selectilux N, combines the well-known advantages of a cyclized polyisoprene-based resist with the capability to resolve features in the 1 to 2 micron range and a high thermal stability. The main properties are: high photospeed to reduce the influence of oxygen and to prevent thickness loss, high contrast to achieve good resolution in projection exposure, low flow distortion up to 240 C to ensure good image stability, low swelling during the development to prevent reduction of the resolution. Photospeed and contrast can be derived from the characteristic curve of Selectilux N which is obtained by the determination of the remaining thickness of the resist vs exposure energy.     

Resolution, overlay, and field size for lithography systems

Resolution, overlay, and field size limits for UV, X-ray, electron beam, and ion beam lithography are described. The following conclusions emerge in the discussion. 1) At 1-µm linewidth, contrast for optical projection can be higher than that for electron beam. 2) Optical cameras using mirror optics and deep UV radiation can potentially produce linewidths approaching 0.5 µm. 3) For the purpose of comparing the resolution of electron beam and optical exposure, it is useful to define the minimum linewidth as twice the linewidth at which the contrast of the exposure system has fallen to 30 percent. 4) X-ray lithography offers the highest contrast and resist aspect ratio for linewidths above about 0.1 µm, but for dimensions below 0.1 µm, highest aspect ratio is obtained with electron beam. 5) With electron beam exposure on a bulk sample, contrast for a 50-nm linewidth is the same as that for 1-µm linewidth, provided the resist is thin. Higher accelerating voltages make it easier to correct for proximity effects and to maintain resolution with thick resist. 6) Ultimately the range of secondary electrons limits resolution in electron beam lithography, just as the range of photoelectrons limits resolution in X-ray lithography. In both cases, minimum linewidth and spacing in dense patterns is about 20 nm. Resolution with ion beams will probably be about the same because the interaction range of the ions will be similar to the electrons.     

Projection photolithography-liftoff techniques for production of 0.2-µm metal patterns

A technique which allows the use of projection photolithography with the photoresist liftoff process, for fabrication of Submicrometer metal patterns, is described. Through-the-substrate (back-projection) exposure of the photoresist produces the undercut profiles necessary for liftoff processing. Metal lines and superconducting microbridges of 0.2-µm width have been fabricated with this technique. Experimental details and process limits are discussed.     

Simulation of focus effects in photolithography

A simple physical model is proposed for bulk imaging effects during latent image formation in a resist layer. The model considers refraction at the air-resist interface, as well as depthwise defocus of the lateral intensity distribution within the resist layer. The approach represents a first-order correction to the vertical propagation model used in conventional photolithography simulation, yet preserves the data structures of simulators such as SAMPLE and SPESA, and requires only a modest increase in computational effort. Comparison of simulated resist profiles with published experimental data shows that this model qualitatively explains the asymmetries in photolithographic response observed as a function of focus offset position in a single layer resist process. The question of the optimum focal position within the resist layer is discussed using simulated focus-exposure diagrams and the concept of effective defocus     

Optical MTF evaluation for the photolitographic technology

Measurements of the modulation transfer function (MTF) for different photolithographic exposure tools, such as contact, proximity, and projection printers have been made by the Fourier analysis of an exposed and developed positive photoresist profile. Through the obtained MTF data, the optical performance of the tools is presented concerning linewidth accuracy, reduction in developed photoresist thickness, and linewidth variations at steps. The general conclusion is that hard contact printers are superior to commercially available projection printers at present in the trend toward smaller pattern geometries.     

应用三维线算法的SU-8胶光刻过程模拟

采用二维法向量作为分量,加权求和近似得到三维网格点上的单位法向量,将经典的二维线算法改进为三维形式。综合SU-8胶光刻过程中衍射、吸收率随光刻胶深度的变化及交联显影等各种效应,应用该三维线算法对SU-8化学放大胶进行光刻过程三维建模。该模型对被加工表面演化过程的模拟较为精确,可在实际应用中对SU-8胶的光刻模拟结果进行有效预测。     

新型高抗粘紫外纳米压印光刻胶的工艺研究

为了减少紫外纳米压印技术脱模过程中的接触粘附力,开发了一种新型高流动、抗粘的紫外纳米压印光刻胶。光刻胶以BMA为聚合单体,添加特定配比的交联剂和光引发剂配置而成。紫外纳米压印实验在本课题组自主研发的IL-NP04型纳米压印机上完成。实验得到光刻胶掩膜膜厚为1.21μm,结构尺寸深246nm,周期937.5nm。实验结果表明,在没有对石英模板表面进行修饰的情况下,该光刻胶依然表现出高可靠性和高图形转移分辨率,有效减少了紫外纳米压印工艺中的模板抗粘修饰的工艺步骤。     

Modeling projection printing of positive photoresists

The accompanying papers "Optical Lithography" and "Characterization of Positive Photoresist" introduce the concepts of modeling using destruction of the photoactive inhibitor compound to describe exposure and a surface-limited removal rate to describe development together with the optical exposure parameters A, B, and C and a rate relationship, R(M), which characterize the photoresist for modeling purposes. This paper applies the model to, the projection exposure environment: exposure and development of photoresist are treated with a simulation model that allows computation of image surface profiles for positive photoresist exposed with a diffraction limited real image.