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.     

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.     

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.     

Fabrication of high aspect ratio gratings for X-ray imaging

Grating-based X-ray imaging system is an important tool to investigate the inner structure of thick samples. The key components of the system consist of three golden gratings. The high aspect ratio gratings are fabricated using the SU-8 material. Considering the grating linewidth broadening varies with exposure dose, the relationship between linewidth broadening and exposure dose is studied experimentally. A series of gratings with different periods and different duty cycles are designed by optimizing the linewidth and exposure dose. Finally, the gratings are successfully fabricated by combining UV lithography and electroplating.     

干涉条纹相位变化对平面全息光栅曝光的影响

为更好地评价平面全息光栅曝光系统的性能,了解干涉条纹相位变化对光栅制作的影响,基于曝光量表达式,结合光栅掩模槽形二元模型,采用理论分析和数值计算的方法,分析了条纹相位变化对曝光对比度、光栅掩模槽形和曝光量相位的影响。各种形式的干涉条纹低频漂移均会降低曝光对比度,导致掩模槽形的可控性下降,其影响具有一致性;为保证曝光对比度达到0.95,低频漂移均方根值应控制在0.05个条纹周期以内;小幅值高频振动对光栅曝光的影响可以忽略;低频漂移造成的曝光量相位误差不影响光栅的衍射特性。结果表明,为获取合格的光栅掩模,应控制光刻胶非线性和曝光量的匹配关系,并将干涉条纹低频漂移均方根值控制在1/20条纹周期以内。可将其作为评价全息光栅曝光系统稳定性的重要指标。     

1 µm MOSFET VLSI technology: Part VI—Electron-beam lithography

This paper discusses the fabrication of 1 µm minimum linewidth FET polysilicon-gate devices and circuits. These were designed for the tight dimensional ground rules (resolution, linewidth control, and overlay) achievable using direct wafer write scanning electron-beam lithography with individual chip registration. The present work focuses on vector-scan electron-beam technology and processing, while other papers in this series discuss other aspects of the work. Different types of 1 µm MOSFET chips were written on 57 mm Si wafers using a totally automated electron-beam system which performs table stepping, registration to fiducial marks, and pattern writing in a vector scan mode (on an individual shape basis) with control of exposure dose for individual shapes. The pattern data were prepared by batch processing which includes proximity correction as well as sorting of shapes to achieve data compaction and minimal distance between shapes. A novel two-layer positive resist system has been developed to achieve reproducible liftoff profiles over topography and better linewidth control. The final results presented here demonstrate that there are no fundamental barriers to the extension of this work to small dimensions.     

Signal linewidth broadening due to fibre nonlinearities in long-haul coherent optical fibre communication systems

Signal spectral linewidth broadening due to the Kerr effect in long-haul coherent optical fibre communication systems is examined theoretically and experimentally. By recirculating loop experiments, it is shown that linewidth broadening becomes serious in a system of more than a few thousand kilometres.<>     

E-beam lithography of computer generated holograms using a fully vectorial 3D beam propagation method

A fully vectorial 3D beam propagation method (BPM) has been applied to obtain a required pattern of computer generated hologram (CGH) with a variable profile of four phase levels. The computer reconstruction of the CGH image having one and two focal spots was performed by application of the fully vectorial 3D BPM method. After transferring the CGH by EBL technique an adequate phase profile was obtained. Inter-level parameter method was developed to obtain the estimated an electron beam dose required for the even topographical patterning. Using this method, an EBL exposure dose determined to achieve the required relief amplitude of 1.29 μm was 43 μC/cm². The manufactured holograms showed that the overall proposed production process, from the 3D BPM computer simulation to e-beam lithography, can be used to obtain good quality product with reasonable time and computational resources.     

Electrical measurement of submicrometer contact holes

A technique to measure submicrometer-sized isolated features electrically is described. P.M. Hall's formula (1967-8) modified with an experimentally determined coefficient is used to obtain the hole diameter and the hole area from the electrically measured linewidth with a higher precision than the precision of the linewidth itself. By incorporating a large number of features in a single test structure, a gain in precision of better than 3 nm is shown analytically. The contact hole diameter and the area of the isolated features obtained in this way, were found to be accurate when compared with physical measurements using a scanning electron microscope. The test pattern used is a combination of two basic four-point probe linewidth-measuring structures and two modified ones. The modified structures contain a large number of contact holes in the 10-μm lines running in the horizontal and vertical directions, which reduce the apparent electrical linewidth. Subtracting the reduced linewidth from that of a solid reference line removes the other contributions to linewidth variation. The size of the contact holes can then be derived from these results. The special test pattern, an analytic expression for interpretation of the results, and hole size data from a variety of exposure dosages are discussed     

Linewidth reduction and frequency stabilization of semiconductorlasers using dispersive losses in an atomic vapor

The effects of a dispersive loss on the linewidth and chirp of a semiconductor laser are discussed. A van der Pol analysis is used to obtain expressions for linewidth reduction and chirp reduction in an arbitrary optical feedback configuration. Specifically, the use of an atomic cesium vapor as a dispersive loss mechanism is considered. The dominant factor in reducing the linewidth and chirp is shown to be the frequency-dependent phase change from the atomic vapor. However, it is shown that under certain operating conditions the frequency-dependent amplitude changes from the atomic vapor also contribute to the reduction. The results are shown to be in agreement with a detailed rate equation calculation     

Phase and amplitude fluctuations in a mode-locked laser

Comparisons are made of the phase and amplitude fluctuations of a laser when it operates single mode and when it is mode locked with the same total average power. Despite the much lower signal-to- (spontaneous emission) noise ratio of the mode-locked laser, the linewidth of each of the locked modes is the same as that of the single mode. The fluctuation of the total intensity of the mode-locked laser, and the linewidth enhancement factor due to intensity fluctuation as recently analyzed by Henry, are the same in both cases.     

Transmission Line Measurement of Narrow Linewidth Ferromagnetic Samples (Correspondence)

Measurement of ferromagnetic resonance linewidths over a range of microwave frequencies is facilitated by the use of a nonresonant waveguide system. The loading effect encountered in such a transmission line system, however, becomes significant when the linewidth is less than a few tens of oersteds. The effect of transmission line loading was avoided by the use of an automatic compensation network.     

Theory of spectral linewidth of external cavity semiconductor lasers

A new formula of the spectral linewidth of external cavity semiconductor lasers is proposed, with which linewidth narrowing with the optical feedback is discussed. It is shown that in the limit of large external cavity length, the linewidth caused by the phase diffusion due to spontaneously emitted photons becomes dominant and the linewidth decreases proportionally to the inverse of the external cavity length.     

Analysis of FM and AM responses of a tunable three-electrode DBRlaser diode

Starting with the wave propagation equation, neglecting spatial and spectral hole-burning effects, and using a solving method based on the Green's function, the rate equations are derived for a multielectrode laser. By applying them to a three-electrode DBR laser, tunability and linewidth performance are evaluated, and the modulation characteristics such as the amplitude response and the frequency response are analyzed. It is shown that amplitude modulation can be completely suppressed, and at the same time, a pure frequency modulation obtained if currents are related by an adequate equation. Since this laser diode has many advantages such as a wide wavelength tunability and a narrow spectral linewidth, it will be suitable as a lightwave oscillator or transmitter in future communication systems     

深亚微米ULSI工艺检测技术——扫描探针显微镜

扫描探针显微镜(SPM)是80年代发展起来的一种具有超高空间分辨率的显微技术。扫描探针显微镜功能强大,在表面科学、生命科学、微电子工业等许多领域得到广泛的运用。随着集成电路的线宽进入深亚微米阶段,扫描探针显微镜在ULSI工艺表征中显得日益重要。本文将介绍扫描探针显微镜及其在微电子工艺和器件微分析中的应用。     

Estimation of resist profile for line/space patterns using layer-based exposure modeling in electron-beam lithography

One of the essential tasks in the dose control for fabrication of 2-D and 3-D patterns using electron-beam lithography is estimation of remaining resist profiles after development. A conventional approach is to compute the exposure distribution for a target pattern through convolution with the point spread function (PSF) and then obtain the resist profile via simulation of the development process based on the exposure distribution. A new approach which does not require calculation of the exposure distribution and simulation of the resist development is proposed. It utilizes a set of experimental results on which estimation of the resist profile is based, and has a good potential to provide an alternative to the conventional approaches. In this paper, the proposed approach is described in detail along with the results obtained from an extensive simulation and also experiments.     

An approximation of the radiation integral for distorted reflectorantennas using surface-error decomposition

The physical optics/aperture integration (PO/AI) formulation is often used to analyze the radiation patterns of reflector antennas. In this study, the PO/AI radiation integrals for distorted reflector antennas are addressed. The surface error of the antennas is approximated by a series of surface expansion functions. The radiation integral is decomposed into a series of radiation-type integrals, each of which corresponds to one of the surface expansion functions. Each of these radiation-type integrals is then weighted by amplitude coefficients. The advantage of performing the decomposition is that each of the radiation-type integrals can be computed and the pattern data stored. The computation of the pattern for a distorted reflector antenna with a changing error profile is performed by recalling the pattern data for each perturbation term and weighting it with the amplitude coefficient. This facilitates rapid evaluation of the radiation integral in cases where the error profile is changing (for example, time-varying errors). The superposition of integrals presented in this paper was shown to be valid for surface-error profiles up to 0.1 λ rms amplitude     

Spectral linewidth of MQW DFB lasers with intensity-modulatedTM-polarized light injection

The spectral linewidth of a multiple-quantum-well (MQW) distributed feedback (DFB) laser is measured when intensity-modulated orthogonally polarized (transverse magnetic (TM) mode) is injected into the laser. The spectral linewidth does not change when the modulation frequency is higher than several hundred megahertz and is almost the same as without light injection. However, it broadens when the injected orthogonally polarized light modulation frequency is close to zero. The line shape of the MQW DFB laser's lasing light becomes non-Lorentzian in shape below a modulation frequency of 500 MHz     

光学光刻工艺中邻近效应的定量分析

随着超大规模集成电路 (VLSI)图形密度的增大 ,邻近效应已成为光学光刻的关键问题之一。通常在平整硅片上对 0 5 μm图形采用 0 5 4NA和传统的单层i线抗蚀工艺时 ,密集图形和孤立图形间的线宽差异大约为 0 0 8μm。然而 ,这一线宽差异已严重地影响了实际生产的工艺稳定性。阐述了邻近效应对图形尺寸、线条与间隙占空比、衬底膜种类、曝光过程的散焦效应、与抗蚀剂厚度变化有关的抗蚀工艺条件和显影时间的依赖性。同时 ,采用 2种不同抗蚀剂实验监测了不同潜像对比度引起的关键尺寸 (CD)偏差。为减小实际图形因抗蚀剂厚度变化引起的CD差异 ,获得最佳抗蚀剂厚度 ,进行了一种模拟研究。     

基于 DMD 的步进式无掩模数字曝光方法及装置

随着器件特征尺寸的继续缩小,所需掩模的成本呈直线上升态势,为降低掩模成本,无掩模光刻技术成为人们研究的热点。介绍了一种基于DMD的步进式无掩模数字曝光方法,并对用于实现该曝光方法装置的设计方案和具体实现进行了论述与分析。最后利用厚的光刻胶进行曝光工艺实验,实验结果表明,本装置可以实现亚微米级线条的刻蚀,较高的侧壁陡度,线条拼接结果良好,可以实现大面积数字式曝光。