US20080047352A1 - Low cost pressure sensor for measuring oxygen pressure - Google Patents

Abstract

A low cost sensor assembly for measuring oxygen pressures contains a transistor header. The transistor header has terminal pins extending therefrom. The transistor header co-acts with a first circuit insulator board. The first circuit board has deposited thereon four hand mirror shaped contact areas each one associated with one of the terminal pins of the transistor header. The top portion of each contact areas has an aperture with the extending arm of the area directed towards the center of the board. The board is epoxied to the transistor header with the terminal pins of the header extending into the apertures of the contact board. A second contact board is then epoxied to the first contact board. The second contact board has a series of four apertures located at the center. Each of the apertures of the second board contacts the handle portion of the mirror patterns of the first board. A leadless piezoresistor sensor assembly is then positioned and secured to the second board whereby the terminals from the sensor assembly align with each of the apertures in the second board. The terminals of the sensor assembly are apertures filled with a conductive glass metal frit and each filled aperture makes contact with a terminal of the sensor. The configuration has all conductive terminals of the entire device completely isolated and insulated from the oxygen environment, thus preventing ignition of the oxygen.

Description

FIELD OF THE INVENTION
• This invention relates to pressure sensors in general and more particularly to a low cost pressure sensor for measuring oxygen pressure and methods of fabricating the same.
BACKGROUND OF THE INVENTION
• Pressure transducers are utilized and employed in a wide variety of applications. One application for the use of a pressure transducer involves measuring oxygen pressures. In aircraft or in other environments oxygen masks are employed in dangerous situations. The oxygen masks can be released and used by a passenger in the event of a technical breakdown whereby the aircraft may lose pressure or in other situations. In employing such devices, one must measure the actual pressure of the oxygen as it enters each mask. One can monitor the entire pressure of the oxygen on the aircraft, but this would not be sufficient and it is preferable that one measure the pressure of the oxygen at each mask. In this manner one can be assured that the proper pressure is being furnished together with the oxygen. In many such devices, the oxygen instead of flowing continuously, is basically pulsed and therefore bursts of oxygen are sent at predetermined repetitive intervals. In this manner, the average pressure imparted to the oxygen mask should be known and therefore a pressure transducer or a sensor is employed in order give an accurate measure of the pressure supplied to the mask. If the pressure is not adequate, a warning light or other indication can be provided indicating that the passenger should use another mask or use other facilities.
• The aspect of monitoring oxygen pressure in an aircraft or other environment is very desirable. It may also be desirable to measure oxygen pressure in other types of oxygen devices such as in hospital environments and so on where oxygen is also supplied to a patient or other user. As one can ascertain, measuring the pressure of oxygen creates certain problems. Any transducer utilized in such an environment must be capable of being cleaned to remove Organic materials and to have no electrical current carrying members in contact with the oxygen. As one can ascertain, any spark or exposure to electrical current can ignite the oxygen thus leading to a disaster. A pressure transducer utilized in such environments must be fabricated to prevent the above-noted problems. It is a further desire to provide such a pressure transducer at low cost due to the large number of transducers that are employed in an aircraft having a large number of passenger seats and positions.
SUMMARY OF THE INVENTION
• A pressure transducer, comprising a header having a top and bottom surface and having a plurality of terminal pins extending from the top surface to the bottom surface, a first circuit board having a top and bottom surface with the bottom surface, positioned on the top surface of the header, the first board having one aperture for each of the terminal pins located on the top surface with the associated aperture surrounding the associated pin, each of the apertures surrounded by a contact area with each contact area having an extending conductive arm directed toward the center of the board, a second circuit board having a top and bottom surface with the bottom surface positioned on the top surface of the first board, the second board having a plurality of apertures located about the center of the second board and with each aperture positioned to overlie an associated conductive arm of the first board, a pressure sensor having an array of pressure responsive devices located thereon and having a top surface for receiving a pressure and a bottom surface positioned on the second board about the center thereof, the bottom surface having terminal pads associated with the array with each pad overlying an associated aperture of the second board, a conductive material deposited in the apertures of the second board to enable conductive contact to the terminal pads of the sensor the conductive material in each aperture contacting the associated extending conductive arm to thereby electrically connect each sensor pad to each of the header terminal pins.
BRIEF DESCRIPTION OF THE FIGURES
• FIG. 1 is a cross-sectional view of the pressure transducer for measuring oxygen according to an embodiment of the invention.
• FIG. 2 is a cross-sectional view of a sensor employed in conjunction with an embodiment of the invention.
• FIG. 3 consists ofFIGS. 3A,3B and3C which respectively show a sensor diaphragm, a cross-sectional view of the sensor and a top-view of the piezoresistive elements found on a sensor.
• FIG. 4 is a cross-sectional perspective exploded view showing each individual part of the sensor depicted inFIG. 1.
• FIG. 5 consists ofFIGS. 5A and 5B which respectively show a top-view of a circuit board and a cross-sectional view of the same circuit board used in an embodiment of the invention.
• FIGS. 6A and 6B show a top-plan view and a cross-sectional view, respectively, of another circuit board used in an embodiment of the invention.
• FIGS. 7A-7E depicts the various steps utilized in fabricating a pressure transducer according to an embodiment of the invention.
• FIG. 8 is a schematic circuit diagram of an oxygen pressure sensor and associated alarm circuitry.
DETAILED DESCRIPTION OF THE INVENTION
• Referring toFIG. 1 and particularly toFIG. 1A there is shown a cross-sectional view of a lowcost pressure sensor10 particularly adapted for measuring oxygen pressures. Thepressure sensor10 has ametallic can18 which basically has atubulation portion27 extending to agreater diameter portion28. Thetubulation portion27 extends into alarger diameter portion28. Theportion28 has an extendingflange19. The metallic can10 as seen inFIG. 1A has a bottle shape having the tubulation orneck27 which typically is on the order of a quarter of an inch in length and one-eighth of an inch in diameter. Thetop opening11 serves as a pressure port for receiving oxygen pressure. Thelarger diameter portion28 surrounds thepressure sensor24 and is secured to atransistor header21. A perspective view of themetallic can18 is shown inFIG. 1B.
• Thetransistor header21 is a typical transistor header and has ametal base portion29 which has extendingterminal pins25 and26. Such transistor headers are well known and for example a TO5 header or a TO8 header can be used forheader21. Such headers typically have at least four pins as25 and26. Typically the metal can18 is discharged welded viaflange19 to theheader flange35. Theheader21 has mounted on the top surface thereof a first circuit board or aninsulative substrate20. Theboard20 has apertures into which the pins as25 and26 extend. The board is mounted and secured to the top surface of the transistor header with an epoxy material. Such epoxy materials are well known. Positioned onboard20 is asecond board30 which is mounted and secured to board20 with an epoxy bond. As will be explained, both thecircuit board20 andcircuit board30 have apertures for accommodating the pins of the transistor header aspins25 and26, which pins eventually will be connected to the outputs of the piezoresistive sensor. The piezoresistive sensor is depicted inFIG. 1A byreference numeral24. Essentially the sensor is a leadless sensor as the type available from Kulite Semiconductor, the assignee herein. Theleadless sensor24 is mounted and secured to board30 by an epoxy bond. There is also a layer ofepoxy31 which surrounds the peripheral edge of the sensor. The sensor contains a glass metal frit16 which frit contacts the terminal pads of the sensor. Aconductive epoxy17 enables one to make contact with theterminal pins25 and26 of thetransistor header21.
• A more detailed view of a typicalpiezoresistive sensor24 is shown inFIG. 2. As seen the piezoresistive sensor has p-typemonocrystalline silicon piezoresistors32. Thepiezoresistors32 are formed on asensor wafer35, which for example may be a wafer of silicon. The entire structure, as indicated, is secured to the top surface of theboard30 by means of an epoxy. The epoxy also forms a surroundingperipheral band31 about thesensor element24. The sensor element includes aglass contact wafer36 which is bonded to thesilicon wafer35. There are terminal apertures as depicted which are filled with a conductiveglass metal frit16. Glass metal frits are conductive and essentially enable one to make contact to the piezoresistors located on the silicon wafer as is known. Theentire sensor24 is bonded to theboard30 by means of an epoxy bond. An epoxy17 is a conductive epoxy and therefore makes contact to the metal glass frits as contained in the semiconductor wafer. The epoxy17 fills the apertures ofboard30 and makes contact with the contact pads of thesensor24.
• The above-noted wafer depicted inFIG. 2 essentially is referred to as a leadless dielectrically isolated sensor chip. The assignee herein has a number of patents which fully describe such a sensor. For example, reference is made to U.S. Pat. No. 5,973,590 entitled “Ultra-Thin Surface Mount Wafer Sensor Structures and Methods of Fabricating the Same”, issued on Oct. 26, 1999 to A. D. Kurtz et al and assigned to the assignee herein. That patent depicts a sensor chip which may be employed as the chip ofFIG. 2. For example, reference is made toFIG. 10 of the above-noted patent which shows a suitable sensing structure which may be employed for the sensor depicted inFIG. 2. Reference is also made to U.S. Pat. No. 6,210,989 which is a divisional of the above-noted patent and which issued on Apr. 3, 2001 having the same title as indicated above and also assigned to the assignee herein. Both patents disclose semiconductor sensor devices which include a semiconductor diaphragm member which is depicted inFIG. 2. The recesses as37 which form an active area for the entire transducer which active area functions as a diaphragm and deflects upon application of a force to thesensor24.
• Also surrounding the transducer is a top-ring member40 which is secured to the sensor structure by conventional means. In any event, as one can ascertain the sensor ofFIG. 2 as further noted in the above-noted patents contain four piezoresistors which are connected in a Wheatstone bridge configuration. The Wheatstone bridge has four terminals which are directed via the associated glass metallicfrit contacts16 to theconductive epoxy terminals17 and eventually come into contact with terminal pins such as25 and26 of thetransistor header21. This therefore enables one to provide an output for the Wheatstone bridge as well as providing a biasing to the bridge. This is conventional and as one can see from the above-noted patents, which are incorporated herein, the devices are referred to as leadless sensing devices because of the fact that there are no wires directed from the devices. The means of making connections to the devices reside in the apertures which contain the glass metal frit which are conductive frits as well as the conductive epoxies which direct the terminals of the Wheatstone bridge to the output connector of the device.FIG. 1B shows a perspective view of the metal can18 depicted inFIG. 1A showing thetubular portion27 which has thepressure port11 at the top end and which as shown inFIG. 1A extends intoportion28 which has theflange19 which is welded to the correspondingflange35 of thetransistor header21 by adischarge weld22.
• Referring toFIG. 3 there is shown inFIG. 3A a top view of the semiconductor sensor depicted inFIG. 1A. Thesemiconductor sensor24 has acentral boss61 which acts as a stop. Surrounding theboss61 are thinned areas as recesses62. These thinned areas will deflect upon application of a force to the device. These areas are known as the active area as compared to, for example,area60 which is a peripheral area known as the inactive area. As seen in FIG.3B a glass wafer or other device is mounted to thesilicon wafer44 by means of a fusion bond. Theglass wafer60 is an insulating wafer and is used for mounting purposes. As seen inFIG. 3B thepiezoresistors41 are shown as well as the glass frit filledapertures42. Theapertures42 are filled with a conductive metal glass frit and the metal glass frit makes contact to the terminals of the Wheatstone bridge as fully described in the above-noted patents.FIG. 3C shows four piezoresistor sensors as50,52,53 and54 connected to associated terminal pads as51 and57 to form a Wheatstone bridge.FIG. 3C is taken fromFIG. 3B with thewafer43 omitted and looking down on the piezoresistive sensors. Typically shown, the sensors as50,52,53 and54 are serpentine and operate to vary resistance upon application of an applied force to the device. The applied force is applied via thepressure port11 which is the top open surface of the tubular portion of themetallic can18. This is shown inFIG. 1B where pressure (P) is applied to theaperture11 which is the pressure port.
• Referring toFIG. 4 there is shown an exploded cross-sectional view of the various components depicted inFIG. 1 prior to installing the same. Thus there is shown the metal can18 having thetubular portion27 with apressure port11 defined by the top open surface. Thetubular portion18 is narrower than theend portion28 which terminates in an extendingflange19. It is noted that the can as depicted inFIGS. 1 and 4 essentially appears like a cross section of a typical bottle where the neck of the bottle is depicted byreference numeral18 and the body of the bottle byreference numeral28. There is shown thechip24 prior to being mounted on theboard30, whichboard30 as indicated, is mounted onboard20. Thetransistor header21 is shown withpins25 and26 emanating therefrom. The pins typically as25 and26 are secured to themetallic header21 by means of glass enclosures as for example,65 and66. Pins may be secured to the transistor header by other means.
• Referring toFIG. 5A in conjunction withFIG. 5B, there is shown a depiction of theboard20. As seen theboard20 has analignment notch71. The board also contains four terminal copper-plated areas as72,73,74 and75. Each area has a central aperture asaperture70 associated witharea74. Theboard20 is typically fabricated from an insulating material such as a Kapton or a ceramic or other insulating material. The areas as for example72,73,74 and75 are copper-gold-plated areas and are formed on the board by means of photolithographic techniques. Such techniques are well known in the semiconductor art. As seen the four pads, each appear as a hand mirror configuration with acentral aperture70. The arms of the mirror as76 and77 extend towards thecenter78 of theboard20. Theboard thickness20 is typically 0.3 inches thick where the apertures are typically 0.02 inches and are equally spaced on theboard20. The board as indicated is metallized by typical techniques and preferably the board is fabricated from Kapton or a ceramic material such as alumina or some other suitable material.FIG. 5B shows a cross-section of the board showing the throughaperture70 each associated with the hand mirror like contact pads as72,73,74 and75.
• Referring toFIG. 6A there is shown a top-plan view of theboard30 which also has analignment notch80. Theboard30 contains apertures as81,84,85 and86 with each aperture bounded by a L-shapedmark82. Theapertures81 are shown more clearly in the cross-sectional view depicted inFIG. 6B. The L-shaped members as82 are alignment marks to enable a proper positioning of thesensor structure24 on theboard30. The board has four apertures (81,84,85, and86) which are in one embodiment, basically 0.015 inches in diameter. These are through holes and are equally spaced. These apertures will accommodate the suitable terminals of thesensor element24. Theboard20 accommodates the terminal pins from the transistor header aspins25 and26 (FIG. 4). This is also clearly depicted inFIG. 1A where the pins extend through the apertures as70. Theelongated arms76 and77 (FIG. 5A) associated with each of the pad areas overlie the apertures as81 and82 of theboard30 whenboards20 and30 are positioned as shown. These apertures are filled with theconductive epoxy17. Therefore, theconductive epoxy17 which fills the apertures as81,82 makes contact to the terminals of the sensor chip as for example those terminals which are filled with theglass metal frit16. Thus, the terminals filled with theglass metal frit16 make contact with the conductive filledapertures81,84 and85. Theconductive epoxy17 makes contact with the elongated ends76 and77 of the hand mirror shapedterminal pads72,73,74 and75 ofFIG. 5A. The apertures ofterminal board20 accommodate contact with thepins25 and26 of the transistor header. Thus,circuit boards20 and30 interface with the sensor chip enabling one to contact each of the terminals of the Wheatstone bridge array which is located on the sensor chip. All the terminals which emanate from the sensor are completely surrounded and enclosed and make no contact with the oxygen environment. This therefore provides a sensor structure which is completely isolated from the environment and if any spark occurs it cannot be directed to the oxygen environment because of the way the sensor is mounted and isolated in the assembly. It is seen that the conductive epoxy terminals as17 are all surrounded by theboard20 and30 while the glass metal frit contacts as16 associated with the sensor are all surrounded by the glass contact wafer of the sensor. It is thus seen that the entire sensor and all electrical carrying leads are totally isolated in the configuration and contact is made as described above via thecircuit boards20 and30 depicted respectively inFIG. 5 andFIG. 6.
• Referring toFIG. 7 there is shown a typical assembly procedure for fabricating the above-noted device.FIG. 7A depicts the first step in the process where atransistor header21 having terminal pins25 and26 is first shown. The transistor header as indicated above may be a TO5 header or a TO8 header or any transistor header. While twopins25 and26 are shown it is noted that there are at least four pins associated with a typical transistor header. As indicated the pins as25 and26 will eventually contact the terminal pads of thesemiconductor sensor wafer24. Referring toFIG. 7B there is shown a second step in the process where theboard20 is now mounted to the transistor header by means of an epoxy bond. Also shown in this second step are conductive epoxy areas as90 and91. Theconductive epoxy areas90 and91 are employed to make contact with the hand mirror likeconductive areas72 and73 (FIG. 5A). Therefore, the conductive areas assure that terminal pins25 and26 for example are all conductively connected via theaperture70 to thecircuit board20.FIG. 7C depicts a third step in the process, where thecircuit board30 havingapertures81 and85 is placed upon thecircuit board20. Thecircuit board30 is again mounted oncircuit board20 with a suitable epoxy. Theapertures81 and85 are shown.FIG. 7D depicts a fourth step in the process. In step4,apertures81 and85 associated withcircuit board30 are filled with aconductive epoxy17. Thesensor24 is placed thereon having its apertures filled with aglass metal frit16. Theglass metal frit16 makes contact with theconductive epoxy17 which in turn makes contact with the extendingarms76,77 associated with the hand mirror liketerminal pads72,73,74 and75 ofboard20, thereby enabling the four terminals of the Wheatstone bridge on the sensor wafer to be connected to an associated terminal pin as25 and26 of thetransistor header21. Step5 shows the final assembly where thecan18 is now welded to the extendingflange35 of thetransistor header21 via aweld22. As one can ascertain,FIG. 7E shows the final transducerstructure depicting boards20 and30 and depicting thepressure port11 associated with the tubular portion of the metal can18.
• Referring toFIG. 8 there is shown a simple circuit diagram depicting one use of an oxygen pressure sensor according to an embodiment of this invention. As seen inFIG. 1reference numeral100 depicts a Wheatstonebridge having piezoresistors101,102,103 and104. This Wheatstone bridge array is the same array as would be fabricated onsensor chip24 shown inFIG. 1A andFIG. 2. As indicated, each of the sensors as101,102,103 and104 is a piezoresistive sensor and located on thesemiconductor wafer24. The piezoresistive sensors may be p-type monocrystalline silicon piezoresistors for example. The Wheatstone bridge array as shown inFIG. 8 is the typical array which is found on the sensor wafer as24 ofFIG. 1. The terminals as shown for example asterminals120,121,122 and123 are the four terminals of the Wheatstone bridge which eventually connect and are directed to the terminal pins of the transistor header. The output of the Wheatstone bridge is directed to anoperational amplifier105 which produces an amplified version of the voltage. When a pressure (P) is applied to the sensor array, the sensors which are positioned on the active area of the diaphragm change resistance as the diaphragm deflects. In this manner, the output voltage is a direct indication of the magnitude of pressure. For an oxygen pressure sensor the output of operational amplifier is directed to a high pressure detector/monitor module107 and alow module108. Each module may be associated with atimer106. The function ofmodules107 and108 is to determine whether the pressure as applied to the Wheatstone bridge is within desirable limits. If the pressure is too low, the pressure monitor108 will recognize the low voltage and if it maintains low for a predetermined period as determined bytimer106 analarm110 will be sounded. Thealarm110 may be a visual alarm such as a light or other indicator and may be located in the cockpit of the plane or at some other remote location. In a similar manner, if the pressure is too high, this will be detected by thehigh pressure detector107 and if the pressure remains too high for a predetermined period determined bytimer106 then ahigh pressure alarm109 will be activated. It is understood that the monitoring of the Wheatstone bridge can be implemented by means of a microprocessor and a suitable program can be written regarding the same.FIG. 8 is merely included to show that thepressure sensor100 which is monitoring the pressure of oxygen in an environment is associated normally with circuitry to determine whether or not the pressure operates within suitable limits. The circuit ofFIG. 8 is merely by way of an example and not intended to be limiting in any respect. It is also again indicated that the oxygen pressure sensor as described above is utilized in thecircuit100 and essentially the main aspect and purpose of the sensor is to assure that there can be no electrical contact with the oxygen environment. Therefore, any current carrying lead or any electrical arc will not be propagated to the oxygen environment because of the structure of the pressure transducer. Again, as shown, one has provided a sensor for measuring pressure in an oxygen environment where all active or current/voltage carrying terminals are completely isolated from the oxygen environment by means of insulating materials as ceramic, Kapton or insulating epoxy. These materials as employed such as Kapton, ceramic and the epoxies are highly insulating materials and therefore the danger of any electrical contact being made with the oxygen environment is totally minimized. It is also seen that the sensor is simple to construct and extremely reliable.
• Thus, it should be apparent to one skilled in the art that there may be alternative configurations and alternate embodiments which may be employed. These include the substitution of different materials. For example the use of glass or glass seals in lieu of epoxy and use of other conductive materials in lieu of conductive epoxy or glass metal conductive frit-like materials are envisioned.
• All such modifications and alterations should be apparent to those skilled in the art and all such configurations are deemed to be encompassed within the spirit and scope of the claims appended hereto.

Claims (10)

1. A pressure transducer, comprising:

a header having a top and bottom surface and having a plurality of terminal pins extending from said top surface to said bottom surface,

a first circuit board having a top and bottom surface, with said bottom surface positioned on said top surface of said header, said first board having one aperture for each of said terminal pins located on said top surface with said associated aperture surrounding said associated pin, each of said apertures surrounded by a contact area with each contact area having an extending conductive arm directed toward the center of said board,

a second circuit board having a top and bottom surface, with said bottom surface positioned on the top surface of said first board, said second board having a plurality of apertures located about the center of said second board and with each aperture positioned to overlie an associated conductive arm of said first board,

a pressure sensor having an array of pressure responsive devices located thereon and having a top surface for receiving a pressure and a bottom surface positioned on said second board about the center thereof, said bottom surface having terminal pads associated with said array with each pad overlying an associated aperture of said second board,

a conductive material deposited in said apertures of said second board to enable conductive contact to said terminal pads of said sensor said conductive material in each aperture contacting said associated extending conductive arm to thereby electrically connect each sensor pad to each of said header terminal pins.

2. The pressure transducer according toclaim 1, further including a tubular member having a top and a bottom opening, with said bottom opening positioned about the periphery of said header, with said top opening serving as a pressure port to enable a pressure to be monitored to be applied to said pressure sensor.

3. The pressure transducer according toclaim 1, wherein said header is a transistor header selected as TO5 or TO8 header.

4. The pressure transducer according toclaim 1, wherein said first circuit board is fabricated from an insulating material, with said first circuit board secured to said top surface of said header by an epoxy bond.

5. The pressure transducer according toclaim 1, wherein said second circuit board is fabricated from an insulating material, with said second circuit board secured to said first circuit board by an epoxy bond.

6. The pressure transducer according toclaim 2, wherein said tubular member has a top narrow tubular section extending to a larger bottom tubular section, with said bottom tubular section secured at the periphery to said header.

7. The pressure transducer according toclaim 1, wherein said pressure sensor is a semiconductor pressure sensor chip having four piezoresistive elements mounted on an active area of said sensor and arranged in a Wheatstone bridge array with each terminal of said bridge directed to an associated terminal pad of said sensor, said sensor having said piezoresistors dielectrically isolated with said chip being secured to said second circuit board by an epoxy bond.

8. The pressure transducer according toclaim 7, wherein said piezoresistive elements are p-type monocrystalline silicon piezoresistors.

9. The pressure transducer according toclaim 1, wherein said conductive material is a conductive epoxy.

10. The pressure transducer according toclaim 1, wherein said pressure to be monitored is the pressure provided by a source of oxygen.

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