Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
2000-02-01
2001-10-23
Utech, Benjamin L. (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
Reexamination Certificate
active
06306773
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a method for selective etching of SiC, a method for producing a SiC diaphragm on a SiC-substrate, a method for producing a MEMS (micro electrical mechanical system)-device of SiC having a diaphragm or cantilever structure, and a method for producing a piezo-resistive pressure sensor of SiC.
There is a great demand in for instance the automotive industry for low cost, high temperature pressure sensors that can operate in combustion engines. By accurately monitoring and controlling the pressure in the combustion chamber, the engine efficiency can be raised and the fuel consumption and pollutant emission reduced. To meet these needs, new and innovative pressure sensors for high temperature applications are being developed by various research groups around the world.
It is well known that silicon carbide (SiC) is a useful material for high temperature applications due to its unique properties, such as its wide band gap, high breakdown electric field and high electron saturation drift velocity. Furthermore, SiC exhibits excellent mechanical properties at high temperature, chemical stability and large piezo-resistive coefficients, which makes it suitable for high temperature piezo-resistive sensors. One difficulty in the production of piezo-resistive pressure sensors in SiC is the achievement of a diaphragm having the degree of accuracy necessary for accurate pressure measurements. One method for fabricating SiC-based pressure sensors utilises 3C—SiC films epitaxially grown on Si substrates, which gives a material system that combines the high temperature properties of SiC with methods for the production of diaphragms in Si. A drawback with this material system is that the junction between SiC and Si conducts leakage currents at temperatures around 300° C. Another problem is degradation of the device after a prolonged time due to thermal mismatch effects.
Another newly developed method for producing SiC-based pressure sensors comprises photo electrochemical etching of 6H—SiC. The production process begins with epitaxial growth of p-type 6H—SiC on n-type 6H—SiC wafers, followed by the epitaxial growth of n-type 6H—SiC on the p-type layer. Photo electrochemical etching is used to micromachine the n-type 6H—SiC wafer from the backside into diaphragms and also to pattern the n-type epitaxial 6H—SiC on top into piezo resistors. A drawback of this production method is that the photoelectrochemical etch process directionality is poor, making lateral dimensional control (e.g. of diaphragm size) difficult. Consequently, components produced by this method show a poor repeatability when used for pressure measurements.
Due to its unique properties, SiC is also useful as material for MEMS (micro electro mechanical system) devices of other types than pressure sensors. However, the use of SiC in MEMS applications has been limited due to the fact that it is difficult or even impossible to micromachine and treat this material by means of conventional and known processes for fabrication of for instance Si-MEMS devices. Consequently, there is a great need for an efficient process for matching SiC, allowing the fabrication of MEMS devices of SiC with a high degree of accuracy, and by means of which micro structural devices of SiC having a complex structure may be produced.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a new method for selective etching of SiC enabling the production of SiC-MEMS devices, in particular MEMS devices having free hanging structures (i.e. diaphragms, cantilevers and beams), with a high degree of accuracy.
According to the invention, this objective is achieved by applying a positive potential to a layer of p-type SiC being in contact with an etching solution containing fluorine ions and having an oxidising effect on SiC. This etching process makes it possible to etch a layer of SiC with a high degree of accuracy. Consequently, this etching process can be very useful in the production of a MEMS device.
According to a preferred embodiment of the invention, said layer of p-type SiC is part of a multilayer structure with at least two layers of SiC with substantially different nature of doping, i.e. different doping concentration and/or different doping type, n and p, so that said etching step results in an etching of said layers to an extent depending upon the nature of doping of the individual layers. In this way, a selective etching of a layer of SiC forming part of a multilayer structure of SiC can be efficiently performed and all types of micro structural designs may be obtained by varying the nature of doping and the positions of the doped regions, thereby the tendency to be etched of the different layers.
According to another preferred embodiment of the invention, the multi-layer structure comprises a first layer of p-type SiC, a second layer of p-type SiC and a third layer of n-type SiC, the p-type layer to be etched constituting said second layer and being located between the first and the third layer, said second layer having a doping concentration which is higher than the doping concentration of the first layer, and the positive potential being applied to the second layer via the first layer. The first p-type layer functions as a means for conducting the necessary current to the second p-type layer, whereas the n-type layer functions as a barrier to the current. Since the first p-type layer has a lower doping concentration than the second p-type layer, the second p-type layer will supply the greater part of the positive charges required for the etching process. During the etching process, the first p-type layer and the third n-type layer will remain almost unaffected by the etching process. Consequently, the etching process will result in a uniform and well defined inter space between the first layer and the third layer.
Another object of the invention is to provide a method for producing a free hanging structure, such as a diaphragm, cantilever or beam on a SiC-substrate with a high degree of accuracy.
According to the invention, this object is achieved in
that the SiC-substrate is formed with a first layer of p-type SiC, a second layer of p-type SiC and a third layer of n-type SiC, the second layer being located between the first and the third layer, and the second layer having a doping concentration which is higher than the doping concentration of the first layer,
that a passage is formed in the substrate for allowing a selective etching solution to come into contact with the second layer,
that the substrate is placed in a selective etching solution containing fluorine ions and having an oxidising effect on SiC, and
that a positive potential is applied to the second layer via the first layer so as to etch said second layer in the etching solution for the formation of an interspace between the first layer and the third layer, whereby said free hanging structure is formed by said third layer.
A further object of the invention is to provide an efficient and accurate method for producing a MEMS device of SiC having a free hanging structure, such as a diaphragm, cantilever or beam.
According to the invention, this object is achieved by a method comprising the following steps:
forming a first layer of p-type SiC on a substrate of p-type SiC by epitaxial growth or ion implantation, said layer being formed with a doping concentration which is higher than the doping concentration of the substrate,
forming a second layer of n-type SiC on the first layer by epitaxial growth or ion implantation,
forming a passage in the second layer for allowing a selective etching solution to come into contact with the first layer,
placing the so formed sample in a selective etching solution containing fluorine ions and having an oxidising effect on SiC, and
applying a positive potential to the first layer via the substrate so as to etch said first layer in the etching solution for the formation of an interspace between the substrate and the second layer, whereby said free hanging str
Adås Christian
Harris Christopher
Hörman Thomas
Karlsson Stefan
Konstantinov Andrei
Anderson Matthew
Dilworth & Barrese LLP
Utech Benjamin L.
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