Earliest Paper with Porous Silicon Based Micromachining Process

Tu Xiang Zheng

The present author published a paper with the title “Fabrication of Silicon Microstructures Based on Selective Formation and Etching of Porous Silicon” in J. Electrochem. Soc, Vol. 135, No. 8, in August 1988. It was the earliest paper that describes silicon microstructures formed based on porous silicon micromachining process.

The next paper with the title “Using porous silicon as a sacrificial layer” by P Steiner, A Richter and W Lang. was published in 1993, in Journal of Micromechanics and Microengineering  Volume 3, Number 1.

The silicon microstructures were tiny mechanical devices such as sensors, valves, gears, mirrors, and actuators embedded in silicon chips. Before porous silicon micromachining or 1988 year, all these devices were produced by bulk micromachining process or surface micromachining processing.

Silicon wafers can be anisotropically wet etched, forming highly regular structures. Wet etching typically uses alkaline liquid solvents, such as potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH) to dissolve silicon. These alkali solvents dissolve the silicon in a highly anisotropic way so as to produce V-shaped grooves.

Surface micromachining process builds microstructures by deposition and etching of different structural layers on top of the silicon wafer. Generally polysilicon is commonly used as one of the layers and silicon dioxide is used as a sacrificial layer which is removed or etched out to create the necessary void in the thickness direction. Added layers are generally very thin with their size varying from a few microns.

The earliest paper provided a new micromachining process for silicon microstructures formation. The process consists of selective anodization of silicon in concentrated HF solution to form porous silicon and etching of the porous silicon in dilute KOH solution to form desired microstructures. In the process a starting material was n-type silicon wafer having resistivity in the range of 3.2 - 4.8 Ω-cm. Proton implantation with post-implantation annealing was employed to produce a high donor concentration layer in the wafer. Then nitrogen implantation was performed to create highly resistive region in the high donor concentration layer. The un-implanted regions provided the entrance windows through which the anodic current was able to reach the underneath layer. Since the donor concentration in the wafer was much lower than that in the proton implanted layer, the anodic reaction could be stopped automatically at the interface between the high donor concentration layer and the un-implanted regions.

The porous silicon micromachining incorporates the advantages of both bulk and surface micromachining:

• The porous silicon layer as a sacrificial layer can be formed in the silicon wafer and processed from the front side.
• Porous silicon is rapidly etched in dilute hydroxide solutions at room temperature.
• Sacrificial layer formation can be patterned both by selective substrate doping, as porous silicon formation is highly selective with respect to different dopant types and concentrations, and by masking of the substrate.
• Deep channels can be formed in the silicon wafer removing a formed porous layer.
• Porous silicon provides a planar sacrificial surface and is formed much more quickly than thermally grown or chemically deposited sacrificial layers.
• It can also be oxidized to form thick sacrificial oxide layers, thick oxide layers for thermal isolation or for SOI applications.
• Using porous silicon as a sacrificial layer also greatly reduces processing time and complexity, as well as device area, over bulk micromachining.
• It is possible to manufacture free-standing structures of high mechanical and electrical quality since the mechanical structures may be constructed from single crystal silicon. 

Using porous silicon micromachining process the present author had developed several MEMS sensors and actuators. Among them are piezoresistive pressure sensors, thermal flow sensors, thermal conducting (vacuum) sensors, capacitive pressure sensors, and ink jet printer heads.