Low pressure sensor device with high accuracy and high sensitivity

Pressure non-linearity in a low pressure sensor device formed from a silicon diaphragm with an embedded piezoresistive transducer is reduced by using a shallow boss or thin stiffener on an ultra-thin diaphragm while the pressure sensitivity of the device is increased with corner trenches.

BACKGROUND

Solid state pressure sensors are well known. U.S. Pat. No. 4,236,137 to Kurtz, et al. discloses a semiconductor pressure transducer. U.S. Pat. No. 5,156,052 to Johnson, et al. also discloses a solid state pressure transducer. U.S. Pat. No. 6,006,607 to Bryzek, et al. discloses a pressure sensor that uses a piezoresistive device. U.S. Pat. Nos. 5,178,016 and 6,093,579 also discloses solid state pressure sensors.

A well-known problem with prior art pressure sensors that use piezoresistive devices formed into a thin silicon diaphragm is pressure non-linearity or PNL. The PNL is a function of the silicon diaphragm's deflection. The greater the diaphragm deflection, the greater degree of output non-linearity, whether the piezoresistance is detected and measured as a voltage or current.

Output non-linearity becomes more problematic in sensors that are intended to detect low pressures, e.g., pressures below 10 kPa. Since low pressure sensing devices require very thin silicon diaphragms, the diaphragm deflection in a thin diaphragm tends to aggravate the PNL in pressure sensors that are designed to measure low pressures. Another problem with thin silicon diaphragms is that they are fragile. A major challenge is to create a diaphragm to lower or reduce PNL while improving pressure sensitivity without increasing the die size for a low pressure sensor. A solid state piezoresistive pressure sensor that can be used at low pressures and which has an improved output linearity and which is more rugged and more sensitive than those in the prior art would be an improvement.

BRIEF SUMMARY

Embodiments of the invention are directed to a pressure sensor comprising: a silicon diaphragm (diaphragm) having a first thickness between about 1 and about 10 microns and having a piezoresistive element formed therein; and a diaphragm stiffener having a second thickness between about 1 and 20 microns, the diaphragm stiffener being attached to the diaphragm and located proximate to a center of the diaphragm. The diaphragm stiffener may have a second thickness between about 1 micron and about 10 microns. The diaphragm stiffener is surface micromachined. The stiffener may have an outer perimeter, and the diaphragm may be etched out and thinner in a plurality of regions that are distributed around the perimeter of the diaphragm. The diaphragm stiffener may be rectangular. The diaphragm stiffener may be square. The diaphragm stiffener is round. An etched-out region may be located at each comer of the stiffener. The pressure sensor may include a sidearm between each etched-out region. The etched-out region may be of a first thickness, the sidearm may be of a second thickness, and the stiffener may be of a third thickness. The first, second, and third thickness may be different from each other. The stiffener may be formed from the diaphragm. The stiffener may be formed from a material that is different from the diaphragm. The pressure sensor may include a housing having a recess. The pressure sensor may include at least one circuit formed in the silicon diaphragm and a passivation layer formed over the diaphragm.

Embodiments of the invention are directed to a method of forming a pressure sensor, the method comprising the steps of: forming a shallow gap on a first side of a first die, the shallow gap having a bottom surface; attaching a stiffener to the shallow gap bottom surface; forming a plurality of etched out regions around the stiffener; bonding the first die to a second die; thinning a second side of the first die to form a diaphragm; and forming a piezoresistive transducer on the second side of the first die. The plurality of etched out regions may have different thicknesses. The method may include the step of forming a hole through the second die. The step of forming a shallow gap and the step of thinning the second side may provide a diaphragm having a thickness between about 1 micron and about 200 microns. The step of forming a hole through the second die may include the step of forming the hole using deep reactive ion etching (DRIE).

Embodiments of the invention are directed to a method of forming a pressure sensor, the method comprising the steps of: forming a shallow gap on a first side of a first die, the shallow gap having a bottom surface and where the shallow gap is formed to provide a stiffener to the shallow gap bottom surface; forming a plurality of etched out regions around the stiffener; bonding the first die to a second die; thinning a second side of the first die; and forming a piezoresistive transducer on the second side of the first die. The plurality of etched out regions have different thicknesses. The method may include the step of forming a hole through the second die. The step of forming a shallow gap and the step of thinning the second side may provide a diaphragm having a thickness between about 1 micron and 20 microns. The step of forming a hole through the second die may include the step of forming the hole using deep reactive ion etching (DRIE). The step of attaching a stiffener may include the step of forming the stiffener from the first die by etching.

DETAILED DESCRIPTION

FIG. 1andFIG. 2are different perspective views of a preferred embodiment of a pressure sensor10for use in automotive and industrial pressure sensing applications.FIG. 3is a cross-sectional diagram of the pressure sensor10shown inFIG. 1andFIG. 2. The sensor10shown inFIGS. 1,2and3includes a piezoresistive pressure sensor element, which is described below and which has a reduced PNL and improved pressure sensitivity through the use of a thin diaphragm that includes a surface-micro machined stiffener, also referred to herein as a “shallow boss.” The sensor element described below is not shown inFIG. 1orFIG. 2as it is inside the housing12.FIG. 3however shows the relative location of the pressure sensor element14in the sensor10.

The injection molded plastic housing12seals a diaphragm-type pressure sensor element14inside a cavity16. An integrated circuit18that includes electronic devices to measure resistance changes of one or more piezoresistive devices in the sensor element14, and generate an electrically measurable output signal in response thereto, is electrically connected to piezoresistive devices in the diaphragm of the pressure sensor element14via lead wires20that extend from the integrated circuit18to metal bond pads on the pressure sensor element14. Power and ground connection wires22provide electrical current to the integrated circuit18.

Electrical signals from the electronic devices inside the integrated circuit18, and which represent deflection of the diaphragm in the pressure sensor element14, are routed through the housing12through a signal lead frame24that extends into a shroud that surrounds signal and power lead frame24.

As described below, the pressure sensor element14is formed of a thin, square or rectangular silicon diaphragm36having top and bottom surfaces or sides, a square being a special type of rectangle having four, equal-length sides and equal interior angles (90 degrees). One or more piezoresistive transducers38are formed into the diaphragm36near its peripheral edge using prior art techniques that are well known to those of ordinary skill. Deflection of the diaphragm in response to pressure applied to the diaphragm surfaces creates stress in a piezoresistive transducer, which causes its resistance to change. Resistance changes of the piezoelectric device are then converted to measurable electrical quantities by circuitry in the integrated circuit (IC)18, i.e., voltage or current, to produce an electrical signal representative of pressure on the diaphragm.

Since the pressure sensor10is for sensing low pressures, the diaphragm thickness should be thin. In a preferred embodiment, the diaphragm thickness is nominally between about three (3) and about (5) microns (micrometers) however, alternate embodiments include diaphragms with thicknesses between one micron and about ten microns in order to enable the diaphragm to respond to pressures below 10 kPa. While a thin diaphragm enables the pressure sensor element14to respond to very low pressures, the use of such a thin diaphragm aggravates PNL. The inventors have overcome the increased PNL caused by a thin diaphragm by the use of a thin or “shallow” stiffener34applied to, or formed as part of the diaphragm36on the side of the diaphragm opposite the piezoresistive transducer38. The stiffener34is also referred to herein as a “boss.” The use of a thin stiffener, of a proper thickness, enables a thin diaphragm to be able to respond to low pressures without over-deflecting the diaphragm.

The thin stiffener34or “boss” is preferably formed as part of the diaphragm36during an etching process that forms the diaphragm itself. Surface micromachining, which is a well-known process of applying multiple layers to each other, one at a time, is optional but preferably used to properly “thin” the diaphragm36and form the stiffener34. In an alternate embodiment, the stiffener34is formed separately and then attached to the diaphragm36.

In a preferred embodiment, the diaphragm36is square or substantially square with a width and length of about seven hundred seventy microns. As shown in the figures, the stiffener34preferably has the same geometric shape as the diaphragm36and the center of the stiffener is located as close to the center of the diaphragm36as possible so that the diaphragm36is supported at its corners and so that the piezoresistive transducer38located on the center of the diaphragm edge is stressed by diaphragm deflections. In the preferred embodiment, the square stiffener had a width and length of about four hundred seventy microns and a thickness that can range from about one micron up to about twenty microns however a preferable range for the stiffener34is between about one micron and ten microns.

FIG. 4is a cross-sectional diagram of a pressure sensor element14configured for use in a pressure sensor, such as the one shown inFIGS. 1,2and3. The pressure sensor element14is comprised of two layers of semiconductor material joined together by wafer bonding. The top layer, which is referred to herein as the device wafer, is formed to have the aforementioned thin diaphragm36. When the diaphragm36deflects in response to pressure applied to it, the diaphragm deformation changes the resistance of the piezoresistive transducer38. Resistance changes are measured electrically by devices (not shown) in the integrated circuit18to generate an output signal proportional to, or representative of the diaphragm deflection. The non-linearity of the piezoresistive element's resistance is reduced by the stiffener34on the opposite side of the diaphragm36.

As can be seen inFIG. 4, the pressure sensor element14is comprised of a device layer28having a top surface29and a bottom surface31and a substrate layer26having a top surface25and a bottom surface27. The substrate layer is preferably made of a single crystalline silicon.

InFIG. 4, the device layer28has its own top and bottom surfaces29and31respectively and is formed of single crystalline silicon. The bottom surface31of the device layer28is etched out using any appropriate etching technique such as deep reactive ion etching (DRIE) to form a cavity30in the bottom surface31. The cavity30is preferably square or rectangular but formed to have L-shaped corners32that are etched deeper and which are best seen inFIG. 7.

After the bottom surface31of the device wafer28is etched, the bottom surface31of the device wafer28is wafer-bonded to the top surface25of the substrate wafer26. After the device wafer28is wafer-bonded to the substrate wafer26, the top surface (identified inFIG. 8by reference numeral45) of the device wafer28is thinned using chemical-mechanical polishing or CMP to form a top surface29of the diaphragm36. Thinning the device wafer28from its top surface (identified inFIG. 8by reference numeral45) produces a secondary or second top surface29shown inFIG. 4. The thickness of the device wafer28away from the cavity30after thinning, i.e., the distance between top surface29and bottom surface31, is about 400 microns. The diaphragm is considered to be the material of the device wafer28that remains after CMP thinning and which is between the top surface29and the bottom surface of the stiffener or shallow boss34. The distance between the top of the stiffener34and surface29defines the diaphragm's thickness. The thickness of the device wafer after the top surface is thinned to form surface29results in the formation of a thin diaphragm that will deflect in response to very low pressures applied to either side of the diaphragm.

The stiffener34described above can be formed during the DRIE process, in which case the stiffener and diaphragm are formed of the same material or the stiffener can be formed separately by either the same material as the diaphragm or a different material. Like the diaphragm36, the stiffener34is also thin, i.e., preferably between two and about seven microns but preferably about four microns thick. As set forth above, the stiffener thickness can range from one to about ten microns.

As shown in the figures, the stiffener34does not extend all the way to the sidewalls37of the trench but is instead centered in the diaphragm. Despite the fact that the stiffener34extends only part way toward the sidewalls37, the stiffener nevertheless reduces the deflection of a thin diaphragm and geometry non-linearity in response to an applied pressure and in so doing reduces the resistance non-linearity of a piezoresistive transducer38formed into the top surface29of the device layer28. The stiffener is thus important to improving the linearity of the pressure sensor element14, (or reducing PNL), the operating pressure range of which is determined by the thickness of the diaphragm36.

FIG. 5is a top view of the pressure transducer38shown in cross section inFIG. 4. In this figure, the diaphragm36is clearly shown in dotted lines as a square portion of the top surface29of the device layer28and located at the geometric center of the diaphragm. The piezoresistive transducer38is formed near the center of one edge of the diaphragm36.

FIG. 6is a bottom view of the device wafer28. The thinned out corner regions or sections32are trenched, which are also shown as being separated from each other by the sidearms42. As shown inFIG. 7, the side arms42are thicker than the corner sections32. The stiffener34can therefore be considered to be additional material thickness, not part of the sidearms42and not part of the corners32. By way of example, the sidearm42thickness could be 5 microns, the corner thickness 3 microns and the thickness of the stiffener34, which is not part of the sidearms42and not part of the corners32, could itself be 4, 5 or 6 microns.

FIG. 7is a perspective view of the bottom of the diaphragm of the device wafer28. In this figure, the stiffener34is shown as being formed as part of the diaphragm36, as can be made to happen during an etching of the cavity30. In an alternate embodiment, however, the stiffener34can be formed as a separate structure applied to the bottom of the diaphragm36, such as surface micromaching.

FIG. 8shows steps of a process to form a pressure transducer application which has a high accuracy and high pressure sensitivity. The fabrication process starts with two separate semiconductor wafers identified in the figures above by reference numerals26and28. As a second step, the bottom surface31of the device wafer28is etched to form a shallow cavity30. A gap or cavity30is formed from the bottom side because the substrate wafer26is etched in a later step to form a through hole that extend all the way through the substrate wafer26and which allows fluid (gas or liquid) to impinge upon the back or bottom side of the diaphragm.

After the device wafer28and substrate wafer26are formed, the device wafer and substrate wafer are wafer-bonded to each other. Once the wafers are bonded to each other, the top surface45is thinned using a chemical-mechanical polishing (CMP) technique, the result of which is a secondary top surface29.

Wafer-to-wafer-bonding provides a hermetic seal between them. The cavity30, after enclosure by the wafer bonding of the substrate26to the device layer28, isolates the cavity30from the outside world during the fabrication of circuits.

In a fourth step, circuits39, which include the piezoresistive transducer38and a metal bond pad43for wire bonding and an interconnect44between the metal pad43and the piezoresistive transducer38, are formed into the secondary top surface29of the thinned device wafer28, using well-known prior art techniques. Circuits in the secondary top surface29allow electrical connections to be made to the piezoresistive transducer38from an external integrated circuit. A passivation layer40is added over the secondary top surface29to protect the circuits39.

In a final step, a through hole is formed all the way through the substrate wafer26from its bottom surface27to its top surface25. The through hole51thus allows the pressure transducer38to operate as a differential transducer, which is to say that the resistance of the piezoresistive transducer38will change in response to a pressure difference between the pressure inside the cavity30and above the top surface29.

The dimensions of the diaphragm and stiffener described above are important because they imbue the pressure sensor element14with characteristics not found in the prior art, namely the ability to measure low pressures with reduced PNL than would otherwise be possible using a thick diaphragm and/or a thick stiffener taught by the prior art.

As used herein, the diaphragm36is considered to be the portion of the device wafer28directly above the cavity30. In a preferred embodiment the diaphragm thickness is between about 3 and about 5 microns. The diaphragm itself is preferably square having an edge-to-edge dimension of about 700 to 800 microns with a preferred embodiment being about 770 microns across.

The stiffener34, which is also referred to herein as a boss, is also preferably square with a nominal width of about 400 microns. The corners32of the diaphragm are thinner and have a thickness of about 3 microns. Alternate embodiments of the stiffener can be rectangular, circular or elliptical, which are well-known geometric shapes and therefore omitted from the figures for brevity.

A hole51formed through the substrate wafer26is preferably formed using deep reactive ion etching or DRIE. The hole allows fluid (gas or liquid) to exert pressure against the bottom or backside of the diaphragm to provide a differential pressure sensor. The sidewalls of such a hole are nearly vertical. Alternate embodiments employ the use of other etching technologies.

As a final processing step, a passivation layer formed of silicone dioxide or silicone nitride is deposited over the secondary top surface29to protect the piezoresistive transducer38and other circuit elements39formed therein. Metal bond pads provide a conductive pathway to which the aforementioned connection wires20can be attached to the pressure transducer.

The foregoing description is for purposes of illustration only. The true scope of the invention is defined by the appurtenant claims.