First commercial application of Porous Silicon Based Micromachining

Tu Xiang Zheng

In the middle of 1988, the present author was first proposed a new process called as porous silicon based micromachining. Using this process, silicon membranes and silicon cantilever beams were successfully fabricated. This process included selective etching of silicon in concentrated HF solution to form porous silicon and selective removing of the porous silicon in dilute alkaline solution to obtain desired silicon microstructures.

In order to do so, proton implantation with post-implant annealing was employed to produce a thicker high donor concentration layer in lightly doped n-type silicon substrates, and nitrogen ion implantation was employed to create thinner highly resistive islands in the formed high donor concentration layer. The etching and the removing were exactly restricted within the region defined by the proton implantation and the nitrogen ion implantation, respectively. The donor states produced by the implanted protons and the radiation damage created by the implanted nitrogen ions were eliminated by annealing at 1000⁰C.

The first commercial application of the porous silicon micromachining was the piezoresistive pressure sensors. The present author was granted a US patent 5,242,863 with title as “Silicon diaphragm piezoresistive pressure sensor and fabrication method of the same” in 1993. The fabrication of the sensor uses porous silicon as a sacrificial layer which is formed in a silicon substrate.

The sensing principle of the present sensor is based on piezoelectric effect of silicon. The sensor is composed of a diaphragm, certain resistors on the diaphragm, a cavity buried under the diaphragm, and a silicon substrate. When a pressure is applied on the sensor, the diaphragm will deform and induce bending stresses that leads to increase of the resistance of the resistors.

According the patent 5,242,863, a silicon diaphragm piezoresistive pressure sensor comprises a diaphragm formed by a single-sided fabrication method. The pressure sensor is made up of a substrate on which there is a diaphragm at or near the surface of the substrate with a chamber under the diaphragm. The pressure sensor is fabricated by undercutting a silicon substrate to form a diaphragm and a cavity within the bulk of the substrate under the diaphragm. The fabricating steps including a) forming a buried low resistive layer under a predetermined diaphragm region; b) converting the low resistance layer into porous silicon by etching of silicon in a concentrated hydrofluoric acid solution; c) removing the porous silicon by selective etching; d) filling the openings formed in the etching of porous silicon with a deposited material to form a sealed reference chamber, and c) Adding appropriate means to the exterior of the diaphragm and substrate to detect changes in pressure between the reference chamber and the surface of the substrate.

The present method enables the sensor to be fabricated by what is called a single-sided processing method wherein all the processing steps are conducted solely on the upper side of the silicon substrate. Accordingly, the diaphragm and the reference pressure chamber are all formed by processing the substrate from one side. This greatly simplifies the manufacturing method compared to the conventional method which, and among other things, leads to a substantial reduction in production costs.

With the present method it is possible to form a diaphragm with a high degree of accuracy and avoid the problems caused by a lack of uniformity of thickness of the silicon substrate, a perennial problem with the present double-sided manufacturing method. Consequently, it is possible to fabricate diaphragms of relatively small and highly accurate dimensions in reference to a predetermined crystal plain dimension of the substrate. The ability to form a diaphragm of a predetermined and reduced thickness and dimension with high accuracy allows the production of sensors of much higher sensitivity and accuracy as compared to those made by current manufacturing methods.

Since the reference pressure chamber is formed within the bulk of the silicon substrate from one side, an absolute pressure sensor is formed with an air tight seal all which can be done by integrated circuit fabrication techniques. The conventional manufacturing method has a persistent problem in providing for air tight bonding between the diaphragm and the base material a serious obstacle to effective and efficient mass production. Obviously with the technique as described herein with its simplified fabrication process the actual cost of manufacturing accurate and small pressure sensors can be substantially reduced.

The present method also enables the silicon pressure sensor to be formed by integrated circuit manufacturing techniques. This is possible because all of the processing steps are conducted by a one-sided processing method as described herein, consequently it is easy to design and treat the silicon pressure sensor itself as one element of an integrated circuit because the techniques of both manufacturing integrated circuits and the pressure sensor as described herein are substantially the same techniques. This allows for manufacture of combined pressure sensor and integrated circuits of predetermined signal processing characteristics with appropriate circuits, amplification and whatever addition devices are necessary for the use of the pressure sensor. 

It is clear that using the porous silicon micromachining many sensors and actuators can be processed with good parameters and a good yield. The sensors include accelerometers, gyroscopes, pressure sensors, humidity sensors, and microphones, The actuators include ink jet printer heads, Fabry–Pérot interferometers, and vaporizers.