EEE435_RIE_Lab_v5

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Arizona State University *

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MISC

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Chemistry

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Nov 24, 2024

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EEE435/591 Fundamentals of CMOS and MEMS Processing 1 RIE Laboratory (Revision 5) Module 6 Laboratory – Reactive Ion Etching Objective : This lab provides an introduction to reactive ion etching (RIE) using a PlasmaTherm 790 tool, see Fig. I. You will use the PT790 to etch an oxide covered wafer and will measure the oxide thickness before and after the etch step using the Filmetrics F40. Based on these measurements you will determine the etch rate of silicon dioxide (SiO2) as well as the etch uniformity across the wafer. Background : Reactive ion etching is a plasma technology that is widely used in semiconductor manufacturing to etch metals, semiconductors, and dielectric insulators such as silicon dioxide and silicon nitride. RIE is a dry etch process that combines chemical etching reactions with physical ion bombardment. The dry etch process can be used to produce an isotropic or anisotropic etch profile, depending on the processing pressure and chemistry. Figure II shows the differences between anisotropic and isotropic etch profiles. The anisotropic etch profile produces similar size transfer (W initial  W final ) and the isotropic etch profile widens open features (W initial < W final ). The goal of most RIE processes is to achieve highly anisotropic etching as this enables high resolution transfer of patterns in the photoresist masking layer into the underlying substrate material. Dry etching does, however, have some disadvantages. For example, the energetic ions used for the dry etch processing can damage materials and adversely affect the properties of the etched layers. This damage includes crystal defects, implanted impurities, damage due to ultraviolet radiation generated in the plasma, and defects due to electrical charging of the sample. Loading effects, or the dependence of etch rate on feature size and local geometry, are also a problem in dry etching. Furthermore, low etch selectivity (e.g. erosion of the photoresist layer) is also a problem. Resists that can withstand dry etch processes by design or by treatment are often difficult to remove after the dry etch process has completed. Figure I: ASU NanoFab PlasmaTherm 790 RIE tool for fluorine based etch chemistry. Figure II: Anisotropic etch and isotropic etch profiles.

EEE435/591 Fundamentals of CMOS and MEMS Processing 2 RIE Laboratory (Revision 5) Typical RIE equipment is built around a vacuum chamber with two electrodes that create an electric field which accelerates ions toward the substrate surface. The plasma is initiated by applying RF power at 13.56 MHz to the cathode, whereas the anode is grounded. The electric field ionizes the gas molecules, creating the plasma. A DC bias voltage, that forms on the bottom RF- powered electrode, extracts ions from the “bulk” plasma and energizes them. The ions possess energies ranging from tens to hundreds of eV. The charged ions react with the material’s surface to aid the chemical etch processes. Experiment : This lab will be divided into two stations: the Filmetrics and the PT790. Start by measuring the thickness of the oxide prior to the etch step using the Filmetrics. Work with the TA to r ecord the initial ‘pre - etch’ oxide thickness at each of the locations shown in table of Figure III below. Next perform a 2 minute etch of the oxide layer using the PT790. Record the RIE process conditions in the table below being sure to include the correct units. Finally, measure the oxide thickness after the etch and record the ‘post - etch’ valu es. The TA will take the measurements and post them on the Module 6 Canvas page. Figure III: Table for recording the pre- and post-etch oxide thickness data at the locations shown across the wafer. CHF 3 gas flow O 2 gas flow Chamber base pressure Chamber process pressure RF Power Anisotropic SiO 2 etch conditions Units Table for recording the PlasmaTherm 790 oxide etch recipe conditions. Site SiO2 Pre-Etch Thickness (Å) SiO2 Post-Etch Thickness (Å) Flat Site 2 Site 3 Site 4 Site 5 Site 6 Site 7 Site 8 Site 9 Site 10 Site 11 Site 12 Site 13 Site 14 Site 15 Site 16 Site 17 Site 18 Site 19 Opposite flat

EEE435/591 Fundamentals of CMOS and MEMS Processing 3 RIE Laboratory (Revision 5) Writing-Up the Report Executive Summary: A short paragraph that summarizes what you did and your main findings. Think of this as the only part your busy manager might read before you meet to explain the details. [10 points] Description of RIE Process: Describe how the etch is performed. How was the wafer loaded into the etch tool? Explain the pump down sequence and the etch ‘recipe’ and gas chemistry used to etch the wafer. (1 -2 pages) [40 points] Analysis of the Etch Data: Calculate the mean etch rate and standard deviation across the wafer for each RIE data set. Discuss the uniformity of the etching. In particular, plot a graph of etch rate across the wafer look for systematic variations across the wafer. Describe what is meant by the RIE ‘bullseye effect’. Does your plot show evidence for the bullseye effect? (2-4 pages) [60 points] Oxidised Wafer Etch: Based on the average etch rate calculate the time required to etch a 200nm thick layer of wet oxide. Estimate the uncertainty in this time. (1- 2 pages) [30 points] Over Etch: Explain why you need to include an ‘over etch’ to make sure all the oxide layer has cleared? Based on the measured data how long should the over etch be to ensure that all the 200nm thick wet oxide has been removed? (<1 page) [50 points] Conclusions: This should be a short summary of the main results you achieved. Again, think of your busy manager skipping through the report to the end to confirm the main findings after he or she has read the executive summary. [10 points]

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