Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
  • G01L9/0041—Transmitting or indicating the displacement of flexible diaphragms
  • G01L9/0051—Transmitting or indicating the displacement of flexible diaphragms using variations in ohmic resistance
  • G01L9/0052—Transmitting or indicating the displacement of flexible diaphragms using variations in ohmic resistance of piezoresistive elements
  • G01L9/0055—Transmitting or indicating the displacement of flexible diaphragms using variations in ohmic resistance of piezoresistive elements bonded on a diaphragm
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
  • G01L9/02—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means by making use of variations in ohmic resistance, e.g. of potentiometers, electric circuits therefor, e.g. bridges, amplifiers or signal conditioning
  • G01L9/06—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means by making use of variations in ohmic resistance, e.g. of potentiometers, electric circuits therefor, e.g. bridges, amplifiers or signal conditioning of piezo-resistive devices

Definitions

• the present invention relates to a pressure sensor. More particularly, the present invention is directed to providing a monolithic sensor having multiple sensing elements on a single substrate and electrically selecting one of the sensing elements.
• 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.
• 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.
• 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).
• 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.
• 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.
• ibridge ⁇ The most common (so-called ibridge ⁇ approach) involves placing four piezoresistors, one on each edge of the square or rectangular diaphragm.
• the piezoresistors are connected together in a wheatstone bridge configuration to sense one specific pressure range.
• the other approach locates a single piezoresistive element at one edge of the diaphragm.
• the piezoresistive element is oriented relative to the silicon crystal structure to provide maximum sensitivity of the sensor output when it is accurately placed at the maximum stress point near the diaphragm edge.
• FIG. 1 is a top view of a multi-element sensor circuit in accordance with the present invention
• FIG. 2 is a top view of a sensor in accordance with the present invention showing multiple diaphragm sizes
• FIG. 3 is a cross-sectional view of FIG. 2 taken along line 3-3
• FIGs. 4-6 are cross-sectional views illustrating a method of forming a sensor in accordance with the present invention
• FIG. 7 is a top view of an alternate embodiment of a sensor in accordance with the present invention
• FIG. 8 is a top view of yet another alternate embodiment of a sensor in accordance with the present invention.
• FIG. 1 shows a sensor 10, in accordance with the present invention.
• Sensor 10 includes a sensor substrate 12, sensing elements 14, 16, 18, and 20, and electronic switching circuit 22.
• the sensing elements 14-20 are preferably formed by implantation, diffusion, thin-film deposition, or other well known techniques.
• Switching circuit 22 is electrically connected to each of the sensing elements 14-20 via lines 28, 30, 32, 34, 36, 38, 40, and 42 for electrically selecting at least one of the sensing elements 14-20.
• Lines 28, 32, 36, and 42 preferably carry a S + (positive sensor output) signal and lines 30, 34, 38, and 40 preferably carry a S " (negative sensor output) signal.
• Switching circuit 22 is preferably formed on substrate 12 by a standard semiconductor fabrication process or other well known techniques. In operation, switching circuit 22 selects the desired sensor element for measurement by electrically (digitally) connecting the S + and S " terminals of the sensor to a signal conditioning circuit which measures the differential output voltage of each defined sensing element 14-20 and amplifies and compensates the desired sensing element output signals for use by an external system.
• sensing elements 14-20 are continuously supplied to all sensing elements 14-20.
• One or more sensing elements 14-20 are selected by connecting the desired output signal terminals S + and S ⁇ Other switching configurations can also be used. For example, all S ⁇ S “ , and V + leads could be connected to circuit 22 and then switching in the appropriate V " connection for the desired element (see Fig. 8 below). Other alternative embodiments are described in detail below.
• Sensing elements 14-20 are preferably deposed at varying distances from an outer edge 44 of a cavity 51. It is well known that sensors provide the best performance when the sensing elements are placed adjacent an edge of the diaphragm 46. Also, a sensoris sensitivity is a function of diaphragm thickness and size.
• the edge of the diaphragm 46 can be formed essentially at one of the edges 43, 45, 47, and 49 as indicated in FIGs. 2 and 3. Therefore, during the timed etch process, a repeated etch and measurement cycle to measure diaphragm thickness and stop etching at a precise nominal diaphragm thickness can be minimized. This is because of the ability of the sensor 10 to operate nominally across a range of four diaphragm 46 thicknesses each corresponding to a different diaphragm edge 43, 45, 47, 49 located relative to one of the corresponding elements 14, 16, 18, 20.
• the etching process timing window is widened by virtue of a wider range of diaphragm thicknesses that still produce usable sensors 10. The possibility of over-etch or under-etch is reduced because of an increase in the target diaphragm thickness window.
• FIGs. 2 and 3 show four possible diaphragm 46 thicknesses with corresponding diaphragm edges 43, 45, 47, and 49 defining the optimal edges of diaphragm 46 for each of the sensing elements 14- 20. It is noted that each of the sensing elements 14-20 are preferably positioned on substrate 12 directly above one of the edges 43-49 for optimum performance.
• the on-chip switching circuit 22 is needed to select the element 14-20 that provides performance closest to the nominal target.
• FIGs. 4, 5, and 6 are cross-sectional views of various stages of a preferred method of manufacture of the sensor 10.
• FIG. 4 shows the first step of the method where a sensing substrate 12 is provided and preferably made of silicon or other suitable material.
• Substrate 12 has a top surface 50 opposed to a bottom surface 52.
• FIG. 5 shows the next step of deposing sensing elements 14-20 and electronic switching circuit 22 on the top surface 50. It is noted that electrical interconnect lines 28-42 are also formed at this time but are not shown.
• FIG. 6 shows the final step of removing a portion of substrate 12 from the bottom surface 52 through an etching process to form a cavity 51 to form a diaphragm 46. It is noted that a timed-etch process is preferred but any etching process could be employed depending on the precision required in the placement of the elements 14-20 and the thickness of the diaphragm 46.
• FIG. 7 discloses an alternate embodiment in accordance with the present invention.
• Electronic switching circuit 24 selects the desired sensing elements 114-120.
• the circuit 24 is preferably on- chip (monolithic), but may be off-chip as well.
• Sensor 55 is similar to sensor 10 described above except that sensor 55 has electronic switching circuit 24 instead of circuit 22.
• Circuit 24 deposed on substrate 57 has a switched electrical connection with the sensing elements via lines 54, 56, 58, and 60 as shown for electronically powering at least one of the sensing elements.
• one of lines 54, 56, 58, and 60 connects the sensor V " terminal (ground terminal) to the power supply by selecting an appropriate digital code that is implemented by the switching circuit 24.
• FIG. 8 discloses yet another preferred embodiment in accordance with the present invention.
• the sensor 66 includes a substrate 68, having a diaphragm 70 defined by edge 72, sensing elements 74, 76, 78 and 80, and a switching circuit defined by box 82.
• Switching circuit 82 includes 12 switches 84, which are preferably FETS as shown. The switches 84 are connected to one of the four input terminals 86, 88, 90, and 92.
• input terminals 86-92 are connected to a microprocessor (not shown) or other logic circuitry and select the sensing element to be used. For example, if sensing element 74 is to be used, a signal would be sent through terminal 86 causing the three transistors 84 associated with the element 74 to turn on causing a signal to be outputted to terminals 94 and 96 from element 74. As can be seen, each of the elements 74-80 is always connected to the high-side voltage terminal 98. Switching the low-side power supply voltage terminal and the two output signal terminals of each element 74-80 ensures a clean signal is supplied to terminals 94 and 96. As disclosed above, the sensor can select an element by switching only the power or switching the output terminals. However, by switching both a power terminal and the output terminals, a cleaner signal at terminals 94 & 96 is caused.
• the technique described above selects elements to accommodate process variations in cavity etch and diaphragm edge location.
• the sensors can also be used to make multiple pressure sensor range die. Instead of placing sensing elements on a substrate to cover a wider range of etch conditions for a single targeted pressure range, sensing elements can be placed so that they are optimized for different pressure ranges. Each sensing element has its own target diaphragm size (cavity edge) and thickness. This may be done in conjunction with the same or different size cavity. Generally, the thicker the diaphragm 46 the higher the pressure range that will be sensed by sensor 10.
• a sensor 10 having a diaphragm edge corresponding with line 43 and having a thicker diaphragm will be used for a higher pressure range than a sensor 10 having a diaphragm edge corresponding to any of lines 45-49 and having a thinner diaphragm.
• An advantage of this invention is that only one integrated sensor design, mask set, and fabrication process is needed for the top side of the substrate. The top side of the substrate is where the complicated and costly signal conditioning and wafer processing is done. By only modifying the etch time and/or the backside cavity opening (this is only one low cost mask), four different pressure range sensors can be manufactured from the same integrated circuit wafers.
• a sensor in accordance with the present invention will: 1 ) Select the best performance element out of four which have been targeted for a nominal pressure range, with allowance for the spread of process variation accounted for in the placement of the four elements.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measuring Fluid Pressure (AREA)
• Pressure Sensors (AREA)
• Semiconductor Integrated Circuits (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (2)

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• 1998
  • 1998-08-21 US US09/137,765 patent/US6142021A/en not_active Expired - Lifetime
• 1999
  • 1999-08-12 DE DE69915651T patent/DE69915651T2/de not_active Expired - Lifetime
  • 1999-08-12 WO PCT/US1999/018474 patent/WO2000011441A2/en not_active Ceased
  • 1999-08-12 EP EP99946594A patent/EP1110067B1/en not_active Expired - Lifetime
  • 1999-08-12 ES ES99946594T patent/ES2217809T3/es not_active Expired - Lifetime
  • 1999-08-12 JP JP2000566649A patent/JP2002523736A/ja active Pending

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