Hua-Tai Lin

A 90-nm CMOS device technology with high-speed, general-purpose, and low-leakage transistors for system on chip applications

A leading edge 90nm bulk CMOS device technology is described in this paper. In this technology, multi Vt and multi gate oxide devices are offered to support low standby power (LP), general-purpose (G or ASIC), and high-speed (HS) system on chip (SoC) applications. High voltage I/O devices are supported using 70/spl Aring/, 50/spl Aring/, and 28/spl Aring/ gate oxide for 3.3V, 2.5V, and 1.5-1.8V interfaces, respectively. The backend architecture is based on nine levels of Cu interconnect with hot black diamond (HBD) low-k dielectric (k<=3.0).

OPC modeling by genetic algorithm

Optical proximity correction (OPC) is usually used to pre-distort mask layouts to make the printed patterns as close to the desired shapes as possible. For model-based OPC, a lithographic model to predict critical dimensions after lithographic processing is needed. The model is usually obtained via a regression of parameters based on experimental data containing optical proximity effects. When the parameters involve a mix of the continuous (optical and resist models) and the discrete (kernel numbers) sets, the traditional numerical optimization method may have difficulty handling model fitting. In this study, an artificial-intelligent optimization method was used to regress the parameters of the lithographic models for OPC. The implemented phenomenological models were constant-threshold models that combine diffused aerial image models with loading effects. Optical kernels decomposed from Hopkin’s equation were used to calculate aerial images on the wafer. Similarly, the numbers of optical kernels were treated as regression parameters. This way, good regression results were obtained with different sets of optical proximity effect data.

Pellicle effect on OPC modeling

As the patterning of IC manufacturing shrinks to the 32-nm node and beyond, high-NA and immersion lithography are required for pushing resolution to its physical limit. To achieve good OPC performance, various physical effects such as polarization, mask topography, and mask pellicle have to be considered to improve the model accuracy. The attenuation and the phase variation of TE and TM wave components induced by the pellicle would impact optical qualities in terms of resolution, distortion, defocus shift, and high-order aberrations. In this paper, the OPC model considering pellicle effects is investigated with Jones pupil. The CD variation induced by the pellicle effect can be predicted accurately. Therefore, the improvement on model accuracy for 32-nm node is demonstrated.

90-nm lithography process characterization using ODP scatterometry technology

CD-SEM and scatterometry are two of the top candidates for CD metrology in 90 nm node. In this study, Optical Digital Profilometry (ODP) based scatterometry was used to evaluate four topics: CD SEM and ODP process resolution comparison, ODP duty ratio limitation study, Poly AEI undercut sensitivity, STI ODP to TEM profile and trench depth matching. The scatterometry results were compared to CD-SEM and TEM results to develop the correlation of different metrology techniques. Scatterometry is able to provide robust uniformity measurement with additional information compared to CD-SEM. The additional information included sidewall angle, photoresist thickness, A°RC layer thickness, and under-layer film thickness. Actual data showed that this extra information was essential to trouble shoot the CD uniformity issue, separate the scanner, track, and thin film deposition impact on final CD uniformity. Scatterometry can be used not only as a metrology tool to measure CD uniformity, but also a useful analytical tool to find out the cause of CD non-uniformity. In small FEM study, scatterometry demonstrated its high resolution and precision. It can clearly identify the CD shift of less than 0.5 nm with exposure energy shift of 0.1mJ. This high resolution enabled a clearer definition of process window, and monitoring of small process shift in the actual production. From the experimental results, current optical tool with ODP technology was well qualified for duty ratio > 30 iso line measurement, detecting Poly undercut, STI profile and depth TEM matching.

OPC segmentation: dilemma between degree-of-freedom and stability with some relieves

It is believed that smaller correction segments could achieve better pattern fidelity, however, some unstable OPC results which are beyond the capability of common OPC correction schemes were found once the segment length is less than a certain threshold. The dilemma between offering more degree-of-freedom by decreasing the correction segment length at the cost of longer correction time and the instability induced by the reduced segment length challenges every OPC engineer. In this paper, 2 indices are introduced; the segmentation index is proposed to determine a reasonable minimum segment length while the stability index can be used to examine whether the correction system is a stiff convergence problem. A compromised correction algorithm is also proposed to consider the OPC accuracy, stability and runtime simultaneously. The correction results and the runtime are analyzed.

Impacts of post OPC shapes on pattern

As feature size get smaller, its crucial to gain depth of focus (DOF) common window in optical lithography. In addition to the DOF of individual patterns, the shift of the best focus between various patterns is significant reducing the common DOF. High-order spatial frequencies diffracted from sharp corners and small patterns on the mask induce additional phase terms and can shift the best focus significantly. We analyzed the correlation between the pattern shape after OPC correction and its corresponding DOF, and found that more complicated shapes lead to more focus shift. Wafer experiment and simulation confirm the predictions. This provides another index for future OPC application.