Casper A. H. Juffermans

Influence of high substrate doping levels on the threshold voltage and the mobility of deep-submicrometer MOSFETs

The high levels of substrate doping needed in deep-submicrometer MOS devices affect device properties strongly. The authors present a detailed experimental study of high-doping effects on the threshold voltage, which is shown to be affected by the quantum-mechanical splitting of the energy levels in the conduction band. A simple expression to account for these effects is proposed and the consequences for device scaling and design are discussed. Furthermore, the increasing levels of substrate doping and high normal electric fields affect the channel mobility through Coulomb and surface-roughness scattering. Several empirical models for the surface mobility are compared with the characteristics of experimental devices. >

Novel aberration monitor for optical lithography

The aberration monitor allows independent determination of spherical, coma, astigmatism and three point in a single experiment using existing equipment. The monitor consists of a circular phase object, with a diameter of approximately (lambda) /NA and a phase depth of (lambda) /2. Due to the relative large diameter, the image prints as a narrow ring into the resist. Without aberrations its contours are concentric circles. Aberrations deform the ring in a characteristic way. A detailed analysis of the ring shape through focus identifies the aberrations of the projection lens. A linear aberration model is compared with simulations. Experimental results of various aberrations are shown and ar correlated to line width measurements and interferometric lens data.

Comparison of various phase-shift strategies and application to 0.35-μm ASIC designs

Phase shifting masks for real circuits have been investigated extensively only for DRAMs. In this paper, we report on the applicability of i-line phase shifting lithography to the production of application specific ICs (ASICs). The performance of several phase shift strategies is compared, using an i-line stepper with a numerical aperture of 0.48. Data preparation and mask technology considerations are taken into account. Emphasis is placed on the two most critical levels: poly gate and contact window. Results on poly topography are shown. For the poly level, the frequency doubling alternating shifter strategy in combination with a positive resist seems to be capable of printing features down to 0.35 micrometers CD, but the development of automatic phase shift level generation software is still in a preliminary phase. Edge contrast enhancement strategies in combination with a negative resist are considerably simpler, in particular the halftone PSM strategy. These strategies are also very useful in combination with a positive resist for the contact level, where a doubling of the process latitudes was obtained.

Impact of high order aberrations on the performance of the aberration monitor

The aberration ring test is used to determine the low and high order lens aberrations. The method is based on two key elements: the linear response of ART to aberrations and the use of multiple imaging conditions. Once the model parameters are determined by means of simulations, the Zernike coefficients are solved from a set of linear equations. The Zernike coefficients thus obtained are correlated to interferometric lens data and to line width measurements.

Aberration retrieval using the extended Nijboer-Zernike approach

We give the proof of principle of a new experimental method to determine the aberrations of an optical system in the field. The measurement is based on the observation of the intensity point-spread function of the lens. To analyze and interpret the measurement, use is made of an analytical method, the so-called extended Nijboer-Zernike approach. The new method is applicable to lithographic projection lenses, but also to EUV mirror systems or microscopes such as the objective lens of an optical mask inspection tool. Phase retrieval is demonstrated both analytically and experimentally. The extension of the method to the case of a medium-to-large hole sized test object is presented. Theory and experimental results are given. In addition we present the extension to the case of aberrations comprising both phase and amplitude errors.

Focus and exposure dose determination using stepper alignment

The standard ASML alignment system in combination with modified marks is used to determine the primary imaging parameters of the stepper: focus and exposure dose. The method uses a standard chromium on glass reticle. The paper discusses system calibration and feedforward process control in production. A lens qualification at the specified resolution and swing curves are shown. The exposure dose measurements are compared with dose to clear measurements. The theory for optimizing the mark layout is presented.

Lithographic process optimization using process capability analysis

A capable process fulfills many requirements on e.g. depth of focus, exposure latitude, and mask error factor. This makes a full optimization complicated. Traditionally only a few parameters are included in the optimization routine, such as the focus-dose process window, while other parameters like the (NA,σ ) illumination conditions are fixed at a specified value. In this paper we present an analytical model for describing the effect of variations in dose, focus and mask CD. We optimize the overall CD distribution, both the target value and the CD variation, taking the statistical variations of focus, dose and mask line width variations into account. The improved CD control is measured quantitatively, using the well-known process capability index (Cpk). The results are compared to traditional optimization schemes and brute force Monte Carlo simulations. Process latitudes can be better optimized while calculating the OPC curve. This is achieved by tuning the mask corrections to the process variations and simultaneously optimizing the global mask bias. Furthermore, the optimization method enables a trade off between mask error and process control. Simulated aerial image data is used to determine the optimum mask bias and illumination condition for different levels of process variation, including mask CD variation. The effect of optimizing the global mask bias is calculated. Finally, the results will be compared to experimental data for a number of illumination settings.

Characterization of a projection lens using the extended Nijboer-Zernike approach

In this paper we give the proof of principle of a new experimental method to determine the aberrations of an optical system in the field. The measurement is based on the observation of the intensity point spread function of the lens. To analyze and interpret the measurement, use is made of an analytical method, the so-called extended Nijboer-Zernike approach. The new method is applicable to lithographic projection lenses, but also to EUV mirror systems or microscopes such as the objective lens of an optical mask inspection tool. Phase retrieval is demonstrated both analytically and experimentally. Theory and experimental results are given.

Optimization of antireflection layers for deep-UV lithography

Increasing swing and notching effects due to the higher reflectivity of silicon with decreasing exposure wavelength are a major problem in deep UV lithography. Top and/or bottom anti reflective layers (ARL) are necessary for sub-half micron lithography. In this article a simple method describes how optimum values of the refractive index and the thickness of bottom ARLs can be determined. Two classes of bottom ARLs exist: (1) materials with low intrinsic reflectivity in resist; and (2) Bottom ARLs based on the combined effect of absorbance and interference, so that ARL thickness control over topography is critical. The advantages and disadvantages of both ARL types are given. On poly-Si the optimum ARL is a combination of the two types. A quantitative comparison of the usefulness of various materials as ARL on (poly)silicon, oxide and aluminum is made. Materials considered are: spin-on organic ARC, TiW, TiN, and plasma-enhanced CVD deposited amorphous layers such as hydrogenated silicon (a-Si:H), carbon (a-C:H), silicon carbide a-SiCx, oxynitride a-SiNxOy, and nitride a-Si3+xN4.

Submicron CCD memory structures fabricated by electron-beam lithography

We present experimental data of the first Charge-Coupled Device (CCD) fabricated in a single-level polysilicon process with sub micron gates separated by submicron gaps. Both electron-beam and optical lithography is used. Further it will be shown that using this technology and fully exploiting the basic simplicity of the CCD-concept, memory structures can be made with extremely high bit densities with 2 to 5 µm2cells.