Ali Ahmadi

Brain-Computer Interface Signal Processing Algorithms: A Computational Cost vs. Accuracy Analysis for Wearable Computers

Brain Computer Interface (BCI) is gaining popularity due to recent advances in developing small and compact electronic technology and electrodes. Miniaturization and form factor reduction in particular are the key objectives for Body Sensor Networks (BSNs) and wearable systems that implement BCIs. More complex signal processing techniques have been developed in the past few years for BCI which create further challenges for form factor reduction. In this paper, we perform a computational profiling on signal processing tasks for a typical BCI system. We employ several common feature extraction techniques. We define a cost function based on the computational complexity for each feature dimension and present a sequential feature selection to explore the complexity versus the accuracy. We discuss the trade-offs between the computational cost and the accuracy of the system. This will be useful for emerging mobile, wearable and power-aware BCI systems where the computational complexity, the form factor, the size of the battery and the power consumption are of significant importance. We investigate adaptive algorithms that will adjust the computational complexity of the signal processing based on the amount of energy available, while guaranteeing that the accuracy is minimally compromised. We perform an analysis on a standard inhibition (Go/NoGo) task. We demonstrate while classification accuracy is reduced by 2%, compared to the best classification accuracy obtained, the computational complexity of the system can be reduced by more than 60%. Furthermore, we investigate the performance of our technique on real-time EEG signals provided by an eMotiv® device for a Push/No Push task.

Yield Forecasting in Fab-to-Fab Production Migration Based on Bayesian Model Fusion

Yield estimation is an indispensable piece of information at the onset of high-volume production of a device. It can be used to refine the process/design in time so as to guarantee high production yield. In the case of migration of production of a specific device from a source fab to a target fab, yield estimation in the target fab can be accelerated by employing information from the source fab, assuming that the process parameter distributions in the two fabs are similar, but not necessarily the same. In this paper, we employ the Bayesian Model Fusion (BMF) technique for efficient yield prediction of a device in the target fab. BMF adopts prior knowledge from the source fab and combines it intelligently with information from a limited number of early silicon wafers from the target fab. Thus, BMF allows us to obtain quick and accurate yield estimates at the onset of production in the target fab. The proposed methodology is demonstrated on an industrial RF transceiver.

Light-weight single trial EEG signal processing algorithms: Computational profiling for low power design

Brain Computer Interface (BCI) systems translate brain rhythms into signals comprehensible by computers. BCI has numerous applications in the clinical domain, the computer gaming, and the military. Real-time analysis of single trial brain signals is a challenging task, due to the low SNR of the incoming signals, added noise due to muscle artifacts, and trial-to-trial variability. In this work we present a computationally lightweight classification method based on several time and frequency domain features. After preprocessing and filtering, wavelet transform and Short Time Fourier Transform (STFT) are used for feature extraction. Feature vectors which are extracted from θ and α frequency bands are classified using a Support Vector Machine (SVM) classifier. EEG data were recorded from 64 electrodes during three different Go/NoGo tasks. We achieved 91% classification accuracy for two-class discrimination. The high recognition rate and low computational complexity makes this approach a promising method for a BCI system running on wearable and mobile devices. Computational profiling shows that this method is suitable for real time signal processing implementation.

Wafer-level process variation-driven probe-test flow selection for test cost reduction in analog/RF ICs

We introduce a methodology for dynamically selecting whether to subject a wafer to a complete or a reduced probe-test flow, while ensuring that the concomitant test cost savings do not compromise test quality. The granularity of this decision is at the wafer-level and is made before the wafer reaches the probe station, based on an e-test signature which reflects how process variations have affected this particular wafer. While the proposed method may offer less flexibility than approaches that dynamically adapt the test flow on a per-die basis, its implementation is simpler and more compatible with most commonly used Automatic Test Equipment. Furthermore, unlike static test elimination approaches, whose agility is limited by the relative importance of the dropped tests, the proposed method is capable of exploring test cost reduction solutions which maintain very low test escape rates. Decisions are made by an intelligent system which maps every point in the e-test signature space to either the complete or the reduced test flow. Training of the system seeks to maximize the number of wafers subjected to the reduced flow for a given target of test escapes, thereby enabling exploration of the trade-off between test cost reduction and test quality. The proposed method is demonstrated on an industrial dataset of a few million devices from a Texas Instruments RF transceiver.

Spatio-temporal wafer-level correlation modeling with progressive sampling: A pathway to HVM yield estimation

Wafer-level spatial correlation modeling of probetest measurements has been explored in the past as an avenue to test cost and test time reduction. In this work, we first improve the accuracy of a popular Gaussian process-based wafer-level spatial correlation method through two key enhancements: (i) confidence estimation-based progressive sampling, and, (ii) inclusion of spatio-temporal features for inter-wafer trend learning. We then explore a new application of the enhanced correlation modeling method in estimating High Volume Manufacturing (HVM) yield from a small set of early wafers and we demonstrate its effectiveness on a large set of actual industrial test data.

Harnessing fabrication process signature for predicting yield across designs

Yield estimation is an indispensable piece of information at the onset of high-volume manufacturing (HVM) of a device. The increasing demand for faster time-to-market and for designs with growing quality requirements and complexity, requires a quick and successful yield estimation prior to HVM. Prior to commencing HVM, a few early silicon wafers are typically produced and subjected to thorough characterization. One of the objectives of such characterization is yield estimation with better accuracy than what pre-silicon Monte Carlo simulation may offer. In this work, we propose predicting yield of a device using information from a similar previous-generation device, which is manufactured in the same technology node and in the same fabrication facility. For this purpose, we rely on the Bayesian Model Fusion (BMF) technique. The effectiveness of the proposed methodology is evaluated using sizable industrial data from two RF devices in a 65nm technology.

Yield prognosis for fab-to-fab product migration

We investigate the utility of correlations between e-test and probe test measurements in predicting yield. Specifically, we first examine whether statistical methods can accurately predict parametric probe test yield as a function of e-test measurements within the same fab. Then, we investigate whether the e-test profile of a destination fab, in conjunction with the e-test and probe test profiles of a source fab, suffice for accurate yield prognosis during fab-to-fab product migration. Results using an industrial dataset of ~3.5M devices from a 65nm Texas Instruments RF transceiver design fabricated in two different fabs reveal that (i) within-fab yield prediction error is in the range of a few tenths of a percentile point, and (ii) fab-to-fab yield prediction error is in the range of half a percentile point.

Wafer-level adaptive trim seed forecasting based on E-tests

Post silicon trimming is extensively used to counter the effects of manufacturing process variation on certain critical electrical parameters of an integrated circuit (IC). Usually, trimming is performed iteratively by adjusting the resistance value of a trim circuit to specific discrete values. Test programs represent those values by codes and apply common search algorithms in order to find a code which makes a device (optimally) compliant to its design specifications. Consequently, manufacturing yield is increased significantly, yet at the expense of added test time and complexity. In this work, we introduce a novel methodology wherein a trained multivariate model is used to predict, adaptively for each wafer, the optimal starting point of the algorithm that searches for the trim code. Thereby, we seek to minimize the number of code changes that the search algorithm has to perform and, by extension, the overall trim time. In order to provide this prediction prior to wafer sort, so that simplicity of test-floor logistics does not get compromised, the predictive model is built using electrical test (e-test) measurements, which are available before wafer sort, and is trained through measurements from a set of early wafers. Effectiveness of the proposed method in reducing trim time is demonstrated on 370 wafers of an high performance device manufactured by Texas Instruments.

Yield Forecasting Across Semiconductor Fabrication Plants and Design Generations

Yield estimation is an indispensable piece of information at the onset of high-volume production of a device, as it can inform timely process and design refinements in order to achieve high yield, rapid ramp-up, and fast time-to-market. To date, yield estimation is generally performed through simulation-based methods. However, such methods are not only very time-consuming for certain circuit classes, but also limited by the accuracy of the statistical models provided in the process design kits (PDKs). In contrast, herein we introduce yield estimation solutions which rely exclusively on silicon measurements and we apply them toward predicting yield during: 1) production migration from one fabrication facility to another and 2) transition from one design generation to the next. These solutions are applicable to any circuit, regardless of PDK accuracy and transistor-level simulation complexity, and range from rather straightforward to more sophisticated ones, capable of leveraging additional sources of silicon data. Effectiveness of the proposed yield forecasting methods is evaluated using actual high-volume production data from two 65-nm RF transceiver devices.

Harnessing process variations for optimizing wafer-level probe-test flow

We propose a methodology for dynamically selecting an optimal probe-test flow which reduces test cost without jeopardizing test quality. The granularity of this decision is at the wafer-level and is made before the wafer reaches the probe station, based on an e-test signature which reflects how process variations have affected this particular wafer. The proposed method offers flexibility by optimizing test flow per process signature and its implementation is simple and compatible with most commonly used Automatic Test Equipment. Furthermore, unlike static test elimination approaches, whose agility is limited by the relative importance of the permanently dropped tests, the proposed method is capable of exploring test cost reduction solutions which achieve very low test escape rates. Decisions are made by an intelligent system which maps every point in the e-test signature space to the most appropriate probe-test flow. Training of the system seeks to optimize the test flow of each process signature in order to maximize test cost reduction for a given target of test escapes, thereby enabling exploration of the trade-off between test cost reduction and test quality. The proposed method is demonstrated on an industrial dataset of a million devices from a 65nm Texas Instruments RF transceiver.