Rainer Pforr

Optical lithography techniques for 0.25 /spl mu/m and below: CD control issues

It is generally accepted that optical lithography will be the technology of choice for the fabrication of devices for several generations to come. In this paper some of the current challenges to print 0.25 /spl mu/m dimensions are addressed. Although the mainstream technologies for 0.35 /spl mu/m and 0.25 /spl mu/m processes are more or less settled, the main challenge will most likely be CD control. Optical proximity effects and CD variations resulting from substrate reflections start to play a major role in the CD budget. Several methods to reduce these contributions (such as the use of optical proximity correction techniques and the use of anti-reflection coatings and surface imaging processes) are presented. Moreover, it will be indicated that considerable improvements in CD control can be obtained by optimization of stepper parameters. Finally, in an attempt to indicate the future of optical lithography, a simulation study is presented, indicating the available process latitudes which can be obtained by combining several of the above listed techniques.

Bleachable dyed resist—investigation of its process latitude in comparison with standard resists

Abstract The process latitude of a resist with a bleachable dye component is investigated theoretically with respect to most interesting lithographic parameters and compared with standard dyed and undyed as well as with heavy PAC-loaded resists. Bleachable dyed resists are found to increase the process latitude especially on highly reflective substrates with topography. This is due to the attenuation of irregular reflections and the interference of the light by a stronger absorption, and by reduction of the bulk effect, which is caused by contrast enhancement due to the bleachable dye component. Calculations are made using a modification of Dills model and the process simulator Lithsim .

New resolution-enhancing mask for projection lithography based on in-situ off-axis illumination

A new technique has been developed for the illumination of a mask in a wafer stepper. A phase-shifting layer (a so-called kinoform) is introduced in the optical path of the projection system between the last lens of the illumination system and the mask absorber. This phase- layer interacts with the incident light in such a way, that the features on the mask get a specific optimized illumination. Therefore the phase distribution has to be generated under consideration of the feature size and distribution on the mask. In that sense a mask feature customized illumination is obtained. In a first application phase, the phase layer is created on the glass side of the mask. Linear as well as chessboard phase gratings are applied. With a chessboard phase grating, a quadrupole-like illumination is generated, which can be optimized to the mask feature distribution by choosing the correct grating frequency, phase-shift and tilt.

In-process image detection technique for determination of overlay and image quality for ASM-L wafer stepper

The possibilities of in-process blue image sensing by using only the implemented darkfield TTL alignment system of a stepper are investigated. It is shown, that all overlay related parameters of a PAS5000/50 and /70 stepper such as red-blue, magnification, rotation and translation can be measured from an enhanced latent image in a special dyed resist but also with some reduction in accuracy in a pure resist only. Results of overlay measurements on typical technological layers and of similar experiments based on latent image sensing in dyed and in pure resist are given, indicating the capability of in-situ overlay control and correction on process wafers. A new alignment target sensing (ATS) technique of focus measurement using the alignment detection system of the stepper for both daily focus control and correction as well as for focus setting on process wafers with typical technological layers is introduced. The method uses special designed alignment markers containing lines and spaces at the working resolution. Results are given for sensing the developed resist pattern as well as the latent image for an i-line and a DUV stepper. The applicability is demonstrated by measuring the influence of varying resist layer thickness and of varying oxide layer thickness of various film stacks of technological layers on optimum focus. The validity of these results is proven by comparisons to other focus measurement techniques such as chessboard and SEM linewidth measurements. A mathematical model based on the diffraction theory of thin gratings has been developed to support the marker design as well as to calculate the Image Quality Signal (IQS) vs. focus/exposure curves of developed images. The model has been verified by experiments.

Contrast enhancement of the resist latent image using exposure induced absorption amplification - fundamentals, modelling, and applicability

Abstract Dyes have been investigated which serve to enhance the resist latent image for exposures with g- and i-line as well as DUV lenses especially in the non-actinic spectral range. Their chemistry and optical properties are described. The usability of these dyed resists for wafer stepper self-calibration methods with regard to overlay and image quality investigations is demonstrated, which resemble- in comparison to known methods - most closely real life processing conditions. The applicability of these resists for measuring the optical transfer behaviour of lenses by using microphotometric means and its limits in accuracy is demonstrated by comparison to theoretical results.

Attenuated phase shifting masks in combination with off-axis illumination: a way towards quarter micron DUV lithography for random logic applications

Abstract The combination of off-axis illumination (OAI) schemes with ‘weak’ phase shifting techniques is promising to improve the process latitudes of both dense and isolated features. In this paper, the performance of this combination to print 0.25 μm poly levels for logic circuits is investigated experimentally and by simulations, using DUV stepper ( λ = 0.248 μm , 0.42 NA ). Various resist types have been addressed, mainly with respect to proximity issues.