Michael K. Templeton

Optimal sampling strategies for sub-100-nm overlay

Overlay control is a critical requirement of the lithographic process, and the challenge will be even greater with 0.18- micron technology, where the overlay budget is expected to shrink to 70 nm. Control of overlay is often achieved by modifying the stepping parameters to remove any correctable overlay errors. The estimated value of these parameters depends on the overlay error between the two layers, the model used, and the overlay-sampling plan. Overlay sampling strategies face the following dilemma: plans that sample overlay at the edge of the wafer or field will show atypically large overlay errors, but these plans can result in more accurate estimates of the correctable terms. Therefore when measuring overlay at these extreme points the lithographer needs to recognize that this type of sampling will typically indicate that overlay is substantially worse than it is in the average field. In this paper, a number of different sampling plans that measure 25 points on a wafer were tested. The results obtained from the various plans have been compared to the results obtained from measuring the entire wafer. The data show that the sampling pattern can have a significant effect on the values of the various correctable parameters, and that an inappropriate sampling plan can consume a significant portion of the overlay budget. We have identified several effective sampling patterns, and the improved performance of these plans is attributed to the fact that these patterns achieve greater coverage of the wafer and measure a large number of wafer (or grid) points than the other sampling plans.

Improving stigmation control of the CD-SEM

Production fads currently rely on CD-SEM metrology for linewidth control in lithography and etch. The quality of the measurement data is therefore directly tied to the quality of the SEM imaging and that of the electron beam. Experience has shown that even within a 12 hour period, the beam alignment can drift sufficiently to cause a shift of several nanometers in the measured CD. Furthermore, the alignment of the electron beam has traditionally been performed manually, with the quality of the SEM image then being dependent on the judgement of the operator. While this has been adequate in the past, the drive towards ever smaller geometries means that even slight changes in the beam spot can lead to unacceptable variation in the CD measurements. In this paper, we will describe how a more consistent electron beam can be achieved by obtaining quantitative feedback on its sharpness and eccentricity. Using this quantitative information removes the subjective nature of the manual beam alignment and requires less training on part of the users. Furthermore, this procedure results in beam conditions that are at least as good as the alignment performed by an experienced operator, and in fact generally improves the alignment when compared to that obtained by subjective judgement of the image quality. Once a baseline is established, SPC charts for the sharpness and eccentricity values can be used to track tool performance. If either value falls out of a specified range, the system can be flagged as having a problem and efforts to restore the image resolution can begin immediately, reducing the risk of erroneous measurements and thus preventing lots from being misprocessed or mistakenly reworked.

Phase-shift mask technology: requirements for e-beam mask lithography

Phase-shifted patterns (alternating, 90-degree, and chromeless) have been incorporated into a reticle layout, fabricated with a MEBESR III system, and evaluated experimentally at 365 nm using steppers with numerical aperture (NA) ranging from 0.4 to 0.48 and partial coherence ranging from 0.38 to 0.62. Test circuit layouts simulate actual circuit designs with critical dimensions ranging from 0.2 micrometers to 1.2 micrometers . These results, combined with experimental measurement of layer to layer registration and aerial image simulations, provide a first-order assessment of e-beam lithography requirements to support phase-shift mask technology.

Suitability of high-numerical-aperture i-line steppers with oblique illumination for linewidth control in 0.35-μm complex circuit patterns

A 2-dimensional scalar aerial image model was used to computationally study i-line imaging with oblique illumination. Limited comparisons between developed photoresist images and aerial images were made. The effects on CD control, exposure latitude, and bias of varying annular and quadrupole geometry were mapped via simulation. Significant improvements in DOF of isolated lines was achieved with oblique illumination. Isolated line to dense line bias could be adjusted by changing the illumination type. Although oblique illumination improved the aerial image contrast at defocus, it caused degradation in the aerial image contrast at best focus. Long and short range proximity effects degraded the simulated CD control of optimized oblique illumination systems. This was observed in simulations of an SRAM gate cell. The imaging performance at .9 micrometers defocus, of an i-line system (NA .48) with oblique illumination, was judged to be worse than a KrF system (NA .42) with standard illumination. Quadrupole illumination was not found to measurably affect lens distortion.

0.5-μm photolithography using high-numerical-aperture I-line wafer steppers

Results are presented from a new high numerical aperture (NA 0. 48) iline 5X reduction lens which resolves 0. 5 micron lines and spaces over greater than 1 micron depth of focus in several commercially available i-line resists. The performance of this lens is contrasted with that of a NA 0. 40 i-line lens. The NA 0. 40 lens has better depth of focus for 0. 7 microns lines and spaces (L/S) and larger while the NA 0. 48 lens has better depth of focus for L/S smaller than 0. 7 microns down to a resolution cutoff near 0. 35 micron L/S. Other characteristics of the lens such as its relative insensitivity to absorption heating effects and its behavior as a function of the overpressure of He gas within the lens are explored. Simulation work suggests that a NA of between 0. 5 and 0. 55 is optimum for printing 0. 5 micron L/S. Further it suggests that there may be sufficient depth of focus at 0. 4 micron L/S to make i-line a competitor to DUV lithography for the 64 Mbit DRAM generation. 1.

A Systematic Approach for I-line Manufacturing Process Optimization

A framework for process optimization and characterization was discussed and applied to the development of an 0.8μm i-line lithography process for CMOS manufacturing. Response surface and characterization data were presented. Metrics for process optimization were discussed. Depth of focus windows for i-line and g-line lithography were compared for resist materials of similar capability run with optimized processes. Depth of focus data on 3 different stepper types were used to draw conclusions: a 0.38 NA g-line and 0.40 NA i-line from Manufacturer 1, and a 0.48 NA g-line from Manufacturer 2. I-line resists from 2 different manufacturers were seen to have similar depth of focus characteristics. Maintaining acceptable wall angle for the resist profile was found to be a more severe constraint on the depth of focus than maintaining critical dimension control. The i-line resist offered better wall angle than g-line resist, but less global process stability. At center of field, i-line lenses and currently available i-line resists have effectively 10 to 20% more depth of focus than g-line lenses and g-line resists at comparable resolution.

Feasibility Study of Silylation Process for Submicron Manufacturing

Surface imaging using silylation treatment is explored with the aim of understanding the size of the process window for submicron lithography. The influences of resist materials and process temperature were investigated. Resist material strongly influenced the quality of imaging. However, the optimum processing conditions for the conventional diazoquinone/novolac resist and specially formulated dry develop resist were similar. Soft bake temperature, presilylation bake temperature, and silylation bake temperature strongly influenced critical dimension (CD) control in the temperature regimes investigated. The large effect of presilylation bake and silylation bake temperature on CD control can probably be overcome by driving the diazoquinone induced cross linking reaction to completion. However, it is unclear whether a similar scheme will control the 0.025μm per degree centigrade line width variation with soft bake temperature. Considering both the depth of focus and exposure latitude data, the dry develop process appears to conservatively extend the manufacturable resolution to a k of about 0.65 in the Rayleigh resolution equation. At this resolution, our data indicate a depth of focus of about 1.5μ to 2.0μ and an exposure latitude of about 30% (for 0.6μ lines/spaces printed with a 0.40 NA i-line stepper).