Thin Film Based Micro-Machined Sensor Development and Mitigation of Process Challenges

Realization of thin film based sensor employing bulk micro-machined process using wet chemistry is challenging due to lateral etching phenomena. This phenomenon of convex corner undercutting is associated with wet micromachining process where differential etching is required. The challenge of having differential etching to achieve varied depth in close vicinity imposes extra constraint on the process. The present requirement simultaneously etching of closely spaced tunnel and cavity from back side having varied depth keeping intact front side layers is the main bottleneck which was circumvented with modified process. Etching trials related combining wet and dry etching techniques for deeper etching are also carried out and critical aspects are presented in this article. Another aspect is to define process steps compatible with CMOS integration. Further study of piezo-electric layer and etch rate of various deposited layer are also presented. This article details process trials, challenges, mitigation techniques and realization aspects of the micro-machined based sensor.

Microstructure evolution mechanism of Al/Mg dissimilar joint during friction stir welding

A butt friction stir welding (FSW) process was performed on 6061 Al and AZ31 Mg plates. The microstructure evolutions of the three main regions in the nugget zone (NZ) retained in the FSW joint were systematically investigated to clarify the joint formation mechanism during FSW. The differential etching of these microstructural features was found to produce very vivid flow features. During FSW, the material in the shoulder affected zone (SAZ) was mainly driven by the shoulder, and only a small amount of it was driven by the pin. A strip of Al transferred by the pin from the retreating side (RS) to the advancing side (AS) contacted and reacted with Mg, thus forming intermetallic compounds (IMCs) (e.g., Mg17Al12 and Al3Mg2). Due to the stirring action and tilted angle of the threaded pin, a banded structure (BS) feature tilted at approximately 45° was produced by the alternating lamellae of IMCs. The appearance of an onion ring structure occurred in the severely deformed zone (SDZ), which could be attributed to the reflection effect of the imaginary die wall. Finally, the overall flow pattern of the joint was obtained.

Differential etching after lithic heat treatment: First results of an experimental study

The practice of lithic heat treatment creates a combination of initial dull flake scars and subsequent smooth flake scars when the implement is finished after heating. Experiments were done to test the susceptibility of dull and smooth flake scars to etching. The points were etched in 40% hydrofluoric acid for either 40 or 60 seconds. In the 40 seconds experiment, the smooth flake scars of 9 (out of 25) heated points showed less etching than the dull flake scars or no etching at all. These artefacts formed a weathering pattern that is similar to double patina in reworked flints. Ten unheated control samples did not form differential weathering between different generations of flake scars. In the 60 seconds experiment 4 (out of 25) heated points showed some parts of the smooth surface to be less affected. Ten unheated control samples did not form differential weathering. The experiments show that sometimes smooth flake scars are more resistant to etching initially. In Scanning Electron Microscopy, flint artefacts are sometimes pre-treated with hydrofluoric acid. Heat treated flints are susceptible to differential weathering by hydrofluoric acid. Thus, pre-treatment with hydrofluoric acid of heat treated SEM samples can lead to surfaces that are etched to a different extent. The chemical etching in this study does not replicate any natural patination process. How heat treated lithics respond to natural weathering processes cannot be predicted. Further studies should produce natural patination and test selected artefacts in contexts of intentional heat treatment for signs of heating.

Influence of Negative Charging on High Rate SiC Etching for GaN HEMT MMICs

We discuss the influence of negative charging on high-rate ICP etching of SiC via-holes for GaN HEMT MMICs. There is large differential etching behavior such as etch rate, etching profile, and RIE lag between S.I.- and n-SiC substrates because of the difference in wafer heating and negative charging of the sidewall during etching between both substrates. We analyze the difference in negative charging between both substrates by simulating the etching profile.