Ultrasonic Micromachining in the fabrication of MEMS Micro-sensors

Transcription

1 Ultrasonic Micromachining in the fabrication of MEMS Micro-sensors Jamil Akhtar Professor AcSIR New Delhi Chief Scientist & Head, CSIR-CEERI, Pilani, INDIA

2 CEERI, Pilani A constituent laboratory of CSIR, New Delhi, India Since labs all over India

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4 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

5 Anisotropic Etching in Si(100) O Kenneth E. Bean, Anisotropic Etching of Silicon, IEEE Trans. On Electron Devices, Vol.ED-25, No.10, pp , 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 , 1998

6 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

7 Thickness Calculation Of Etched thickness From micrometer readings, the value of x =.014 mm Y= x tan54.74 Y= mm Y= mm Y= μm (thickness ) 7

8 Etching of Si(100) in Aq. KOH

9 Tetra Methyl Amonium Hydroxide (TMAH)

10 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

11 Convex and Concave

12 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 µms

13 Bulk micromachined pressure sensor

14 Output Voltage (mv) Output Voltage (mv) Polysilicon piezoresistive pressure sensor Volts Volts Differential Pressure (bar) Differential Pressure (bar)

15 Non-conventional machining Ultrasonic Milling Electro Discharge Machining Electro-Chemical Etching Laser machining FIB etching RIE/DRIE

16 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

17 To Investigate/optimize Energy distribution from one to many tools Control of roughness Control over sharp geometries

18 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.

19 USM Etching rates for pyrex, Quartz, Silicon, Ceramic and SiC

20 Pyrex Category Time (sec) Thickness (um) Etching rate(um/sec) Continuous Time Discrete Time (time not sure) (seethrough hole); and breakage

21 quartz Category Time (sec) Thickness (um) Etching rate (um/sec) Continuo us Time Discrete Time (seethrough hole)

22 Silicon Category Time (sec) Thickness (um) Etching rate(um/sec) Continuous Time (thr oughout ) 15.76

23 Ceramics Category Time (in sec) Thickness (in um) Etching rate(um/sec) Continuo us Time Discrete Time (seethrough hole)

24 Simulation results Angle Maximum Stress(*10 5 N/m 2 ) Alumina Silicon carbide Silicon Quartz Sapphire

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26 Surface analysis of Materials Category Surface Roughness(nm) Surface of Inside hole(um) quartz silicon Pyrex ceramics

27 Schematics of circular diaphragm pressure sensor chip Resistance line length um Resistance width - 10 um Metal line width - 20 um Center resistance - 90 um away from the center Edge resistance um from the edge Contact pad um X 200 um Contact pad location um away from the diaphragm Chip size - 4mm X 4mm

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29 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.

30 Work piece preparation of USM

31 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

32 Hole making by DRIE

33 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 Fig a Fig b

34 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

35 Control pads 7. Metal line formation in wheat stone bridge configuration 7. PECVD SiO2 passivation and Pad opining

36 Fabrication process of pressure sensor

37 Processed wafer USM based diaphragm Metal line formed Poly silicon resister formed View of single pressure sensor device

38 USM based circular diaphragm silicon pressure sensor

39 Typical glimpses of USM based fabrication process & measured characteristics

40 Typical glimpse of Quartz Pressure Sensor Process

41 Manufacturers SONIC-MILL, USA for USM Mikro Tool, Singapore for Micro-EDM

42 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

43 THANK YOU