Diaphragm Thickness Control
Etching a sensor diaphragm to a desired thickness allows the diaphragm to deflect properly upon exposure to the source of pressure to be sensed. The deflection of the diaphragm is dependent on the pressure being exerted and the thickness and size of the diaphragm. Therefore, in order to have sensors that produce a consistent response from sensor-to-sensor, the prior art diaphragm thickness must be maintained within relatively strict tolerances.
The prior art typically used one of several well known processes to etch the diaphragm, including timed-cavity-etch, oxide etch-stop, and electrochemical etch-stop.
The timed-cavity-etch process is performed by repeatedly interrupting the etching for thickness measurements and resuming etching until a specified diaphragm thickness is achieved. If a cavity is over-etched or under-etched, then the diaphragm is not the desired thickness to provide the proper structural response of the diaphragm and therefore must be discarded. Due to the constant stopping, measuring, and resuming, the process introduces many opportunities for the substrate to break, increasing the possibility of low manufacturing yields. In addition, the timed-etch process typically requires greater labor and more time than other etch techniques that employ an etch-stop layer.
Two other processes besides timed etch are available to create the sensor cavity with defined diaphragm thickness. They are oxide etch-stop using bonded wafers, also known as silicon-on-insulator (SOI), and electrochemical etch-stop. These processes have the benefit of providing a well-defined diaphragm thickness, but also have cost and processing trade-offs.
The oxide etch-stop process uses a starting material for the sensor substrate that consists of two silicon wafers bonded together, with a silicon dioxide layer in between. (The internal oxide layer is created by growing it on one of the wafers before they are bonded together.) When the cavity is etched by immersion in a chemical bath which is exposed to one surface of the bonded wafer/substrate, silicon is removed until it reaches the oxide layer which resists removal by the etchant. The diaphragm thickness is defined by precisely polishing the other surface of the wafer substrate to the desired thickness. The oxide etch stop process is simpler than timed etch, but is offset by the high cost of the bonded and polished starting material wafers used as the sensor substrate. The availability of bonded/SOI wafers is typically limited. Also, the diaphragm thickness must be pre-determined and therefore the flexibility to change pressure ranges if product mix changes is severely limited.
The electrochemical etch-stop process also provides a precise diaphragm thickness. The starting material used as the sensor substrate consists of a silicon wafer which has undergone a wafer processing step to add a P-type (in an N-substrate) or N-type layer (in a P-substrate) by means of diffusion, ion implantation, or epitaxial growth or other known process. To create a diaphragm, the wafer is etched in a fixture which applies bias to the N and P regions such that an etch stop layer is created when the chemical etchant reaches the layer which was deposited to define the desired diaphragm thickness (e.g. N epitaxial layer deposited on P substrate). Similar to the oxide etch-stop method, the diaphragm thickness must be pre-determined and sets the pressure range of the sensor well ahead of the etch process. A different diaphragm thickness requires a wafer with an N or P layer added at a different level in the wafer. The extra processing to create the etch stop layer adds cost to the wafer starting material compared to the bulk silicon wafers used in the timed etch process. Also, etch process fixturing to electrically make contact with the wafer and properly bias it to create an etch stop at the PN junction complicates the manufacturing process, and presents the potential for electrical leakage which could inhibit or damage the etch process.