David Ziger

Linesize effects on ultraviolet reflectance spectra

We have investigated the effect of resist thickness, linewidth, and pitch on UV reflectance spectra. This technique exploits the property that conventional novolak resists are very absorptive from 200 to 300 nm, while substrates are significantly more reflective. For line-space developed resist features of constant pitch, we observe that the reflectance in this wavelength range varies periodically with ? and increases linearly with decreasing linesize. The dominant factor in wavelength dependence is the constructive/destructive interference of the measurement light from the air-resist and air-substrate interfaces. The linesize dependence at constant pitch and resist thickness is predominantly controlled (within proper boundary-condition regimes) by the percentage of the substrate exposed. The gross periodicity of the deep UV (DUV) reflection spectra for patterned films is correlated with resist thickness in a manner similar to the resist-thickness dependence of UV reflection spectra for unpatterned films. Simulation of DUV reflectance from patterned films showed semiquantitative agreement with experimental results.

Focus characterization using end of line metrology

A new method is introduced to measure relative focus using conventional optical overlay instruments. Optical end of line metrology (OELM), is based on patterning a wide frame in which adjacent sides are constructed of submicron sized lines that run perpendicular to the center opening. Because truncation is size dependent, line and space features exhibit significantly more line shortening effects than the solid sections. When measured with a conventional optical overlay tool, the difference in line shortening between the solid and line and space sections are measured as an alignment offset, which is a relative measure of actual line shortening. Reproducibility for measuring the apparent misalignment of these modified box-in-box structures was 3 nm (3/spl sigma//sup est/), which is similar to the repeatability for measuring conventional resist box in box alignment boxes. Truncation is sensitive to focus and the utility for using OELM toward characterizing focus-dependent lens parameters was investigated. Estimated 3/spl sigma/ for calculating best focus and astigmatism were 0.04 /spl mu/m and 0.01 /spl mu/m, respectively. Focus corrections were accurate to 0.05 /spl mu/m within /spl plusmn/0.3 /spl mu/m of best focus. Additionally, a general method is presented to estimate the error in the calculated best focus.

Understanding optical end of line metrology

Optical end of line metrology, OELM, is a new method to measure relative line shortening effects using conventional optical overlay instruments. In this technique, a frame which has two adjacent sides that are constructed of lines and spaces is imaged onto a wafer. Since sub 0.5 micrometers gratings cannot be resolved using conventional optics,the alignment tool sees the sides compared of lines and spaces as solid edges. The purpose of this paper is to characterize errors implicit with this approach. First we show a general error analysis for determining best focus using OELM measurements. From this, we introduce the concept of local image quality as the inverse of the minimum lien shortening, and curvature of line shortening with focus.

Complementary alignment metrology: a visual technique for alignment monitoring

Complementary alignment metrology (CALM) is a new metrology technique to visually measure stepper alignment correctable factors such as horizontal, vertical and rotation offsets as well as magnification errors. CALM is based on the concept that a line and space pattern exposed into resist will e completely cleared if, prior to development, it is exposed a second time by shifting the grating by exactly its half- pitch. We have used this principle to fabricate test wafers that visually indicate correctable factors. The estimated 3(sigma) accuracy of CALM readings compared to box-in-box measurements is 0.03 micrometers . Linearity between CALM readings and box-in-box measurements is maintained for misalignments of +/- 0.13 micrometers . Using such a technique allows baseline corrections to be performed on a more frequent basis.

Linesize effects on UV reflectance spectra

We have investigated the effect of resist thickness, linewidth and pitch on UV reflectance spectra. This technique exploits the property that conventional novolak resists are very absorptive from 200 - 300 nm while substrates are significantly more reflective. For line/space developed resist features of constant pitch, we observe that the reflectance in this wavelength range varies periodically with (lambda) and increases linearly with decreasing linesize. The dominant factor in wavelength dependence is the constructive/destructive interference of the measurement light from the air/resist and air/substrate interfaces. Linesize dependence at constant pitch and resist thickness is predominantly controlled (within proper boundary condition regimes) by the percentage of the substrate exposed. The gross periodicity of the DUV reflection spectra for patterned films is correlated with resist thickness in a manner similar to the resist thickness dependence of UV reflection spectra for unpatterned films. Simulation of DUV reflectance from patterned films showed semiquantitative agreement with experimental results.