Ultrasonic Micromachining in the fabrication of MEMS Micro-sensors Jamil Akhtar Professor AcSIR New Delhi Chief Scientist & Head, CSIR-CEERI, Pilani, INDIA
CEERI, Pilani A constituent laboratory of CSIR, New Delhi, India Since 1953 37 labs all over India
Anisotropic etching of Silicon Crystal plane dependent chemical etching Limited shapes of 3-D structures Restrictions on cavity walls Angle between etched surface and side walls Highly controllable and repeatable Highly uniform etched surface Slow etching rates Toxic
Anisotropic Etching in Si(100) O Kenneth E. Bean, Anisotropic Etching of Silicon, IEEE Trans. On Electron Devices, Vol.ED-25, No.10, pp 1185-1193, October 1978 O Kurt E. Petersen, Silicon as a Mechanical Material, Proceedings of the IEEE, Vol.70, No.5, May 1982 O Irena Zubel and Irena Barycka, Silicon anisotropic etching in alkaline solutions I. The geometric description of figures developed under etching Si(100) in various solutions, Sensors and Actuators A 70, pp.250-259, 1998
KOH based Anisotropic Etching For cavity formation KOH is the most popular etchant. Etch Rate: {110} > {100} >> {111} Used at elevated temperature (70-80 ºC) Etching mask Resist will not survive Oxide is attacked slowly Nitride is not attacked, best masking material
Thickness Calculation Of Etched thickness From micrometer readings, the value of x =.014 mm Y= x tan54.74 Y=.014 1.412 mm Y=.019796 mm Y= 19.796 μm (thickness ) 7
Etching of Si(100) in Aq. KOH
Tetra Methyl Amonium Hydroxide (TMAH)
Micromachining of Si(100) Boiling point of KOH solution with varying concentration Etching rate of (100) silicon with varying KOH concentration and temperatures AFM scan of the etched (100) silicon surface at 80 0 C in (a) 10% wt KOH and (b) 20% wt KOH
Convex and Concave
Alignment with crystal planes Square size 800 µm Inner Sq. size 80 µm Inner rec. size 80 and 160 µm After Etch: 776 µm and 91 µm 793 µm and 84 µm 800 and 80 160 µms
Bulk micromachined pressure sensor
Output Voltage (mv) Output Voltage (mv) Polysilicon piezoresistive pressure sensor 5 0 2 Volts 20 0 5 Volts -5-10 -20-15 -20-25 -40-60 -30 0 5 10 15 20 25 30 Differential Pressure (bar) 0 5 10 15 20 25 30 Differential Pressure (bar)
Non-conventional machining Ultrasonic Milling Electro Discharge Machining Electro-Chemical Etching Laser machining FIB etching RIE/DRIE
Integration of USM with MEMS Arrays of tool Alignment marks Silicon wafer compatibility Process sequence optimization Steep edges of the cavity Arrays of 3-D structures Any shape Economic in time Non toxic Integration with Microelectronics and MEMS processes
To Investigate/optimize Energy distribution from one to many tools Control of roughness Control over sharp geometries
USM- an overview, effort made for micromachining in pressure sensor Main component of USM consists of power supply, amplitude maintainer, horn, cutting tools, slurry hose and recirculation pump. USM principle states that a power supply that generates a 20 KHz signal when applied to piezoelectric transducer, converts high frequency electrical signal into mechanical motion. This mechanical motion from the converter is amplified using amplifier maintainer and transmitted to horn. Then it causes horn and milling tool vibrated perpendicularly to the wafer face 20,000 times per second. A recirculation pump forces slurry of abrasive material boron carbide between the tool face and work piece where flow speed of slurry can be controlled by slurry hose switch. The abrasive particles propelled by the tool strike the work piece at 150,000 times at their own weight. In this way USM etch the surface of the material as per applied tip.
USM Etching rates for pyrex, Quartz, Silicon, Ceramic and SiC
Pyrex Category Time (sec) Thickness (um) Etching rate(um/sec) Continuous Time Discrete Time 30 314.43 10.48 60 270.95 4.51 90 335.66 3.72 120 387.53 3.22 150 384.08 2.56 180 346.92 1.93 30+30 564.75 60+30 573.62 90+30 589.63 120+30 585.86 150+30 592.18 180+30 616.74 240+30 601.56 300+30 599.86 300+30 +30 360+30 +30 (time not sure) 901.78 958. 68 (seethrough hole); and breakage
quartz Category Time (sec) Thickness (um) Etching rate (um/sec) 30 321.6 10.72 Continuo us Time 60 331.50 5.53 90 347 2.89 120 364.17 3.03 150 355 2.36 180 309.74 1.72 30+30 568.68 Discrete Time 60+30 602.11 90+30 680.32 120+30 701.22 150+30 646.30 180+30+ 30 986.62 (seethrough hole)
Silicon Category Time (sec) Thickness (um) Etching rate(um/sec) Continuous Time 5 245.54 49.11 10 237.94 23.79 15 281.75 18.78 20 315.23(thr oughout ) 15.76
Ceramics Category Time (in sec) Thickness (in um) Etching rate(um/sec) 30 201.10 6.70 Continuo us Time 60 243.79 4.06 90 341.50 3.70 120 354.28 2.95 150 376.15 2.50 Discrete Time 30+30 439.91 60+30 745.28 (seethrough hole)
Simulation results Angle Maximum Stress(*10 5 N/m 2 ) Alumina Silicon carbide Silicon Quartz Sapphire 54.7 2.578 2.597 2.492 2.661 2.526 60 2.710 2.732 2.608 2.806 2.660 65 2.621 2.640 2.534 2.704 2.578 70 2.670 2.695 2.568 2.766 2.620 75 2.695 2.718 2.589 2.794 2.640 80 2.707 2.731 2.597 2.811 2.653 85 2.716 2.740 2.601 2.823 2.660 90 2.728 2.750 2.612 2.838 2.672
Surface analysis of Materials Category Surface Roughness(nm) Surface of Inside hole(um) quartz 18 2.2 silicon 16 2.8 Pyrex 25.6 3 ceramics 20 1.86
Schematics of circular diaphragm pressure sensor chip Resistance line length - 400 um Resistance width - 10 um Metal line width - 20 um Center resistance - 90 um away from the center Edge resistance - 100 um from the edge Contact pad - 200 um X 200 um Contact pad location - 250 um away from the diaphragm Chip size - 4mm X 4mm
Cleaning process I. Degreasing : Trichloro ethylene Acetone Methanol II. Piranha: Different ratios of H 2 SO 4 and H 2 O 2 as per cleaning requirement (3:1, 5:1, 7:1). Dilute HF ( HF: DI H 2 O = 1:50) to remove the thin oxide layer.
Work piece preparation of USM
Post cleaning of USM process wafer and result Sample Step thickness (µm) Surface roughness (µm) Pyrex 1000 (587) 1.2 Silicon 380 (344) 1-2
Hole making by DRIE
Principle of piezoresistive pressure sensors When applied pressure from top towards the surface of the device, where diaphragm deforms under applied pressure, the thickness of the silicon diaphragm usually strain to a few micrometers. Above the diaphragm, we placed piezoresistors (polysilicon) in the form of Wheatstone bridge, where these piezoresistors convert the stresses induced in the silicon diaphragm by applied pressure into a change in electrical resistance, which is then converted into output voltage by Wheatstone bridge circuit as shown in given figure b. We get the output voltage by equation V R R 2 4 out V in R R R R 1 2 3 4 Fig a Fig b
Poly-Si Si Substrate 1. Silicon Substrate 4. LPCVD/EBPVD Poly-Si deposition SiO₂ Layer 2. Thermally Grown of SiO₂ Alignment mask 3. Cavity formation by ultrasonic milling 6. Ti/Au metalization
Control pads 7. Metal line formation in wheat stone bridge configuration 7. PECVD SiO2 passivation and Pad opining
Fabrication process of pressure sensor
Processed wafer USM based diaphragm Metal line formed Poly silicon resister formed View of single pressure sensor device
USM based circular diaphragm silicon pressure sensor
Typical glimpses of USM based fabrication process & measured characteristics
Typical glimpse of Quartz Pressure Sensor Process
Manufacturers SONIC-MILL, USA for USM Mikro Tool, Singapore for Micro-EDM
Acknowledgments Kulwant Singh ; Ph.D Student (NIT Calicut) Rajesh Saha; Project Fellow ( Left for M.Tech) Ayon Roychaudhuri ; Ph.D Student (AcSIR) Himani Sharma; Ph.D Student (Banasthali) Pradeep Kumar; Project Fellow Director CSIR-CEERI
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