Question

Draw a layout for Micro-Electro Mechanical Systems (MEMS) pressure sensore. For each layer, indicate it using a different color. Also, mention the dimensions.

Answer 1

The nanowire pressure sensor has been fabricated using the INTELLIFAB module. The sequence of steps in the fabrication process is described in the process table shown in fig 1. To increase the sensitivity of the pressure sensor,a double nanowire pressure sensor is designed in such a way that it replaces the conventional piezoresistor of the piezoresistive pressure sensor . The nanowire assembly forms a connecting bridge between the diaphragm and the pressure sensor. The nanowire pressure sensor diaphragm has been fabricated with circular diaphragm. The sequence of steps that are adopted to fabricate the nanowire piezoresistive pressure sensor are tabulated in Fig 1

The design of the MEMS nanowire piezoresistive pressure sensor starts from drawing the mask layout of different layers involved in the fabrication process by using Intellimask module. Intellimask has been specially designed for constructing and editing MEMS device level masks.The layout has an area of about 1400?m X 1400?m. The mask is used to bulk etch the wafer from the bottom to create a diaphragm structure and nanowire assembly. The actual area of the diaphragm after fabrication is about 1000?m X 1000 ?m.

The wafers on which the device has to be fabricated, has to be bulk etched. The isotropic etchants and anisotropic etchants are two types of methods to do bulk etching on silicon. Isotropic etchants like HNA (mixture of hydrofluoric acid, nitric acid and acetic acid) will etch silicon at the same rate in all the directions whereas anisotropic etchants like TMAH (tetraethyl ammonium hydroxide) and KOH (potassium hydroxide) etch silicon much faster in the [100] direction than in the [111] direction given by Bonnie Gray.L.(1996). Based on the geometry of the structure, decision is made to select the appropriate etchant. Potassium being a contaminant for CMOS processes and could probably attack the oxide layer, they cannot be used as etchant.

Literature predicts that the most optimistic etch-rate of silicon by TMAH is 1?m/min which means that in order to etch a regular 500±25 ?m thick wafer to create a diaphragm of 20?m, the etch time would be around 8 hours of continuous etching. ours of continuous etching. A smooth surface could be very difficult to achieve with a continuous uniform etch rate and hence it was decided that in order to reduce the etch time of silicon and maintain a uniformity in the etch rate, pre-thinned wafers would be used.

These wafers are available in 200 um thicknesses and would take a maximum of about 3 hours of time for bulk-etching.The wafer that is available has been got from the Czochralski technique. Bulk silicon oxide of about 1000nm was deposited conformal over the bare silicon wafer. This conformal deposition helps to spread the silicon oxide uniformly on the upper surface of the silicon. The oxide layer acts as a mask for silicon etchants. The process of growing silicon oxide layer over the silicon substrate is known as thermal oxidation process.

On the layer of SiO2, a thin layer of polysilicon has been deposited. The polysilicon was the structural material used in the pressure sensor. The thickness of the polysilicon layer was about 10 nm. Conformal deposition is used so that polysilicon spreads uniformly over the insulator.The film was deposited by a method called Low Pressure Chemical Vapor Deposition (LPCVD). The process has been carried out in very low atmospheric pressure, so that the surface reaction was limited. The method also provided a higher throughput and has very low deposition rate as the low temperature is maintained.

Then to obtain a silicon germanium double nanowire pressure sensor a silicon germanium layer has been deposited using chemical vapor deposits and the layer has been doped through ion implanation of boron with dosage between 4x103 cm-2.A silicon nitride film of 200nm has been deposited on the silicon wafers in order to act as an insulating layer to the polysilicon layer that was deposited previously. Silicon nitride has been mainly used for insulation purpose because it behaves as a nearly impervious barrier to diffusion and protects the layer beneath it. The film was deposited by a method called Low Pressure Chemical Vapor Deposition (LPCVD). The silicon nitride film deposited on the wafer acts as a barrier for the polysilicon layer.

The silicon nitride layer has to be etched in order to make the polysilicon layer contact with the aluminum layer. Before etching, the whole substrate has to undergo a process called lithographic process. Photolithography was carried out on the wafers to transfer the patterns onto the wafer for further etching.The pattern formed is used to etched Si3N4 from inside of the wafer. The etching was carried out using Reactive Ion Etching (RIE) method. Reactive Ion Etching (RIE) is an etching technology used in microfabrication of microsystems. Dry etching was typically 5 times faster than the wet etching. Aluminum of 1000nm thickness was deposited to make contact with the nanowire which acts as resistors of the pressure sensor. On the aluminum, the photoresist layer was coated and the lithographic masking has been done, which when exposed to UV radiation, helps to etch the aluminum. Lithographic masking has been done from the bottom of silicon substrate and by using RIE process silicon substrate along with SiO2 was etched out.

The process of etching has been done to develop the diaphragm for the pressure sensor. The polysilicon diaphragm released was masked again using the mask layout . The polysilicon was patterned to make the thin wire like structure that forms a bridge between the diaphragm and the silicon substrate. The nanowire formed connects the diaphragm with the aluminum metal. After masking, using lithographic process, the polysilicon was etched from outside. The Fig 2 shows the complete pressure sensor after performing reactive ion etching (RIE) to release the nanowire assembly