David J. Stein

Revisiting the Removal Rate Model for Oxide CMP

This study seeks to explain removal rate trends and scatter in thermal silicon dioxide and PECVD tetraethoxysilane-sourced silicon dioxide (PE-TEOS) CMP using an augmented version of the Langmuir-Hinshelwood mechanism. The proposed model combines the chemical and mechanical facets of interlevel dielectric (ILD) CMP and hypothesizes that the chemical reaction temperature is determined by transient flash heating. The agreement between the model and data suggests that the main source of apparent scatter in removal rate data plotted as rate versus pressure times velocity is competition between mechanical and thermochemical mechanisms. A method of visualizing removal rate data is described that shows, apart from any particular interpretative theory, that a smooth and easily interpretable surface underlies the apparent scatter.

High voltage with Si series photovoltaics.

A monolithic crystalline Si photovoltaic device, developing a potential of 2,120 Volts, has been demonstrated12. The monolithic device consists of 3600 small photovoltaic cells connected in series and fabricated using standard CMOS processing on SOI wafers. The SOI wafers with trenches etched to the buried oxide (BOX) depth are used for cell isolation. The photovoltaic cell is a Si pn junction device with the n surface region forming the front surface diffused region upon which light impinges. Contact is formed to the deeper diffused region at the cell edge. The p+ deep-diffused region forms the contact to the p-type base region. Base regions were 5 or 10 μm thick. Series connection of individual cells is accomplished using standard CMOS interconnects. This allows for the voltage to range from approximately 0.5 Volts for a single cell to above a thousand volts for strings of thousands of cells. The current is determined by cell area. The voltage is limited by dielectric breakdown. Each cell is isolated from the adjacent cells through dielectric-filled trench isolation, the substrate through the SOI buried oxide, and the metal wiring by the deposited pre-metal dielectric. If any of these dielectrics fail (whether due to high electric fields or inherent defects), the photovoltaic device will not produce the desired potential. We have used ultra-thick buried oxide SOI and several novel processes, including an oxynitride trench fill process, to avoid dielectric breakdown.

Recent advances in endpoint and in-line monitoring techniques for chemical-mechanical polishing processes

We present a summary of the recent advances in endpoint and in-line monitoring techniques for chemical-mechanical polishing (CMP) processes. We discuss the technical challenges and review some of the approaches that have been published and/or patented. These methods include optical, thermal (pad temperature), friction (torque motor current), electrochemical, chemical, electrical, and acoustic (vibration). We also present experimental data obtained in our laboratory using selected endpoint methods for metal and oxide CMP.

Estimating the Effective Pressure on Patterned Wafers during STI CMP

Removal rate results obtained from a 150 mm Speedfam-IPEC 472 polisher, coupled with a proven removal rate model has allowed for the determination of effective pressure (i.e., the actual pressure exerted on the structuresof a patterned wafer) during chemical mechanical planarization (CMP) of high-density plasma-filled shallow trench isolation (STI) wafers. Results showed that the ratio of derived effective pressure to applied wafer pressure was 2.2, 1.7, and 1.3 for 10, 50, and 90% density wafers, respectively. The relative consistency of these ratios indicates that the effective pressure experienced during polishing is not impacted by pattern density in a proportionate manner.

Prediction of tungsten CMP pad life using blanket removal rate data and endpoint data obtained from process temperature and carrier motor current measurements

Several techniques to predict pad failure during tungsten CMP were investigated for a specific consumable set. These techniques include blanket polish rate measurements and metrics derived from two endpoint detection schemes. Blanket polish rate decreased significantly near pad failure. Metrics from the thermal endpoint technique included change in peak temperature, change in the time to reach peak temperature, and the change in the slope of the temperature trace just prior to peak temperature all as a function of pad life. Average carrier motor current before endpoint was also investigated. Changes in these metrics were observed however these changes, excluding time to peak process temperature, were either not consistent between pads or too noisy to be reliable predictors of pad failure.

In-line monitoring of chemical-mechanical polishing processes

We present an overview of in-line monitoring of chemical- mechanical polishing (CMP) processes. We discuss the technical challenges and review many of the approaches that have been published. Several methods are currently under investigation including optical, thermal (pad temperature), friction (torque motor current), electrochemical, and acoustic (vibration).

Micromachined conformal electrode array for retinal prosthesis application

Retinal prosthesis projects around the world have been pursuing a functional replacement system for those with retinal degeneration. In this paper, we will outline the concept for a micromachined conformal electrode array and present preliminary fabrication results. Individual electrodes are designed to float on micromachined springs on a substrate that will enable the adjustment of spring constants and therefore contact force by adjusting the dimensions of the springs at each electrode. This will also allow us to accommodate the varying curvature/topography of the retina. We believe that this approach will provide several advantages by improving the electrode/tissue interface as well as generating some new options for in-situ measurements and overall system design.

Analysis of in-situ vibration monitoring for end-point detection of CMP planarization processes

This paper details the analysis of vibration monitoring for end-point control in oxide CMP processes. Two piezoelectric accelerometers were integrated onto the backside of a stainless steel polishing steel polishing head of an IPEC 472 polisher. One sensor was placed perpendicular to the carrier plate and the other parallel to the plate. Wafers patterned with metal and coated with oxide material were polished at different speeds and pressures. Our results show that it is possible to sense a change in the vibration signal over time during planarization of oxide material on patterned wafers. The horizontal accelerometer showed more sensitivity to change in vibration amplitude compared to the vertical accelerator for a given polish condition. At low carrier and platen rotation rates, the change in vibration signal over time at fixed frequencies decreased approximately 1/2 to 1 order of magnitude. At high rotation speeds, the vibration signal remained essentially constant indicating that other factors dominated the vibration signal. These results show that while it is possible to sense changes in acceleration during polishing, more robust hardware and signal processing algorithms are required to ensure its use over a wide range of process conditions.

Dependence of Oxide Pattern Density Variation on Motor Current Endpoint Detection during Shallow Trench Isolation Chemical Mechanical Planarization

In this study, we evaluate the limitations associated with variable shallow trench isolation (STI) oxide pattern densities for accurate motor current endpoint detection during chemical mechanical planarization (CMP). Results indicate that repeatable motor current endpoint detection can be achieved for STI wafers with oxide pattern density variations of up to 17.4%. Furthermore, results show that a dependence exists between the STI oxide pattern density variation and motor current endpoint success during polishing. According to the findings of this study, a suitable motor current endpoint detection system could yield successful termination points for STI polishing, as well as reduce the need for polishing reworks.

High voltage series connected Si photovoltaic cells

This report describes the features of monolithic, series connected silicon (Si) photovoltaic (PV) cells which have been developed for applications requiring higher voltages than obtained with conventional single junction solar cells. These devices are intended to play a significant role in micro / mini firing systems and fuzing systems for DOE and DOD applications. They are also appropriate for other applications (such as micro-electro-mechanical-systems (MEMS) actuation as demonstrated by Bellew et. al.) where electric power is required in remote regions and electrical connection to the region is unavailable or deemed detrimental for whatever reason. Our monolithic device consists of a large number of small PV cells, combined in series and fabricated using standard CMOS processing on silicon-on-insulator (SOI) wafers with 0.4 to 3 micron thick buried oxide (BOX) and top Si thickness of 5 and 10 microns. Individual cell isolation is achieved using the BOX layer of the SOI wafer on the bottom. Isolation along the sides is produced by trenching the top Si and subsequently filling the trench by deposition of dielectric films such as oxide, silicon nitride, or oxynitride. Multiple electrically isolated PV cells are connected in series to produce voltages ranging from approximately 0.5 volts for a single cell to several thousands of volts for strings of thousands of cells.