Materials for MEMS. Dr. Yael Hanein. 11 March 2004 Materials Applications Yael Hanein

Transcription

1 Materials for MEMS Dr. Yael Hanein

2 Materials for MEMS MEMS (introduction) Materials used in MEMS Material properties Standard MEMS processes

3 MEMS The world s smallest guitar is about 10 micrometers long about the size of a single cell. It was built to illustrate how microelectro-mechanical technology can be used to create structures. Cornell.

4 MEMS Tiny MEMS pressure sensors

5 What are MEMS? MEMS (micro electro mechanical systems) or MST (micro system technology) The fabrication of devices with at least some of their dimensions are in the micrometer range (Madou) A portfolio of techniques to design and create miniature systems. (Maluf)

6 1 micron scale Atoms 1 µm Cells Visible Infrared Microwaves WaveLength (m)

7 MEMS applications Mechanical Applications Pressure sensors Accelerometers (air bag sensors)

8 Optical Application Optical switches Digital Light Processing

9 Inkjet nozzles Step1 - Nucleation Ink-filled chambers are heated by tiny resistive heating elements Step2 - Bubble Growth By heating the liquid ink a bubble is generated Step3 - Drop Ejection The vaporized part of the ink is propelled towards the paper in a tiny droplet Step4 - Refill Chambers are filled again by the ink through microscopic channels

10 MEMS applications Commercial Biomedical sensors Drug delivery systems Neurological disorders Telecom optical fiber switching Monitoring structural health Automotive safety Military Biochemical warfare detection Inertial systems for guidance and navigation

11 U.S. MEMS markets Year Automotive* Medical Informatio tech.&inds** Military Total In $1,000,000 * 80% air bags ** 1998: inkjets 75%, displays 5.4 Updates info:

12 Why MEMS? A MEMS solution is attractive if: A new function Significant cost reduction Both ** Size reduction is seldom sufficient as the sole reason MEMS is justified when: Added value Increased productivity Cost competitiveness (batch process) Revenue and profit

13 MEMS: Evolution from Si IC technology VLSI (FET as an example)

14 Evolution from Si IC technology ocamac/newsletters/newsletter%209/april96_5of6.html oemagazine.com/fromthemagazine/ may01/mems.html lectures/history/history_computing.html asci/mems.html

15 Photolithography

16 IC - Jack Kilby After proving that integrated circuits were possible (1958), I headed teams that built the first military systems and the first computer incorporating integrated circuits. I also worked on teams that invented the handheld calculator and the thermal printer, which was used in portable data terminals.

17 Moore's Law Gordon Moore (co-founder of Intel) predicted in 1965 that the transistor density of semiconductor chips would double roughly every 18 months.

18 Main IC Benefits High quality materials (SC silicon, GaAs) CAD Possessing infrastructure Batch processes

19 MEMS History Original technology was very similar; same materials (silicon), similar processing (photolithography) but different applications: Pressure sensors, accelerometers Silicon for MEMS SC (integration with VLSI) Extensive experience Piezo-resistive High quality material available Infrastructure available mechanically strong

20 MEMS vs IC Some Issues are relevant to both IC and MEMS design: Stress, selective etching, Pattern transfer, cleanliness, structure release. Some are unique: wet environment, 3D, moving parts. Integration requires considering limitations of both technologies. MEMS evolved and now use various materials, processes for a wide range of applications:

21 MEMS Today Gripper end 8 µm Nikolas Chronis and Luke P. Lee, MEMS2004

22 Standard MEMS sensors/actuators Piezo-resistors Comb drives Thermal expansion Surface chemistry

23 MEMS today Materials: SC (Si,GaAs,SiGe), Glass, Silicone. Application: Bio, RF, Chemistry, Optical, Mechanical. Physics/engineering: Elasticity, piezoresistivity, surface properties, capacitive actuators Processes: Bulk machining, surface machining, DRIE, LIGA, Polymers

24 Functionality Transducer Converts energy from one form to another Sensors (Transducers) A device that detects or measures Actuator

25 SiO2/Quartz/glass The stable oxide is one of the key elements for the success of silicon in IC Excellent thermal and electrical insulation Sacrificial layers in surface micromachining processes (selectively etched in HF)

26 Relevant Material Properties Electrical (SC, metals, insulators) Mechanical (elasticity) Thermal (Heat conductivity) Chemical, electro-chemical Biological (bio-compatibility) Optical (roughness) Processing

27 MEMS and materials Mechanical (elasticity) Processing Chemical, electro-chemical Thermal (Heat conductivity) Cost Optical (roughness) Biological (bio-compatibility) Electrical (SC, metals, insulators)

28 Silicon* One of very few materials that can be economically manufactured in single crystal substrates Diamond lattice * Not to be confused with silicone

29 Silicon

30 Czochralski Crystal Growth Process

31 Float zone pulling

32 Silicon boules

33 Silicon 1. Crystal Growth Polysilicon Seed crystal Crucible 6. Edge Rounding Heater 7. Lapping 2. Single Crystal Ingot 8. Wafer Etching 3. Crystal Trimming and Diameter Grind Slurr y Polishing head 4. Flat Grinding 9. Polishing Polishing table 5. Wafer Slicing 10. Wafer Inspection

34 Silicon Summary Properties: Extensive studies and documentation Suitable for electronic, mechanical, thermal, and optical integration Can sustain harsh (mechanical) handling conditions Crystalline: mechanical properties are uniform across wafer lots Structure: Crystalline, Polycrystalline-polysilicon amorphous Conductivity: Semiconductor

35 Silicon Mechanical: Hard and brittle material, deforms elastically, robust Tensile yield strength 7 GPa Maintain mechanical integrity up to 500 C. >500 C plastic deformation. Properties independent of doping (stress when impurities reach 1020 cm -3 ) Polycrystalline and amorphous: properties vary with deposition conditions, but similar to crystalline silicon. Polycrystalline and amorphous: high levels of intrinsic stress, requires annealing (>900 C). Polycrystalline and amorphous: unstable, >250 C.

36 Silicon Fabrication: Crystalline: Wafers Polysilicon: thin film deposition Amorpous : thin film deposition Optics: Not an active optical material (indirect band gap) Transparent at IR <0.4 µm reflects 60% of incedent light Chemistry/biology Stable and resistant (brake fluid, biological medium) Suitable for high purity gases Benign in the body, does not release toxic substances. Cost: Low ultra pure electronic grade silicon wafers are available for IC

37 SiO 2 SiO 2 Silica Fused silica is a purer version of Fused quartz that is made synthetically from various Silicon gasses. 17 crystalline phases Quartz single crystal material, low impurity concentration Fused quartz is the amorphous form of quartz. Fused quartz is made from natural crystalline quartz, usually quartz sand that has been mined. Glass - amorphous solid, impurities, low melting temperature Borosilicate glass is an "Engineered" glass developed specifically for use in environments such as laboratories and heating applications where Thermal, mechanical and chemical conditions are too much for standard, household type glass. Some common names are Pyrex by Corning, and Duran by Schott Glass. Like most glasses, the dominant component of Borosilicate glass is SiO2 with boron and various other elements added to give it its excellent qualities.

38 SiO 2 Properties Color table (Sze) Fabrication Thermally grown by oxidizing silicon at temperature > 800 C. Spin on glass Bonding Mechanical High stress (difficult to control or anneal) limited use as beams or membranes Uses Cost

39 Metals Aluminum Basic electrical interconnections (common and easy to deposit) Non-corrosive environment only T < 300 C (melting temperature =? ) Good light reflector (visible light) Gold/ titanium/tungsten Better for higher temperature Harsher environments Gold is good light reflector in the IR Platinum and palladium Stable for electrochemistry

40 Common Metals in MEMS Metal Ag Al Au Cr, Ti, TiW Cu ITO Ir, Pt W ρ (10-6 Ω cm) , 42, , Applications Electrochemistry Elect interconnects Optical reflection High T elect interconnect Optical refl IR electrochemistry Intermediate adhesion layer Elect interconnects Transparent interconnects Electrochemistry Bio-potential sensors High T elect interconnects Maluf table 2.3, 24

41 Polymers Properties Spin coated with varying thickness; few nm hundreds of microns Used in sensing of chemical gases and humidity Used as Photoresists, SU8: Epoxy based photoresist can form layers up to 100 µm Polyimide Fabrication Spin-on,molding Cost Low

42 Thermal conductivity Material At 300K (W/m K) Convection W/m 2 K Si (SCS) 156 Si 3 N SiO W Al Au Water Air

43 Thermal Expansion Material At 300K (10-6 / C) Si (SCS) Si N SiO W Al

44 Processing Bulk machining vs surface machining Etching, selective etching, sacrificial layers, isotropic vs anisotropic Release processes Deposition processes

45 Si Etching HF:HNO 3 :CH 3 COOH KOH Ethylene diamine pyrochatechol (EDP) (CH 3 ) 4 NOH (TMAH) SF 6 SF 6 /C 4 F 8 (DRIE) Type Wet Wet Wet Wet Plasma Plasma Anisotropic No Yes Yes Yes varies Yes Rate (µm/min) {111}/{100} selectivity None 100:1 35:1 50:1 None None Nitride etch Low <1 0.1 < SiO 2 etch (nm/min) < P ++ etch stop No Yes Yes Yes No No hazard high

46 MEMS Processing Processes (geometry/design) are material specific

47 Standard MEMS sensors/actuators Piezo-resistors Comb drives Thermal expansion

48 Standard MEMS Processes Piezo-resistive Pressure sensor

49 Standard MEMS Processes Comb driver

50 Bio-potential electrodes

51 MEMS applications Mechanical Applications: Actuators

52 Micro-fabricated cilia