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.     

Analysis of three-dimensional proximity effect in electron-beam lithography

In most of the proximity effect correction schemes, a two-dimensional model of proximity effect is employed by ignoring or averaging the variation of exposure along the depth dimension in the resist. However, as the feature size continues to decrease, the relative variation becomes significant so that it may need to be taken into account in proximity effect correction. In this study, the three-dimensional (3-D) proximity effect is analyzed in detail through computer simulation as a first step toward developing a 3-D proximity effect correction scheme. Effects of the parameters such as beam energy, resist thickness, feature size, developing threshold, etc., on the 3-D spatial distribution of exposure in the resist, in particular, depth-dependent proximity effect, are considered in the analysis. Results from the extensive simulation are presented in this paper.     

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.     

厚层抗蚀剂曝光模型及其参数测量

针对厚层抗蚀剂曝光过程中存在诸非线性因素的影响，更新Dill曝光参数的定义，建立了适合描述厚层抗蚀剂曝光过程的增强Dill模型．光刻过程模拟的准确性与曝光参数的测量精度有很大关系，为此，建立了实时曝光监测实验装置，测量了不同工艺条件、不同厚度抗蚀剂的曝光透过率曲线，并演绎计算出曝光参数随抗蚀剂厚度和工艺条件的变化规律．最后给出了采用增强Dill模型进行曝光过程的模拟和实验结果的分析．     

Proximity-effect correction for VLSI patterns in electron-beam lithography

A proximity-effect correction method for VLSI patterns has been developed. In this method, a dose ratio has been introduced as a control parameter for the negative- resist thickness after development, in addition to the proximity parameters.A new technique has been used to obtain the proximity parameters. By using the dose ratio and the proximity parameters, both the exposure dose and the size of the irradiated shape are easily determined.A pattern accuracy of ±0.1 μm and a uniform resist of the desired thickness were obtained. The computation time is proportional to 1.2 power of pattern density, and is 100 seconds on a 1.5-MIPS computer when correcting for 10⁴ shapes in a pattern whose pattern density is 10⁴.     

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     

一种提取电子束光刻中电子散射参数的新方法

在电子散射能量沉积为双高斯分布的前提下,提出了一种提取电子束光刻中电子散射参数α,β和η的新方法.该方法使用单线条作为测试图形.为了避免测定光刻胶的显影阈值,在实验数据处理中使用归一化方法.此外,用此方法提取的电子散射参数被成功地用于相同实验条件下的电子束临近效应校正.     

Ultrasonic sensor for photoresist process monitoring

An ultrasonic sensor has been developed to monitor photoresist processing in situ, during semiconductor manufacturing. Photoresist development, pre-exposure bake, and post-exposure bake were monitored for the Shipley 1800 series I-line resists, and the pre-exposure bake of Shipley APEX-E deep-uv (DUV) resist was monitored as well. Development monitoring was achieved by measuring thickness changes in the resist as it was removed. Data regarding dependence of development rate on exposure dose was obtained for the I-line resist with exposure doses varying from 20 to 68 mJ/cm². Measurements showed an increase in average development rate from 0.04 to 0.155 μm/s, with the rate leveling off at around 55 mJ/cm². Pre-exposure bake monitoring results demonstrated the ability of the sensor to measure the glass transition temperature of the resist film during prebake as well as the ability to invert out the elastic constants of the film using reflection theory. The glass transition temperature (T_g) is an important parameter in both the pre- and post-exposure bakes and therefore could be useful in monitoring these processes. Results of pre-exposure bake T_g measurements are presented for both-I-line and DUV resists. The glass transition temperature during prebake was found to be higher for the DUV resist than for the I-line series. The I-line resist post-exposure bake measurement of glass transition temperature confirmed the reported T_g of 118°C for the I-line novolac resin. The multiple uses of this sensor make it suitable for integration into a manufacturing setting     

Analysis of pattern accuracy in submicrometer electron-beam direct writing

To create submicrometer patterns with high accuracy on thick single-layer negative resist, error factors that degrade pattern accuracy have been investigated. Pattern accuracy was analyzed using a new evaluation method based on the difference between the resist development energy and the exposure energy at points on the edge of each shape. By introducing a new evaluation parameter, we were able to clarify error factors from the exposure conditions, the proximity effect correction method, and the machine exposure fluctuation. The evaluation parameterKisQ/Q_{0}whereQis the exposure dose appropriate for the desired resist thickness and Q0is the interface gel dose. It was found that the resist resolution and the rounding error of the exposure dose were serious error factors, especially in delineation on submicrometer patterns. To achieve 0.5-µm patterns with ±0.1-µm accuracy on 1-µm-thick negative resist, the resist evaluation parameterKmust be less than 2, the rounding error of the exposure dose must be less than 2.5 percent of the dose, and the beam addressing unit (LSB) must be less than 0.025 µm.     

厚胶光刻中光敏化合物浓度空间分布研究

厚胶光刻过程是一个复杂的非线性过程，其光刻胶内光敏化合物(PAC)浓度空间分布是影响显影面形的主要因素。根据厚层胶光刻的特点，结合光化学反应机理，利用角谱理论，分析了在曝光过程中光刻胶内衍射光场和PAC浓度的空间分布随时间的动态变化，以及后烘(PEB)过程对PAC浓度空间分布的影响。该方法数值计算结果准确，且速度快。数值模拟表明，其内部衍射光场分布与PAC浓度分布是一个动态的、非线性的相互影响过程；后烘工艺可平滑PAC浓度空间分布；PAC浓度空间分布是影响浮雕面形边沿陡度的一个重要因素。     

Photolithographic linewidth control

A simple model for the image formation process in photoresist is proposed based on Fresnel diffraction from the edge of a mask feature. It is shown theoretically and verified experimentally that the dimensional difference between a clear mask feature and its image in photoresist is given by aL^½[ ln (Ei) - ln (ET) + ln (K) ] where a and K are constants peculiar to the exposure optics, L is the separation between mask and silicon wafer, Eiis the exposure energy, and ETis the effective exposure energy threshold of the photoresist. The model is used to show the existence of "optimum" exposure ratios Ei/ETwhich minimize image variability in hard-, soft-, and near-contact printing. Control of this exposure ratio is found to be the key to successful use of off-contact printing. Based on this model, a photolithographic process control system is outlined in which exposure tool operation and photoresist processing parameters are characterized and monitored with a single silicon wafer. The system is equally applicable to photomask fabrication.     

Photosolubility of diazoquinone resists

The photochemical properties of a diazoquinone/novolak resist system were studied over a wide range of material and processing parameters in order to determine the optimum values for a given application. These resists are representative of the commercial positive-working photoresists being used for high-resolution lithography. The solubility of the resist in an alkaline developer depends on exposure E asDelta g^{-1} = [g_{0}(E/E_{e})^{m}]^{-1} + [Delta g_{infin}]^{-1}where the net development rate (Delta g) is the difference between solubility rates of the exposed (g) and unexposed (g0) sample. Three of these parameters characterize the lithographic response of the resist. They depend mainly on the resist composition but not on the processing conditions. Eeis a measure of the sensitivity and ranges from 10 to 20 mJ/cm²of 405-nm light for useful resist formulations. The contrast parameter (m) increases slowly with sensitizer concentration while the saturated solubility ratio (g_{infin}/g_{0}) inereases very rapidly. The fourth parameter (g0) depends strongly on processing parameters. It can readily be set to provide the desired development time, e.g., by adjusting the developer strength. On a more fundamental level, it is found that the dependence of the solubility on exposure can be expressed in a unified manner for all the resist formulations studied asg approx g_{0} exp (2 times 10^{-20} n_{e})where neis the number of exposed sensitizer molecules per cubic centimeter.     

X-ray exposure characteristics of diazo-type photoresist using synchrotron radiation

X-ray exposure characteristics of photoresist AZ1350J have been studied using synchrotron radiation. A novel behaviour of the X-ray exposed resist is discovered. X-ray decomposes the photo-sensitizer of AZ1350J but only changes it slightly into a 3- indenecarboxylic acid. Although this is not preferable to making a positive image in the resist, image reversal by subsequent flood UV-light exposure after X-ray replication of mask patterns can be succesfully performed. The negative images fabricated by this technique have a very high resolution in the half micron range for lines and spaces. This is based on higher γ-value and a higher development rate ratio between Z-ray exposed and unexposed areas than those in positive image processing.     

光刻制程参数对光刻胶DICD和锥角的影响

光刻胶经过曝光、显影后的锥角(Taper)和关键尺寸(Develop Inspection Critical Dimension,DICD)是光刻工艺的重要参数。明确影响锥角和DICD的工艺参数,进而控制锥角和DICD,这对工艺制程至关重要。本文结合光刻制程,探究了光刻胶厚度、曝光剂量、Z值、显影时间对锥角和DICD的影响,并结合蒙特卡罗算法对显影制程进行评估。实验结果表明:光刻胶厚度每增加1μm,DICD增加约2.6μm。同时,厚度增加会导致光刻胶顶部的锥角逐渐由锐角向钝角演变。曝光剂量每增加10mJ/s,DICD则减小约0.8μm,锥角则呈阈值跳跃式上升趋势。基板在最佳焦平面曝光,DICD和锥角均一性最好。显影时间每增加10s,DICD下降约0.3m,锥角则增加约1.7°。最终,DICD和锥角呈负相关关系,可以通过调节光刻工艺参数对锥角和DICD进行控制。     

Image reversal of positive photoresist: Characterization and modeling

We report on an investigation of a so-called image-reversal process where a negative tone image can be obtained with a positive resist. The reversal process is achieved by addition of a base to the exposed photoresist and a subsequent flood exposure. First, we characterize the image reversal by means of the characteristic curves, thus optimizing the process. A simple analytical model and a two-dimensional numerical simulation program have been developed in order to evaluate the dependence of the edge slope of the image-reversed resist line on resist and exposure parameters. Experimental investigation confirmed the simulation result that both positive and negative line edge slopes can be achieved with the image-reversal process by properly adjusting the exposure parameters. Finally, the image-reversal process has been compared experimentally to the standard positive process, considering linewidth control on profiled surfaces.     

Development of positive photoresist

A new model is proposed to describe the development of positive photoresist over the full range of exposure. The model includes the depth dependence of development rate and is capable of fitting measured data of all resists examined to date. A measurement system for determining the exposure and development model parameters is described. Several types of photoresist and developer have been characterized under a number of processing conditions. The effect of the development model parameters on developed resist profiles is illustrated using simulation.     

AZ4620光刻胶的喷雾式涂胶工艺

对于一些MEMS应用,需要在形貌起伏很大的晶圆表面均匀地涂布光刻胶。喷雾式涂胶工艺满足了这些要求。研究了几种稀释的AZ4620光刻胶溶液的雾化喷涂性能,在沈阳芯源微电子设备有限公司KS-M200-1SP喷雾式涂胶机上进行了雾化喷涂试验,分别对裸片及深孔不同尺寸的晶圆进行喷雾式涂胶实验;特别研究了决定喷涂薄膜膜厚和均匀性...     

Characterization of positive resist development

The development of positive photoresist is assumed to be a surface limited reaction whose development rate is determined by the local inhibitor concentration M, as defined by Dill et al. This paper describes a technique for determining development rate as a function of inhibitor concentration and demonstrates the usefulness of the technique for quality control. Resist development parameters for the simulation of line edge profiles in resist are presented.     

Performance analysis and optimal parameter selection for PYRAMID

Previously, we have proposed PYRAMID, a hierarchical, rule-based scheme for proximity effect correction in electron-beam lithography. In this paper we present a performance analysis of PYRAMID for a variety of different system parameters (resist thickness, substrate composition, etc.). We also discuss the optimal choice of two key correction parameters: global exposure block size and local exposure window size.     

A model for the thermal degeneration of a positive opticalphotoresist

Positive optical photoresists have been used extensively in the semiconductor industry and in the manufacture of thin-film disk- and tape-drive heads. Mathematical models for the exposure and development phases of the lithography process have been well defined, but scant information on modeling of the pre-bake phase is available. A pre-bake model which describes the deterioration of the photoactive component in a positive optical resist is derived. A mass balance and the kinetics of the situation are used to describe the change in the Dill exposure model parameters. Experimental verification of the model is shown with three photoresists whose parameters are calculated for different wavelengths of light. The effect of the resist solvent on the exposure model parameters is discussed