Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Physical stress responsive
Reexamination Certificate
2000-11-27
2002-10-29
Picardat, Kevin M. (Department: 2822)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Physical stress responsive
C438S050000
Reexamination Certificate
active
06472244
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating integrated microstructures of semiconductor material and, in particular, to a method of fabricating an integrated piezoresistive pressure sensor having a diaphragm of polycrystalline semiconductor material. The present invention also relates to an integrated piezoresistive pressure sensor having a diaphragm of polycrystalline semiconductor material.
2. Discussion of the Related Art
Micromachining, which is based on microelectronic fabrication techniques, provides fabrication methods for microsystems such as microsensors, microactuators and special micromechanisms. In recent years, micromachining techniques have been developed for producing integrated pressure microsensors of semiconductor material. These sensors provide numerous advantages in comparison to traditional sensors: reduced cost, high degree of performance and reliability, better signal-to-noise ratio, possibility of integration with memory circuits for producing intelligent sensors, possibility of on-line self-testing, and greater reproducibility. As such, integrated pressure microsensors are increasingly used in the automotive industry, contributing to greater safety and environmental protection with absolutely no increase in vehicle cost.
Currently marketed semiconductor pressure microsensors are typically based on three physical effects: 1) a piezoresistive effect whereby the pressure-induced inflection of a silicon diaphragm unbalances a Wheatstone bridge comprising resistors diffused in the diaphragm; 2) a capacitive effect whereby pressure induces a displacement of a diaphragm forming the movable electrode of a capacitor (thus resulting in a variation in capacitance); and 3) a resonant effect whereby the pressure inflects a diaphragm to which a resonant structure is fixed and this mechanical deformation varies the oscillation frequency of the structure. For the sensor to operate effectively, the diaphragms must be of uniform, accurately controlled thickness with no intrinsic mechanical stress. These characteristics are typically achieved by forming the microstructures by plasma or wet etching, isotropically (for profiles coincident with the crystal faces) or anisotropically (for generating profiles which are independent from the crystal orientation).
Many specialized techniques have been developed to fabricate such microstructures. One such fabrication technique employs etching solutions such as ethyldiaminapyrocatechol (EDP), whereby a structure is formed and separated from the substrate by etching the bulk silicon from the front of the wafer using what is known as the “front bulk micromachining” technique.
In the mid 80's, “surface micromachining” was proposed, whereby the sensitive element or micromechanism was formed of polycrystalline silicon, and which provides for forming suspended structures by depositing and subsequently removing sacrificial layers of different types, e.g. silicon oxide or nitride, porous silicon, aluminum, photoresist, polyimide, etc. Surface micromachined suspended structures, however, are characterized by poor flexural rigidity, and have a tendency to collapse onto the underlying layer, thus impairing thermal or mechanical isolation. A general review of surface micromachining technology (as well as bulk micromachining and the characteristics of each) is to be found, for example, in the article entitled “Micromachining and ASIC Technology” by Axel M. Stoffel, in Microelectronics Journal, 25 (1994), p. 145-156.
Several industrial laboratories and research centers have produced prototype integrated microstructures using the surface micromachining technique. Details of these are to be found, for example, in the articles: “Novel fully CMOS-compatible vacuum sensor”, by O. Paul, H. Baltes, in Sensors and Actuators A 46-47 (1995) p. 143-146, in which a diaphragm of dielectric material is formed on a sacrificial metal layer; “Surface-Micromachined Piezoresistive Pressure Sensor” by T. Lisec, H. Stauch, B. Wagner, in Sensor 95 Kongressband, AO1.2, p. 21-25, in which both the sacrificial layer and the diaphragm are of polysilicon and separated by a small layer of silicon oxide; and “Surface-Micromachined Microdiaphragm Pressure Sensors” by S. Sugiyama, K. Shimaoka, O. Tabata, in Sensors and Materials 4, 5 (1993), p. 265-275, in which use is made of a sacrificial polysilicon layer and a silicon nitride layer as the diaphragm.
Though they do in fact provide for better integrating the devices, the above surface micromachining techniques pose serious problems with regard to the quality of the deposited films (amorphous or polycrystalline) to form the diaphragms, stiction of the suspended structures on the silicon substrate, and packaging difficulties.
In the early 90's, a further microstructure fabrication technique, known as “silicon fusion bonding”, was devised, whereby a cavity is formed in a monocrystalline silicon wafer onto which a further monocrystalline silicon wafer, in which the sensor is formed, is bonded.
A similar microstructure fabrication technique employs dedicated or nondedicated SOI (Silicon-on-Insulator) substrates.
Other highly specialized techniques, such as “wafer dissolving,” provide for forming silicon microstructures by means of dedicated processes which are totally incompatible with standard planar microelectronics technology. In a sense, these “ad hoc” processes simply consist of transferring onto silicon what is currently done using other materials, and only provide for fabricating the sensitive portion, so that the processing and control circuit must be formed on a separate chip.
Yet another highly specialized technique is the LIGA method—a German acronym for Lithographie Galvanoformung Abformung—which comprises three processing steps: synchrotron x-ray lithography; galvanic deposition of metal films; and formation of plastic molds (see, for example, S.M.Sze's “Semiconductor Sensors”, John Wiley & Sons, Inc., Chapter 2, p. 75-78).
At present, diaphragms of semiconductor material (silicon) are produced using the bulk micromachining technique, which is described in detail, for example, in the articles “CMOS Integrated Silicon Pressure Sensor” by T. Ishihara, K. Suzuki, S. Suwazono, M. Hirata and H. Tanigawa, IEEE Journal Sol. St. Circuits, vol. Sc-22, April 1987, 151-156, and “Micromachining and ASIC Technology” by A. M. Stoffel, Microelectronics Journal 25 (1994) 145-156. Bulk micromachining typically involves processing a silicon wafer on both faces to exploit the excellent mechanical properties of monocrystalline silicon. However, front-to-rear processing, and the need for particular handling of the wafers make bulk micromachining incompatible with current integrated circuit fabrication technology.
Another method, known as the electrochemical stop method using a PN junction, provides for more accurately controlling the thickness of the diaphragm and eliminating any process-induced tensile or compressive stress. The electrochemcial stop method involves forming a diaphragm in an N-type monocrystalline semiconductor layer (e.g. the epitaxial layer) on a P-type substrate. The N-type layer is masked except for a previously implanted anode contact region. The rear of the substrate is masked with a mask presenting a window aligned with the region in which the diaphragm is to be formed; a positive potential difference is applied between the N-type layer and the substrate, via the anode contact region, after which the P-type substrate is chemically etched for a few hours at a relatively low temperature (e.g. 90° C.). The chemical etching terminates automatically at the PN junction, forming the diaphragm from the N-type layer at the removed substrate region.
FIGS. 1A
,
1
B and
1
C illustrate typical fabrication steps for forming an absolute piezoresistive pressure microsensor using the electrochemical stop method. The initial steps are those commonly adopted in the fabrication of integrated circuits. In particular, starting from a wafer
1
of mono
Ferrari Paolo
Vigna Benedetto
Villa Flavio
Morris James H.
Picardat Kevin M.
SGS--Thomson Microelectronics S.r.l.
Wolf Greenfield & Sacks P.C.
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