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Diamond as an active sensor material
Diamond and Related Materials 8 (1999) 1741–1747 www.elsevier.com/locate/diamond
Diamond as an active sensor material J.L. Davidson a, *, W.P. Kang a, Y. Gurbuz a, K.C. Holmes b, L.G. Davis b, A. Wisitsora-at a, D.V. Kerns a, R.L. Eidson c, T. Henderson c a ECE Department, Vanderbilt University, Nashville, TN 37235, USA b MSE Department, Vanderbilt University, Nashville, TN 37235, USA c Physitron, Inc., Huntsville, AL 35805, USA Accepted 30 November 1998
Abstract Diamond has attractive properties as an advanced electronic material. Its combination of high carrier mobility, electric breakdown, and thermal conductivity results in the largest calculated figures of merit for speed and power of any material. Previously (J.L. Davidson, W.P. Kang, Examples of diamond sensing applications, Proceedings 3rd International Symposium on Diamond Film (ISDF-3), Polytechnical Institute of Russian Academy of Science, St. Petersburg, Russia, 16–19 June 1996) we reported the discovery and development of useful ‘secondary’ effects in diamond and applying them to interesting sensor applications. For example, boron-doped diamond piezoresistors for strain micro-gauges on rugged MEMS (microelectromechanical structures) pressure and acceleration sensors. This paper will present some recent developments with chemically vapor-deposited diamond for microelectromechanical sensing applications such as a new design all diamond pressure microsensor that measures pressure at high temperatures and an accelerometer with over 45 kHz resonant frequency. Also, presented are recent results on layered diamond films that behave as chemical sensors measuring hydrogen, oxygen and many other chemicals’ concentration. For example, a diamond-based chemical gas sensor using Pt/SnO /i-diamond/p+-diamond metal–insulator–semiconductor diode x structure for oxygen sensing is described. In addition, the latest emission properties of fabricated diamond microtips for field emitters are reviewed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Applications; Diamond films; Electrical properties; Micromachining
1. Introduction Following is an introductory overview of the diamond examples that are the subject of this paper. 1.1. Microelectromechanical piezoresistance (PZR) -based effects and microdevices Boron-doped diamond will change electrical resistance with strain (piezoresistance), meaning it can be used as a strain gauge on rugged electronic microsensors for pressure and acceleration sensing. We have studied the PZR effect in p-doped diamond [1] and built an all diamond pressure microsensor [2] that measures pressure at more than 300°C. Diamond deposition processing and silicon photolithographic and etching techniques are used to create undoped diamond diaphragms a few * Corresponding author. Fax: +1 6153436614. E-mail address: [email protected] (J.L. Davidson)
millimeters or less in diameter and 10–20 mm thick. Delineated and electrically isolated doped diamond resistors nominally 50 mm wide with metal interconnects are fabricated on top of the diaphragm. Zero strain values of the resistors are nominally a few hundred to a few thousand ohms. The isolation ratio is greater than 105. As the membrane is flexed by pressure, the DR/R piezoresistance (PZR) of the diamond resistors is measured. Various PZR configurations and temperature behavior are examined. 1.2. Thin film diamond diode chemical/gas sensors We have observed that layered diamond films can behave as chemical sensors measuring hydrogen, oxygen and many other chemicals’ concentration [3]. For example, a diamond-based chemical gas sensor using Pd/i-diamond/p+-diamond metal–insulator–semiconductor diode structure was made and the hydrogen sensing characteristics investigated as a function of
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hydrogen partial pressure and temperature. The hydrogen sensitivity was found to be large, repeatable and reproducible. A diamond-based chemical gas sensor using Pt/SnO /i-diamond/p+-diamond metal–insulator– x semiconductor diode structure for oxygen sensing will be described in this paper. The adsorption/desorption of oxygen gas by the sensor causes large changes in its current–voltage (I–V ) characteristics. The operating principle and the sensing mechanism are discussed. Analysis of the steady state reaction kinetics confirm that the sensing mechanism is attributable to barrier modulation effects on the diamond diode from oxygen adsorption/desorption, further demonstrating that diamond-based gas detectors with better performance and temperature tolerance than those based on conventional semiconductor technology are feasible. 1.3. Micro-patterned diamond field emitter vacuum diode arrays Electron field emission from arrays of patterned pyramids of polycrystalline diamond for vacuum diode applications is being investigated. High current emission from the patterned diamond microtip arrays is obtained at low electric fields. An emission current from a diamond microtip of several micro-amps was observed for a field of <10 V mm−1. Field emission for these diamond microtips exhibits significant enhancement in total emission current compared to emission from Si, Ge, GaAs, and metal surfaces. The fabrication process utilizes selective deposition and molding of polycrystalline diamond film in a silicon cavity mold and subsequent creation of a free standing polycrystalline diamond diaphragm with diamond pyramidal microtip array. The effect of doping, tip sharpening and sp2 content on the electron emission characteristics are investigated. The possibility of using the emission microstructures in MEMS type configurations where the tip to anode spacing varies due to, e.g. force is being explored for sensing. Flexing of a free standing anode due to pressure or acceleration modulates the cathode–anode distance thereby changes the emission characteristics. Measurable change in the emission current can be correlated to the change in pressure or acceleration. The emission effect is relatively insensitive to temperature and radiation and the emission current is very sensitive to the cathode to anode spacing, providing an interesting ‘microplatform’ for pressure sensors, accelerometers and other similar sensors.
2. Discussion and results 2.1. Microelectromechanical piezoresistance (PZR) -based effects and microdevices Chemically vapor-deposited diamond films, processed similar to conventional semiconductor devices, are used
to fabricate microelectromechanical systems (MEMS). Diamond MEMS (DMEMS) derive response from the piezoresistive (PZR) property of doped diamond [4,5]. The operation of these types of DMEMS is based on the principle that there is a thin diamond diaphragm on which doped diamond resistors are fabricated. When the diaphragm is flexed, the resistors undergo a change in resistance in response to the applied stress. The change in resistance in a resistor bridge configuration provides a corresponding voltage change that can be modeled and calibrated to measure pressure or acceleration. A diamond pressure sensor with specific performance goals was required. The goals for the mechanical design and operation of the diamond pressure sensor (DPS ) were: resonant frequency above 200 kHz, capability of operating in a thermal environment >600°C, measuring pressure variations of ~0.002 psi while surviving at 15.0 psi pressure levels, and fit within a 0.1 in (2.54 mm) diameter envelope. For the first time, a sensor performance goal was used to design a diamond device. Furthermore, initial failure stress data were obtained from pressure burst tests on test samples [6 ]. These samples exhibited a significant amount of geometric non-linear response in analytical evaluations. Using linear analysis techniques, the diamond failure stress was predicted from the rupture pressure to be in the (1–5)×106 psi range. However, using non-linear methods on the same data, the average failure stress calculated for eight samples was 1.23×105 psi. Other data [7], for twelve samples with diamond diaphragms possessing linear behavior under burst pressures, indicated a failure stress of 1.45×105 psi. Natural diamond was reported to have a measured tensile strength of 5.0×105 psi [8] and a theoretical tensile strength of 1.6×107 psi. From these design requirements and limit estimates, a conceptual DPS was defined as presented in Fig. 1. The 2.5 mm dimension is the breadth of an entire single sensor. The diaphragm diameter, the circle, is 1.5 mm. For the intended performance, the thickness of the diamond is 13.9 mm. The diaphragm has a natural frequency of 210 kHz and reaches a stress of 100 000 psi in the diamond at a transient pressure level of 23.6 psi. The B dimension (0.5 mm) was dictated by packaging restrictions. The A dimension (0.15 mm), a maximum resistor length of 20% of the membrane radius (0– 0.75 mm), will be discussed next. The sensing mechanism of the DPS is dependent on the strain induced at the surfaces of the diaphragm where the strain produces changes in PZR electrical resistivity. Doped diamond semiconductor’s piezoresistive properties can be used as a strain gage similar to silicon resistors on a silicon pressure sensor. Hence, the electrical design of the DPS consists of four resistors in a Wheatstone bridge arrangement with two resistors
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Fig. 2. Linear normalized strain versus normalized radius. Fig. 1. DPS conceptual design ( light grey=resistor, dark grey=metal, circle=diaphragm).
located on the membrane and two resistors on the area surrounding the membrane. Resistors on the membrane are strained as a result of flexing, while the off-board resistors are not strained and do not change value. This causes an imbalance in the bridge that produces a voltage output change for the sensor signal. The sensor configuration shown in Fig. 1 uses radial sensing elements near the supported edge of the diaphragm. Pressure is applied from the cavity side of the diamond membrane. Static analyses were performed to determine the induced strain in the diaphragm. For design analysis, the transient pressure level was assumed to be one-half the static pressure level to account for the transient dynamic nature of the applied loading. Examining the peak tensile radial strain in the diamond membrane as a function of pressure indicates that the strain level is nearly linear, even up to a static pressure of 50 psi. Since the presented strains are located at the boundary, and the radial stress is the primary stress in this area, stress and strain are related through the modulus of elasticity (1.65×108 psi). The radial strain varies along the radius of the diaphragm. In the linear range, a normalized distribution of strain, both radial and tangential, is presented in Fig. 2. It is observed that radial sensing elements located in the last 20% of the radius will optimize signals in the linear or near linear response regime. However, some geometric nonlinearity of response is expected. The failure stress for diamond is well above 100 ksi, possibly more than 200 ksi depending on film quality. Failure stress levels determined from experimentation have indicated average values in the 140–160 ksi range. This pressure sensor is expected to operate above 600°C. Heat generated from operating the device with biases as high as 100 V should have little or no effect on its operation. The resistors are made of diamond, as
is the membrane. The diamond resistors will be sized and doped to give adequate resistance at high temperatures. The predicted output voltage for a signal of 1 psi with a 10 V bias is on the order of 10 V, for 0.01 psi it is 0.1 V. The fabrication process begins with a clean IC grade 3 in silicon wafer (typically p-type, lightly doped, with
100 orientation and resistivity of 30–100 V cm). Intrinsic diamond (i-diamond) is grown (using chemical vapor deposition (CVD) microwave plasma-assisted deposition) to a thickness of about 14 mm. The gases used for this deposition are hydrogen and methane, typically at 10:1 H :CH volume ratio. Boron-doped 2 4 diamond (p-diamond) is deposited directly on the i-diamond to a thickness of 3–5 mm. To achieve p-typedoped diamond (p-diamond ) the diamond film is doped in situ using a solid source boron compound wafer. At temperatures between 700 and 1000°C, boron outgasses and is incorporated in the diamond as it is deposited. The resistivity is controlled by the doping level during the deposition that is varied in magnitude from light to heavy, 1016 to 1020 B cm−3 respectively. To minimize potential alignment obstacles, the diamond membrane is created next. The mask layer is sputtered on the backside and patterned with the membrane mask pattern. The silicon is etched with a vapor reactive ion etch, creating a cylindrical hole and a circular diamond membrane. Fig. 3 is a scanning electron micrograph from the backside of a partially etched sensor with a 1.5 mm diameter opening. Resistor delineation proceeds by coating and patterning the front side of the diamond with an etch protective layer such as patterned spin on glass. The doped diamond layer is then reactively ion etched for a time sufficient to etch completely through the p-diamond but not the i-diamond, thereby achieving doped diamond resistors isolated on an intrinsic diamond membrane and the off-membrane reference doped diamond
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Fig. 3. Scanning electron micrograph of etched diamond pressure sensor membrane cavity.
As of this writing, these fabrication developments suggest that microelectronic technology can be applied to diamond films, resulting in rugged diamond MEMS devices (such as pressure sensors) with the potential for extended performance. The mechanical and electrical design analyses of the diamond pressure sensors indicate resonant frequency in the 200 kHz range with survivability pressure limits in the 46 psi range. Previous research [9] has indicated the capability to achieve gage factors in the 100 range even at elevated temperatures. Judicious selection of resistor doping levels and lengths can provide high gage factors and proper total resistance to allow operation of the pressure sensor at high temperature. The electrical analysis indicates that sufficient signals will be available to measure very small pressure variations. The complete fabrication of these sensors is in process at this time and those results are the subject of a future paper. 2.2. Thin film diamond diode chemical/gas sensors, a diamond-based diode sensor for the detection of oxidizing and reducing gases
Fig. 4. Accelerometer layout configuration (element shading as in Fig. 2).
resistors. Metallization follows to connect resistors in the bridge configuration and achieve bonding pads. Metal systems used include aluminum and Ti/Au for high temperature performance. The wafers are then probed, die scribed, good die attached to a package with a pre-existing hole and wire bonded for testing, per conventional silicon assembly techniques. In a similar manner, a diamond accelerometer (DAS, diamond accelerometer sensor) is in preparation. The mechanical analysis, design layout, and process steps were considered as a whole and the final design is shown in Fig. 4. For this device, the membrane diameter is 4 mm and its thickness is 23 mm, which should result in a resonant frequency of 200 MHz. It is configured to operate at a high vibration frequency and nominal ambient temperatures. The device configuration is again a bridge configuration. The resistors are placed as close to the edge of the membrane as is feasible. The closer the resistors are to the edge the larger the average strain. The resistors are placed within 100 mm of the edge allowing 50 mm tolerance in alignment of resistors to membrane and for 50 mm of etch tolerance.
The sensor structure consisted of a Pt/SnO / x i-diamond/p+-diamond CAIS (catalyst/adsorptiveoxide/insulator/semiconductor) multilayer on a variety of supporting substrates such as tungsten, molybdenum, and p++-silicon (Fig. 5). Major advantages of using diamond-based structures with an adsorptive-oxide electrode such as SnO for gas sensing are higher operatx ing temperature range, reliable sensing performance in harsh environments, simplicity in fabrication process, flexibility in the choice of substrate, and compatibility with silicon microfabrication technology. The use of the adsorptive oxide (SnO ) in conjunction with a catalyst x (Pt) enables the sensor to detect oxidizing and reducing gases (such as oxygen, carbon monoxide, hydrogen, and hydrocarbons) at elevated temperatures with a lower activation energy. It also enhances the sensor performance such as reproducibility, stability, selectivity and sensitivity over a wide temperature range. Fig. 6 shows the I–V characteristics of the device, operated at 300°C, in air ambient and the subsequent change in the I–V characteristics upon exposure to
Fig. 5. Diamond-based CAIS sensor.
J.L. Davidson et al. / Diamond and Related Materials 8 (1999) 1741–1747
Fig. 6. Device current in O (open air), CO and hydrogen gases. 2
Fig. 7. Sensitivity, oxygen partial pressure, and temperature.
carbon monoxide (CO), and hydrogen gases (flow rate, 10 ml min−1). As seen in the figure, the reduction reaction between pre-adsorbed oxygen and carbon monoxide or hydrogen in the device leads to an increase in device current. The I–V characteristic of the device goes back to its value in air upon turning carbon monoxide and hydrogen gases off. The gas sensitivity of a sensor can be measured in terms of the change in device current at a fixed voltage and constant temperature versus gas partial pressure at different temperatures. Fig. 7 shows the change in the CAIS device current, DI, versus partial pressure of oxygen at T=205, 240, and 280°C for V=−3 V. The negative direction of the curves indicates a decrease in device current upon oxygen adsorption. The curves shows a rapid increase in DI at low oxygen concentrations, followed by a saturation trend at high oxygen concentrations. Also observed is the increase in oxygen sensitivity of the device with increasing temperature. The changes in device current, DI, are −0.4, −1.8, and −4.08 mA, for a fixed (10 Torr) oxygen concentration at T=205, 240, and 280°C, respectively.
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Fig. 8. Repeatability, reproducibility, and fast response.
Typical repeatability and reproducibility test of the CAIS sensor for oxygen (air), carbon monoxide and hydrogen gases at T=300°C is shown in Fig. 8. The same transient behavior of the device is observed over several consecutive pulses of gas exposure, indicating the response of the device to oxygen (air), carbon monoxide, and hydrogen gases is repeatable and reproducible. The device also shows a response and recovery time in seconds, revealing that the response is fast and highly sensitive at T=300°C. The oxygen detection mechanisms of the diamondbased CAIS device have been found [10] to be due to the modification of the oxygen vacancies in the SnO x film upon adsorption of oxygen gas. When oxygen gas is introduced from the atmosphere at high temperatures, atomic oxygen can diffuse into the bulk from the surface, leading to a decrease in the number of oxygen vacancies in SnO . Since the conductivity of SnO arises from the x x oxygen vacancies, the decrease of oxygen vacancies also decreases the conductivity. This modifies the voltage distribution across SnO /i-diamond junction, leading to x the decrease of the CAIS device current. The CO and hydrogen detection mechanisms of the diamond-based CAIS device are attributed to the reduction reaction between oxygen and CO or hydrogen, leading to the decrease of oxygen atoms in SnO . Increasing the oxygen x vacancies will also increase the conductivity of SnO , x causing restoration of the voltage distribution across the SnO /i-diamond structure. This will, in turn, cause x the current of the CAIS device to increase in the presence of CO or hydrogen gas. The objective of this study was to utilize the advantages of both microelectronic and resistive-type gas sensors in a single structure by using CVD diamond technology. We achieved sensitivity over a temperature range of 23–500°C, which can be further extended up to 650°C (or possibly to 1000°C with an appropriate passivation of CVD diamond). The resistive-type gas
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sensors, such as a stand-alone thin/thick film of SnO , x are not sensitive at temperatures below 300°C while silicon- and GaAs-based microelectronic gas sensors are sensitive only up to 150 and 200°C, respectively. In this effort we combined the operating temperature ranges of resistive-type and microelectronic gas sensors in a single microstructure, the diamond-based CAIS gas sensor. While diamond is achieving a high temperature tolerance, the microelectronic configuration of the structure (diode in this case) enables the detection of very small changes occurring in/on the SnO film via the exponenx tial relation between current and voltage of the device. The enhanced gas sensing performance coupled with the wider operating temperature range of diamond-based CAIS gas sensors can have advantage over, for example, Si- or GaAs-based microelectronic gas sensors for use in many applications requiring high sensitivity, fast response and recovery, wider operating temperature range, harsh operating environment, durability and dependability.
Fig. 10. The effect of doping on Fowler–Nordhiem (F–N ) plot of diamond tips with different sp2 content.
2.3. Micro-patterned diamond field emitter vacuum diode arrays, efficient electron emitter utilizing boron-doped, sp2 content diamond tips Diamond has recently emerged as a promising material for electron field emission applications. This section describes a practical technique to enhance the electron emission of diamond by incorporation of boron and sp2 content in the diamond tips. The effects of boron doping on electron field emission from an array of micro-patterned polycrystalline pyramidal diamond microtips ( Fig. 9) with varying sp2 content have been systematically studied. The boron doping concentration is approximately 1019 cm−3. The field emission characteristics of undoped and boron-doped diamond tips (Fig. 10) are significantly improved by increasing sp2
Fig. 9. Scanning electron micrograph of a diamond tip.
Fig. 11. The effect of doping on I–E plot of diamond tips with different sp2 contents.
content of diamond tips. By increasing sp2 content, the turn-on electric field can be reduced more than 50% for both undoped and boron-doped diamond tips. Furthermore, the turn-on electric field of the sp2-containing diamond tips decreases with boron doping. The effects of boron doping and sp2 content on the field emission characteristics were examined using F–N analysis. The F–N plots of low and high sp2 content diamond tips with different doping ( Fig. 11) show that the F–N slope of low sp2 content diamond tips is steeper with boron doping, but the F–N slope of high sp2 content diamond tips is shallower with boron doping. The time dependence of the emission current ( Fig. 12) shows stable emission behavior with small current fluctuations of less than 10%. The enhanced field emission characteristic of the boron-doped diamond tip with high sp2 content is significantly better than those reported for planar diamond and silicon emitters in terms of emission current, operating voltage, and
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their potential to become rugged sensitive inertial type sensors. We are exploring MEMS configurations, for example, where the arrays are placed on or face a flexing membrane or microbeam, subject to acceleration forces, or changes in pressure, that alter the spacing between tip and anode and provide sensitive and band-gapindependent operation (e.g. high temperature or radiation situations). Specifics of these devices and their response is the subject of a future paper.
References
Fig. 12. Emission current stability versus time.
stability. We have found [11] the contributing factor for the improved emission characteristics of the diamond tips to be directly related to the overall field enhancement factor b, which is the product of the following field enhancement components. In this diamond tip design b=b b b b , where b , b , b , b are field enhanceg sp2 t p g sp2 t p ment factors due to the tips’ geometry, sp2 content, surface treatment, and boron doping respectively. Isolating and optimizing these collective parameters, such as b of a diamond tip results in continuing p, increases in emission efficiency. A new field emission mechanism is proposed [11] to explain the effects of boron doping and sp2 content on the enhanced field emission characteristics of diamond tips. The enhanced emission behavior is attributed to an increase in the field enhancement factor due to hole accumulation and the formation of cascading sp2–diamond–sp2 embedded microstructures in diamond tips. We are examining and attempting to enhance emission from these field emitter arrays partly because of
[1] D.R. Wur, J.L. Davidson, W.P. Kang, D.L. Kinser, Third International Symposium on Diamond Materials, 183rd Meeting of the Electrochemical Society, Honolulu, Hawaii, May, 1993. [2] D. Wur, J.L. Davidson, W.P. Kang, A polycrystalline diamond film piezoresistive microsensor, Transducers 1993, Yokohoma, Japan, July, 1993. [3] Y. Gurbuz, W.P. Kang, J.L. Davidson, D.L. Kinser, D.V. Kerns, Sensors Actuators B (1996) 100–104. [4] D. Wur, J.L. Davidson, W.P. Kang, D.V. Kerns, Proceedings of International Applied Diamond Conference ’95, NIST, MD, August, 1995. [5] M. Deguchi, N. Hase, M. Kitabatake, H. Kotera, S. Shima, M. Kitagawa, Diamond Relat. Mater. 6 (1997) 367. [6 ] J.L. Davidson, W.P. Kang, L. Davis, K. Holmes, T.G. Henderson, R.L. Eidson, M. Howell, D.V. Kerns, ECS Joint International Meeting, September, Paris, France, 1997. [7] M.P. D’Evelyn, K. Zgonc, Diamond Relat. Mater. 6 (1997) 812. [8] D.M. Jassowski, Report AL-TR-89-044 from Aerojet Techsystems to Air Force Astronautics Lab., November, 1989. [9] D.R. Wur, J.L. Davidson, W.P. Kang, Polycrystalline diamond pressure sensor, J. Microelectromech. Syst. 4 (1) (1995) 34–41. [10] Y. Gurbuz, W.P. Kang, J.L. Davidson, D.V. Kerns, A novel high temperature diode for oxygen gas detection, Third International High Temperature Electronics Conference, Albuquerque, NM, 9–14 June, 1996. [11] W.P. Kang, A. Wisitsora-at, J.L. Davidson, Q. Li, J.F. Xu, D.V. Kerns, The effects of sp2 content and surface treatment on micropatterned pyramidal diamond tips, in: Technical Digest of International Vacuum Microelectronics Conference, Kyungju, South Korea, 17–21 August, 1997, pp. 107–111.
Diamond as an active sensor material J.L. Davidson a, *, W.P. Kang a, Y. Gurbuz a, K.C. Holmes b, L.G. Davis b, A. Wisitsora-at a, D.V. Kerns a, R.L. Eidson c, T. Henderson c a ECE Department, Vanderbilt University, Nashville, TN 37235, USA b MSE Department, Vanderbilt University, Nashville, TN 37235, USA c Physitron, Inc., Huntsville, AL 35805, USA Accepted 30 November 1998
Abstract Diamond has attractive properties as an advanced electronic material. Its combination of high carrier mobility, electric breakdown, and thermal conductivity results in the largest calculated figures of merit for speed and power of any material. Previously (J.L. Davidson, W.P. Kang, Examples of diamond sensing applications, Proceedings 3rd International Symposium on Diamond Film (ISDF-3), Polytechnical Institute of Russian Academy of Science, St. Petersburg, Russia, 16–19 June 1996) we reported the discovery and development of useful ‘secondary’ effects in diamond and applying them to interesting sensor applications. For example, boron-doped diamond piezoresistors for strain micro-gauges on rugged MEMS (microelectromechanical structures) pressure and acceleration sensors. This paper will present some recent developments with chemically vapor-deposited diamond for microelectromechanical sensing applications such as a new design all diamond pressure microsensor that measures pressure at high temperatures and an accelerometer with over 45 kHz resonant frequency. Also, presented are recent results on layered diamond films that behave as chemical sensors measuring hydrogen, oxygen and many other chemicals’ concentration. For example, a diamond-based chemical gas sensor using Pt/SnO /i-diamond/p+-diamond metal–insulator–semiconductor diode x structure for oxygen sensing is described. In addition, the latest emission properties of fabricated diamond microtips for field emitters are reviewed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Applications; Diamond films; Electrical properties; Micromachining
1. Introduction Following is an introductory overview of the diamond examples that are the subject of this paper. 1.1. Microelectromechanical piezoresistance (PZR) -based effects and microdevices Boron-doped diamond will change electrical resistance with strain (piezoresistance), meaning it can be used as a strain gauge on rugged electronic microsensors for pressure and acceleration sensing. We have studied the PZR effect in p-doped diamond [1] and built an all diamond pressure microsensor [2] that measures pressure at more than 300°C. Diamond deposition processing and silicon photolithographic and etching techniques are used to create undoped diamond diaphragms a few * Corresponding author. Fax: +1 6153436614. E-mail address: [email protected] (J.L. Davidson)
millimeters or less in diameter and 10–20 mm thick. Delineated and electrically isolated doped diamond resistors nominally 50 mm wide with metal interconnects are fabricated on top of the diaphragm. Zero strain values of the resistors are nominally a few hundred to a few thousand ohms. The isolation ratio is greater than 105. As the membrane is flexed by pressure, the DR/R piezoresistance (PZR) of the diamond resistors is measured. Various PZR configurations and temperature behavior are examined. 1.2. Thin film diamond diode chemical/gas sensors We have observed that layered diamond films can behave as chemical sensors measuring hydrogen, oxygen and many other chemicals’ concentration [3]. For example, a diamond-based chemical gas sensor using Pd/i-diamond/p+-diamond metal–insulator–semiconductor diode structure was made and the hydrogen sensing characteristics investigated as a function of
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hydrogen partial pressure and temperature. The hydrogen sensitivity was found to be large, repeatable and reproducible. A diamond-based chemical gas sensor using Pt/SnO /i-diamond/p+-diamond metal–insulator– x semiconductor diode structure for oxygen sensing will be described in this paper. The adsorption/desorption of oxygen gas by the sensor causes large changes in its current–voltage (I–V ) characteristics. The operating principle and the sensing mechanism are discussed. Analysis of the steady state reaction kinetics confirm that the sensing mechanism is attributable to barrier modulation effects on the diamond diode from oxygen adsorption/desorption, further demonstrating that diamond-based gas detectors with better performance and temperature tolerance than those based on conventional semiconductor technology are feasible. 1.3. Micro-patterned diamond field emitter vacuum diode arrays Electron field emission from arrays of patterned pyramids of polycrystalline diamond for vacuum diode applications is being investigated. High current emission from the patterned diamond microtip arrays is obtained at low electric fields. An emission current from a diamond microtip of several micro-amps was observed for a field of <10 V mm−1. Field emission for these diamond microtips exhibits significant enhancement in total emission current compared to emission from Si, Ge, GaAs, and metal surfaces. The fabrication process utilizes selective deposition and molding of polycrystalline diamond film in a silicon cavity mold and subsequent creation of a free standing polycrystalline diamond diaphragm with diamond pyramidal microtip array. The effect of doping, tip sharpening and sp2 content on the electron emission characteristics are investigated. The possibility of using the emission microstructures in MEMS type configurations where the tip to anode spacing varies due to, e.g. force is being explored for sensing. Flexing of a free standing anode due to pressure or acceleration modulates the cathode–anode distance thereby changes the emission characteristics. Measurable change in the emission current can be correlated to the change in pressure or acceleration. The emission effect is relatively insensitive to temperature and radiation and the emission current is very sensitive to the cathode to anode spacing, providing an interesting ‘microplatform’ for pressure sensors, accelerometers and other similar sensors.
2. Discussion and results 2.1. Microelectromechanical piezoresistance (PZR) -based effects and microdevices Chemically vapor-deposited diamond films, processed similar to conventional semiconductor devices, are used
to fabricate microelectromechanical systems (MEMS). Diamond MEMS (DMEMS) derive response from the piezoresistive (PZR) property of doped diamond [4,5]. The operation of these types of DMEMS is based on the principle that there is a thin diamond diaphragm on which doped diamond resistors are fabricated. When the diaphragm is flexed, the resistors undergo a change in resistance in response to the applied stress. The change in resistance in a resistor bridge configuration provides a corresponding voltage change that can be modeled and calibrated to measure pressure or acceleration. A diamond pressure sensor with specific performance goals was required. The goals for the mechanical design and operation of the diamond pressure sensor (DPS ) were: resonant frequency above 200 kHz, capability of operating in a thermal environment >600°C, measuring pressure variations of ~0.002 psi while surviving at 15.0 psi pressure levels, and fit within a 0.1 in (2.54 mm) diameter envelope. For the first time, a sensor performance goal was used to design a diamond device. Furthermore, initial failure stress data were obtained from pressure burst tests on test samples [6 ]. These samples exhibited a significant amount of geometric non-linear response in analytical evaluations. Using linear analysis techniques, the diamond failure stress was predicted from the rupture pressure to be in the (1–5)×106 psi range. However, using non-linear methods on the same data, the average failure stress calculated for eight samples was 1.23×105 psi. Other data [7], for twelve samples with diamond diaphragms possessing linear behavior under burst pressures, indicated a failure stress of 1.45×105 psi. Natural diamond was reported to have a measured tensile strength of 5.0×105 psi [8] and a theoretical tensile strength of 1.6×107 psi. From these design requirements and limit estimates, a conceptual DPS was defined as presented in Fig. 1. The 2.5 mm dimension is the breadth of an entire single sensor. The diaphragm diameter, the circle, is 1.5 mm. For the intended performance, the thickness of the diamond is 13.9 mm. The diaphragm has a natural frequency of 210 kHz and reaches a stress of 100 000 psi in the diamond at a transient pressure level of 23.6 psi. The B dimension (0.5 mm) was dictated by packaging restrictions. The A dimension (0.15 mm), a maximum resistor length of 20% of the membrane radius (0– 0.75 mm), will be discussed next. The sensing mechanism of the DPS is dependent on the strain induced at the surfaces of the diaphragm where the strain produces changes in PZR electrical resistivity. Doped diamond semiconductor’s piezoresistive properties can be used as a strain gage similar to silicon resistors on a silicon pressure sensor. Hence, the electrical design of the DPS consists of four resistors in a Wheatstone bridge arrangement with two resistors
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Fig. 2. Linear normalized strain versus normalized radius. Fig. 1. DPS conceptual design ( light grey=resistor, dark grey=metal, circle=diaphragm).
located on the membrane and two resistors on the area surrounding the membrane. Resistors on the membrane are strained as a result of flexing, while the off-board resistors are not strained and do not change value. This causes an imbalance in the bridge that produces a voltage output change for the sensor signal. The sensor configuration shown in Fig. 1 uses radial sensing elements near the supported edge of the diaphragm. Pressure is applied from the cavity side of the diamond membrane. Static analyses were performed to determine the induced strain in the diaphragm. For design analysis, the transient pressure level was assumed to be one-half the static pressure level to account for the transient dynamic nature of the applied loading. Examining the peak tensile radial strain in the diamond membrane as a function of pressure indicates that the strain level is nearly linear, even up to a static pressure of 50 psi. Since the presented strains are located at the boundary, and the radial stress is the primary stress in this area, stress and strain are related through the modulus of elasticity (1.65×108 psi). The radial strain varies along the radius of the diaphragm. In the linear range, a normalized distribution of strain, both radial and tangential, is presented in Fig. 2. It is observed that radial sensing elements located in the last 20% of the radius will optimize signals in the linear or near linear response regime. However, some geometric nonlinearity of response is expected. The failure stress for diamond is well above 100 ksi, possibly more than 200 ksi depending on film quality. Failure stress levels determined from experimentation have indicated average values in the 140–160 ksi range. This pressure sensor is expected to operate above 600°C. Heat generated from operating the device with biases as high as 100 V should have little or no effect on its operation. The resistors are made of diamond, as
is the membrane. The diamond resistors will be sized and doped to give adequate resistance at high temperatures. The predicted output voltage for a signal of 1 psi with a 10 V bias is on the order of 10 V, for 0.01 psi it is 0.1 V. The fabrication process begins with a clean IC grade 3 in silicon wafer (typically p-type, lightly doped, with
100 orientation and resistivity of 30–100 V cm). Intrinsic diamond (i-diamond) is grown (using chemical vapor deposition (CVD) microwave plasma-assisted deposition) to a thickness of about 14 mm. The gases used for this deposition are hydrogen and methane, typically at 10:1 H :CH volume ratio. Boron-doped 2 4 diamond (p-diamond) is deposited directly on the i-diamond to a thickness of 3–5 mm. To achieve p-typedoped diamond (p-diamond ) the diamond film is doped in situ using a solid source boron compound wafer. At temperatures between 700 and 1000°C, boron outgasses and is incorporated in the diamond as it is deposited. The resistivity is controlled by the doping level during the deposition that is varied in magnitude from light to heavy, 1016 to 1020 B cm−3 respectively. To minimize potential alignment obstacles, the diamond membrane is created next. The mask layer is sputtered on the backside and patterned with the membrane mask pattern. The silicon is etched with a vapor reactive ion etch, creating a cylindrical hole and a circular diamond membrane. Fig. 3 is a scanning electron micrograph from the backside of a partially etched sensor with a 1.5 mm diameter opening. Resistor delineation proceeds by coating and patterning the front side of the diamond with an etch protective layer such as patterned spin on glass. The doped diamond layer is then reactively ion etched for a time sufficient to etch completely through the p-diamond but not the i-diamond, thereby achieving doped diamond resistors isolated on an intrinsic diamond membrane and the off-membrane reference doped diamond
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Fig. 3. Scanning electron micrograph of etched diamond pressure sensor membrane cavity.
As of this writing, these fabrication developments suggest that microelectronic technology can be applied to diamond films, resulting in rugged diamond MEMS devices (such as pressure sensors) with the potential for extended performance. The mechanical and electrical design analyses of the diamond pressure sensors indicate resonant frequency in the 200 kHz range with survivability pressure limits in the 46 psi range. Previous research [9] has indicated the capability to achieve gage factors in the 100 range even at elevated temperatures. Judicious selection of resistor doping levels and lengths can provide high gage factors and proper total resistance to allow operation of the pressure sensor at high temperature. The electrical analysis indicates that sufficient signals will be available to measure very small pressure variations. The complete fabrication of these sensors is in process at this time and those results are the subject of a future paper. 2.2. Thin film diamond diode chemical/gas sensors, a diamond-based diode sensor for the detection of oxidizing and reducing gases
Fig. 4. Accelerometer layout configuration (element shading as in Fig. 2).
resistors. Metallization follows to connect resistors in the bridge configuration and achieve bonding pads. Metal systems used include aluminum and Ti/Au for high temperature performance. The wafers are then probed, die scribed, good die attached to a package with a pre-existing hole and wire bonded for testing, per conventional silicon assembly techniques. In a similar manner, a diamond accelerometer (DAS, diamond accelerometer sensor) is in preparation. The mechanical analysis, design layout, and process steps were considered as a whole and the final design is shown in Fig. 4. For this device, the membrane diameter is 4 mm and its thickness is 23 mm, which should result in a resonant frequency of 200 MHz. It is configured to operate at a high vibration frequency and nominal ambient temperatures. The device configuration is again a bridge configuration. The resistors are placed as close to the edge of the membrane as is feasible. The closer the resistors are to the edge the larger the average strain. The resistors are placed within 100 mm of the edge allowing 50 mm tolerance in alignment of resistors to membrane and for 50 mm of etch tolerance.
The sensor structure consisted of a Pt/SnO / x i-diamond/p+-diamond CAIS (catalyst/adsorptiveoxide/insulator/semiconductor) multilayer on a variety of supporting substrates such as tungsten, molybdenum, and p++-silicon (Fig. 5). Major advantages of using diamond-based structures with an adsorptive-oxide electrode such as SnO for gas sensing are higher operatx ing temperature range, reliable sensing performance in harsh environments, simplicity in fabrication process, flexibility in the choice of substrate, and compatibility with silicon microfabrication technology. The use of the adsorptive oxide (SnO ) in conjunction with a catalyst x (Pt) enables the sensor to detect oxidizing and reducing gases (such as oxygen, carbon monoxide, hydrogen, and hydrocarbons) at elevated temperatures with a lower activation energy. It also enhances the sensor performance such as reproducibility, stability, selectivity and sensitivity over a wide temperature range. Fig. 6 shows the I–V characteristics of the device, operated at 300°C, in air ambient and the subsequent change in the I–V characteristics upon exposure to
Fig. 5. Diamond-based CAIS sensor.
J.L. Davidson et al. / Diamond and Related Materials 8 (1999) 1741–1747
Fig. 6. Device current in O (open air), CO and hydrogen gases. 2
Fig. 7. Sensitivity, oxygen partial pressure, and temperature.
carbon monoxide (CO), and hydrogen gases (flow rate, 10 ml min−1). As seen in the figure, the reduction reaction between pre-adsorbed oxygen and carbon monoxide or hydrogen in the device leads to an increase in device current. The I–V characteristic of the device goes back to its value in air upon turning carbon monoxide and hydrogen gases off. The gas sensitivity of a sensor can be measured in terms of the change in device current at a fixed voltage and constant temperature versus gas partial pressure at different temperatures. Fig. 7 shows the change in the CAIS device current, DI, versus partial pressure of oxygen at T=205, 240, and 280°C for V=−3 V. The negative direction of the curves indicates a decrease in device current upon oxygen adsorption. The curves shows a rapid increase in DI at low oxygen concentrations, followed by a saturation trend at high oxygen concentrations. Also observed is the increase in oxygen sensitivity of the device with increasing temperature. The changes in device current, DI, are −0.4, −1.8, and −4.08 mA, for a fixed (10 Torr) oxygen concentration at T=205, 240, and 280°C, respectively.
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Fig. 8. Repeatability, reproducibility, and fast response.
Typical repeatability and reproducibility test of the CAIS sensor for oxygen (air), carbon monoxide and hydrogen gases at T=300°C is shown in Fig. 8. The same transient behavior of the device is observed over several consecutive pulses of gas exposure, indicating the response of the device to oxygen (air), carbon monoxide, and hydrogen gases is repeatable and reproducible. The device also shows a response and recovery time in seconds, revealing that the response is fast and highly sensitive at T=300°C. The oxygen detection mechanisms of the diamondbased CAIS device have been found [10] to be due to the modification of the oxygen vacancies in the SnO x film upon adsorption of oxygen gas. When oxygen gas is introduced from the atmosphere at high temperatures, atomic oxygen can diffuse into the bulk from the surface, leading to a decrease in the number of oxygen vacancies in SnO . Since the conductivity of SnO arises from the x x oxygen vacancies, the decrease of oxygen vacancies also decreases the conductivity. This modifies the voltage distribution across SnO /i-diamond junction, leading to x the decrease of the CAIS device current. The CO and hydrogen detection mechanisms of the diamond-based CAIS device are attributed to the reduction reaction between oxygen and CO or hydrogen, leading to the decrease of oxygen atoms in SnO . Increasing the oxygen x vacancies will also increase the conductivity of SnO , x causing restoration of the voltage distribution across the SnO /i-diamond structure. This will, in turn, cause x the current of the CAIS device to increase in the presence of CO or hydrogen gas. The objective of this study was to utilize the advantages of both microelectronic and resistive-type gas sensors in a single structure by using CVD diamond technology. We achieved sensitivity over a temperature range of 23–500°C, which can be further extended up to 650°C (or possibly to 1000°C with an appropriate passivation of CVD diamond). The resistive-type gas
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sensors, such as a stand-alone thin/thick film of SnO , x are not sensitive at temperatures below 300°C while silicon- and GaAs-based microelectronic gas sensors are sensitive only up to 150 and 200°C, respectively. In this effort we combined the operating temperature ranges of resistive-type and microelectronic gas sensors in a single microstructure, the diamond-based CAIS gas sensor. While diamond is achieving a high temperature tolerance, the microelectronic configuration of the structure (diode in this case) enables the detection of very small changes occurring in/on the SnO film via the exponenx tial relation between current and voltage of the device. The enhanced gas sensing performance coupled with the wider operating temperature range of diamond-based CAIS gas sensors can have advantage over, for example, Si- or GaAs-based microelectronic gas sensors for use in many applications requiring high sensitivity, fast response and recovery, wider operating temperature range, harsh operating environment, durability and dependability.
Fig. 10. The effect of doping on Fowler–Nordhiem (F–N ) plot of diamond tips with different sp2 content.
2.3. Micro-patterned diamond field emitter vacuum diode arrays, efficient electron emitter utilizing boron-doped, sp2 content diamond tips Diamond has recently emerged as a promising material for electron field emission applications. This section describes a practical technique to enhance the electron emission of diamond by incorporation of boron and sp2 content in the diamond tips. The effects of boron doping on electron field emission from an array of micro-patterned polycrystalline pyramidal diamond microtips ( Fig. 9) with varying sp2 content have been systematically studied. The boron doping concentration is approximately 1019 cm−3. The field emission characteristics of undoped and boron-doped diamond tips (Fig. 10) are significantly improved by increasing sp2
Fig. 9. Scanning electron micrograph of a diamond tip.
Fig. 11. The effect of doping on I–E plot of diamond tips with different sp2 contents.
content of diamond tips. By increasing sp2 content, the turn-on electric field can be reduced more than 50% for both undoped and boron-doped diamond tips. Furthermore, the turn-on electric field of the sp2-containing diamond tips decreases with boron doping. The effects of boron doping and sp2 content on the field emission characteristics were examined using F–N analysis. The F–N plots of low and high sp2 content diamond tips with different doping ( Fig. 11) show that the F–N slope of low sp2 content diamond tips is steeper with boron doping, but the F–N slope of high sp2 content diamond tips is shallower with boron doping. The time dependence of the emission current ( Fig. 12) shows stable emission behavior with small current fluctuations of less than 10%. The enhanced field emission characteristic of the boron-doped diamond tip with high sp2 content is significantly better than those reported for planar diamond and silicon emitters in terms of emission current, operating voltage, and
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their potential to become rugged sensitive inertial type sensors. We are exploring MEMS configurations, for example, where the arrays are placed on or face a flexing membrane or microbeam, subject to acceleration forces, or changes in pressure, that alter the spacing between tip and anode and provide sensitive and band-gapindependent operation (e.g. high temperature or radiation situations). Specifics of these devices and their response is the subject of a future paper.
References
Fig. 12. Emission current stability versus time.
stability. We have found [11] the contributing factor for the improved emission characteristics of the diamond tips to be directly related to the overall field enhancement factor b, which is the product of the following field enhancement components. In this diamond tip design b=b b b b , where b , b , b , b are field enhanceg sp2 t p g sp2 t p ment factors due to the tips’ geometry, sp2 content, surface treatment, and boron doping respectively. Isolating and optimizing these collective parameters, such as b of a diamond tip results in continuing p, increases in emission efficiency. A new field emission mechanism is proposed [11] to explain the effects of boron doping and sp2 content on the enhanced field emission characteristics of diamond tips. The enhanced emission behavior is attributed to an increase in the field enhancement factor due to hole accumulation and the formation of cascading sp2–diamond–sp2 embedded microstructures in diamond tips. We are examining and attempting to enhance emission from these field emitter arrays partly because of
[1] D.R. Wur, J.L. Davidson, W.P. Kang, D.L. Kinser, Third International Symposium on Diamond Materials, 183rd Meeting of the Electrochemical Society, Honolulu, Hawaii, May, 1993. [2] D. Wur, J.L. Davidson, W.P. Kang, A polycrystalline diamond film piezoresistive microsensor, Transducers 1993, Yokohoma, Japan, July, 1993. [3] Y. Gurbuz, W.P. Kang, J.L. Davidson, D.L. Kinser, D.V. Kerns, Sensors Actuators B (1996) 100–104. [4] D. Wur, J.L. Davidson, W.P. Kang, D.V. Kerns, Proceedings of International Applied Diamond Conference ’95, NIST, MD, August, 1995. [5] M. Deguchi, N. Hase, M. Kitabatake, H. Kotera, S. Shima, M. Kitagawa, Diamond Relat. Mater. 6 (1997) 367. [6 ] J.L. Davidson, W.P. Kang, L. Davis, K. Holmes, T.G. Henderson, R.L. Eidson, M. Howell, D.V. Kerns, ECS Joint International Meeting, September, Paris, France, 1997. [7] M.P. D’Evelyn, K. Zgonc, Diamond Relat. Mater. 6 (1997) 812. [8] D.M. Jassowski, Report AL-TR-89-044 from Aerojet Techsystems to Air Force Astronautics Lab., November, 1989. [9] D.R. Wur, J.L. Davidson, W.P. Kang, Polycrystalline diamond pressure sensor, J. Microelectromech. Syst. 4 (1) (1995) 34–41. [10] Y. Gurbuz, W.P. Kang, J.L. Davidson, D.V. Kerns, A novel high temperature diode for oxygen gas detection, Third International High Temperature Electronics Conference, Albuquerque, NM, 9–14 June, 1996. [11] W.P. Kang, A. Wisitsora-at, J.L. Davidson, Q. Li, J.F. Xu, D.V. Kerns, The effects of sp2 content and surface treatment on micropatterned pyramidal diamond tips, in: Technical Digest of International Vacuum Microelectronics Conference, Kyungju, South Korea, 17–21 August, 1997, pp. 107–111.