Micro and Smart Systems Lecture - 39 (1)Packaging Pressure sensors (Continued from Lecture 38) (2)Micromachined Silicon Accelerometers Prof K.N.Bhat, ECE Department, IISc Bangalore email: knbhat@gmail.com 1
Topics for Discussions Continued from Lecture -38 on pressure sensor Packaging and testing Accelerometer operating principle and types SOI based accelerometer a case study Surface micromachined comb type accelerometer structure of ADXL type and the force balance circuit 2
Photograph of Integrated pressure sensor chip in a TO39 header 3
Integrated pressure sensor in a package header Pressure port 4
N 2 pressure from cylinder Jig Male part Metal Washer Pressure port Jig Female part Metal cap welded to the header Schematic of packaged pressure sensor mounted in a Jig for testing Teflon O ring Diced pressure sensor Aluminium wire bonding Lead from the header 5
Pressure Vs Output Voltage of the packaged Integrated Pressure Sensor 2500 2000 Sensitivity at the sensor output = 60 mv / bar / 10 V Sensitivity at the amplfier output = 270 mv / bar / 10 V Diferential gain of the amplifier = 4.5 Output ( mv ) 1500 1000 Amplified sensor output sensor output 500 0 0 1 2 3 4 5 6 7 Pressure ( bar ) 6
Applications of Pressure sensors Mapping Pressure on the wings of aircraft Automobile industry Micro-fluidics for Flow sensing Biomedical applications Blood Pressure measurement and Intracranial Pressure (ICP) monitoring Oceanography for Depth measurement. Packaging is critical 7
Flow Sensor Flow rate = P/R Capacitive Pressure sensor Pyrex P 1 Silicon P 2 Pyrex Inlet Outlet R - Fluidic resistance P = (P 1 P 2 ) 8
Summary and Conclusions Despite several engineering challenges, MEMS offer high performance and are small, low power, and relatively cheap. Parameters of pressure sensor are sensitivity, nonlinearity, offset voltage and Maximum operating pressure and range Micro machined Sensors have been fabricated along with Electronics by the batch process of silicon wafers using SOI approach The pressure sensors find wide range of Applications for industrial, automotive, biomedical and aerospace and defense establishments. Pressure sensors constitute about 60% of the microsystem market 9
References K.N.Bhat, Silicon Micromachined Pressure Sensors,Journal of Indian Institute of Science, Vol.87,pp210-213, 2007 S.R. Manjula, Teweldebhran Kifle, E. Bhattacharya, and K.N. Bhat 'Physical Model for the Resistivity and Temperature Coefficient of Resistivity in Heavily Doped Polysilicon' IEEE Transactions on Electron Devices (USA), July 2006 V.Vinoth Kumar, Amitava DasGupta and K.N.Bhat, Processoptimization for monolithic integration of piezoresistive pressure sensor and MOSFET amplifier with SOI approach Journal of Physics, Institute of Physics Publishing, Vol.34, pp210-215, 2006 10
Acceleration Sensors Reference plane Displacement x Steady State condition, x MASS Spring. Spring constant = k Force, F Fixed electrode F Ma kx Displacement,, is a measure of acceleration Sensitivity = x a M k a 11
Approaches to Measure Acceleration Piezoresistive: by integrating a piezoresistor on the spring element Capacitive by measuring the change in capacitance between the proof mass and a fixed electrode. The mass is also referred to as seismic mass 12
Piezoresistive Silicon Micro Accelerometer V in 1 Top view 1 3 Frame Seismic Mass V o 3 A A 2 Beam 2 Resistor cross section at A-A Frame Seismic Mass Frame Beam 13
Micro structure of an accelerometer with air damping and bumpers to serve as over-range stop mechanism. Leads Top cap Bumper Damping gap Frame Beam Seismic Mass Frame Bottom cap Bumper 14
Capacitive Accelerometer Mass Metal Contact Beam Si (10 μm) SiO 2 (1 μm) Bulk Si 15
SOI based Accelerometer Fabrication 1. SOI or polysi 16
2. Patterned mechanical structure (polysi or single crystal ) over the sacrificial SIO 2 17
3. Oxide Etch, Release the mechanical strycture Mass Beam Si or PoySi SiO 2 (1 μm) Bulk Si 18
SOI accelerometer fabrication 4. Metallize Mass Electrodes (Cr / Au) Beam Si or PolySi SiO 2 (1 μm) Bulk Si 19
KOH Etching of Si Convex corners 20
MICROPHOTOGRAPH OF PATTERENED SAMPLE AFTER ETCHING 50 m 21
Microphotograph Of Bent Portion Of Beam (a) 30% KOH at 65 C (b) 30% KOH solution containing 30% tertiarybutanol solution Undercutting Of Convex Corners is present (411) Planes are exposed Undercutting Of Convex Corners is Absent 22
Alternate Approach to prevent endercutting at convex corners Modify the etch- mask pattern as shown Corner compensation scheme The width of the corner beam must be about twice the thickness of the square pattern Programs that simulate the structure to achieve by etching with a given mask are commercially available 23
Thickness =14.45 m Resonance frequency 35.5 KHz measured using laser dopler vibrometer 24
Results obtained by electrostatic actuation of the structure fabricated in the laboratory Capacitance(Pf) 15.55 15.50 15.45 15.40 15.35 15.30 15.25 15.20 15.15 Sensitivity=0.2 pf/ V/ g UZ1 Capaci tance C oa d SiO 2 anchor 15.10 0.0 0.5 1.0 1.5 2.0 Voltage(volts) Released SOI (Mass) Si substrate Support Beam 25
Dynamic test result using Laser Doppler vibrometer (Vibration Amplitude versus frequency) Resonance frequency=35.5khz 26
Vibration test result obtained on silicon microaccelerometer fabricated with SOI wafer processing fabricated as part of E3-222 course in the lab at CEN IISc Bangalore Resonance frequency =35.5kHZ frequency, 27
Applications of Micromachined Accelerometers (a) Acceleration Measurement Front and side airbag crash sensing Vehicle and traction control systems Inertial Measurements and Navigation Human Activity for pacemaker control (b) Vibration Measurement Engine management and condition monitoring Monitoring Seismic activity ( Earth quake) Shock and impact monitoring Security Devices 28
Summary We have discussed the following Pressure sensor packaging and testing Accelerometer operating Principle and approaches Fabrication of accelerometer with Silicon on Insulator wafer Testing by Electrostatic actuation and frequency response for testing capacitive accelerometer 29