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Surface Micromachining Chang Liu Micro Actuators, Sensors, Systems Group University of Illinois at Urbana-Champaign Chang Liu MASS UIUC Outline • Basic surface micromachining process • Most common surface micromachining materials - polysilicon and silicon oxide – – – – LPCVD deposition of polysilicon, silicon nitride, and oxide plasma etching for patterning structural layer micromachined hinges - fabrication process and assembly technique micromachined dimples and scratch drive actuators • Other sacrificial processing systems – metal sacrificial layer, plastics materials, etc. • Stiction and anti-stiction solutions • Multi User MEMS Process (MUMPS) – process definition and layer naming conventions Chang Liu MASS UIUC Basic Sacrificial Layer Processing • • • • • Step 1: Deposition of sacrificial layer Step 2: patterning of the sacrificial layer Step 3: deposit structural layer (conformal deposition) Step 4: liquid phase removal of sacrificial layer Step 5: removal of liquid - drying. Sacrificial wet etching Chang Liu drying MASS UIUC Process and Chemical Compatibility • For a two layer process • The deposition of the structural layer must not damage the sacrificial layer – Thermal stability • The pattering of the structural layer must not damage the sacrificial layer – Chemical and thermal stability • The removal of the sacrificial layer must not damage the structural layer – Chemical and thermal stability Chang Liu MASS UIUC Surface Micromachined Inductor • Air bridge can be formed using sacrificial etching. Chang Liu MASS UIUC Micro Gears with Free-standing Chains Chang Liu MASS UIUC Inductor - By Lucent Technologies Chang Liu MASS UIUC Surface Micromachined, Out of Plane Structures Chang Liu MASS UIUC Hinges • Used in micro optics component assembly. Chang Liu MASS UIUC Hinge Fabrication • Step 1: deposition of sacrificial layer. • Step 2: deposition of structural layer. • Step 3: deposition of second sacrificial layer. • Step 4: etching anchor to the substrate. • Step 5: deposition of second structural layer. • Step 6: patterning of second structural layer • Step 7: Etch away all sacrificial layer to release the first structural layer. Chang Liu MASS UIUC For a four layer process … • Sacrificial layers (Sac1, sac2) • Structural layers (str 1, str 2) • • • • • • • • • • • • Chang Liu Str1 deposition must not affect sac1 Str1 patterning must not affect sac1 Sac2 deposition must not affect sac1 Sac2 deposition must not affect str1 Sac2 patterning must not affect str1 Sac2 patterning must not affect sac1 (if sac 1 is exposed) Str 2 deposition must not affect sac2 Str 2 deposition must no affect str1 Str 2 deposition must not affect sac1 Str 2 patterning must not affect sac2, str1, sac1 Sac 1 removal must not affect str 2, str1 Sac 2 removal must not affect str 2, str1 MASS UIUC To make things more complex and challenging • Certain layers need to be made of a certain material; • Stress control issues may dictate certain layer materials; • Electrical performances may dictate certain layer materials; • Economic issues may dictate certain layer materials; Chang Liu MASS UIUC Chang Liu MASS UIUC This is helpful but … • Every lab is different. • Every machine is different. • Every run may be different. Chang Liu MASS UIUC Metal Sacrificial Layers Aluminum (0.3m) Chang Liu Permalloy (>20m) PR 4620 (10m each) copper (9m each) MASS UIUC Out Of Plane Devices Chang Liu MASS UIUC Three Pillars of MEMS Goals: Better performance Better yield Unique advantages Lower cost Higher yield … Design (physics, Principle) Materials Chang Liu Fabrication MASS UIUC Hybrid Fabrication Process • Combine the following processing styles into a single fabrication sequence – – – – Chang Liu Bulk machining Surface machining Three dimensional assembly Wafer bonding (low temperature or high temperature) MASS UIUC Example: Scanning Probe Microscopy • Problems with existing processes – Etching of positive pyramids • Difficult tot control etch stop point; uniformity difficult to obtain – Utilizes silicon substrate • Can be replaced by lower costs substrates – Bulk etching • Long etching time involved to etch through the wafer • Anisotropic etching: 1-2 micrometere/min • DRIE: 1 micrometere/min, high costs of equipment and consumables Chang Liu MASS UIUC A hybrid method to fabricate SPM probes Chang Liu MASS UIUC Chang Liu MASS UIUC Chang Liu MASS UIUC Chang Liu MASS UIUC Chang Liu MASS UIUC Chang Liu MASS UIUC Example: Solenoid • One way of realizing surface micromachined solenoid Chang Liu MASS UIUC A New Method Chang Liu MASS UIUC Chang Liu MASS UIUC Chang Liu MASS UIUC LPCVD Process • • • • Chang Liu Temperature range 500-800 degrees Pressure range 200 - 400 mtorr (1 torr = 1/760 ATM) Gas mixture: typically 2-3 gas mixture Particle free environment to prevent defects on surface (pin holes) MASS UIUC A Laboratory LPCVD Machine Chang Liu MASS UIUC LPCVD Recipes for Silicon Nitride, Polysilicon, and Oxide • Polycrystalline silicon – Polysilicon is deposited at around 580-620 oC and can withstand more than 1000 oC temperature. The deposition is conducted by decomposing silane (SiH4) under high temperature and vacuum (SiH4> Si+2H2). – Polysilicon is used extensively in IC - transistor gate • Silicon nitride – Silicon nitride is nonconducting and has tensile intrinsic stress on top of silicon substrates. It is deposited at around 800 oC by reacting silane (SiH4) or dichlorosilane (SiCl2H2) with ammonia (NH3) - SiH4+NH3 -> SixNy+ H. • Silicon oxide – The PSG is knows to reflow under high temperature (e.g. above 900 oC); it is deposited under relatively low temperature, e.g. 500 oC by reacting silane with oxygen (SiH4+O2-> SiO2+2H2). PSG can be deposited on top of Al metallization. – Silicon oxide is used for sealing IC circuits after processing. – The etch rate of HF on oxide is a function of doping concentration. Chang Liu MASS UIUC Other Structural or Sacrificial Materials • Structural layers – – – – evaporated and sputtered metals such as Gold, Copper electroplated metal (such as NiFe) plastic material (CVD plastic) silicon (such as epitaxy silicon or top silicon in SOI wafer) • Sacrificial layers – photoresist, polyimide, and other organic materials – copper • copper can be electroplated or evaporated, and is relatively inexpensive. – Oxide by plasma enhanced chemical vapor deposition (PECVD) • PECVD is done at lower temperature, with lower quality. It is generally undoped. – Thermally grown oxide • relatively low etch rate in HF. – Silicon or polysilicon • removed by gas phase silicon etching Chang Liu MASS UIUC A PECVD Machine Processing gases Reaction chamber RF plasma generator Chang Liu MASS UIUC Electroplating • Electroplating process description Chang Liu MASS UIUC Gas Phase Silicon Etching • XeF2 – liquid phase under room temperature – 2XeF2+Si => 2 Xe + SiF4 – vapor phase under low pressure – etches silicon with high speed – http://www.xactix.com/ • BrF3 – solid phase under regular pressure and room temperature – vapor phase (sublimation) under low pressure – BrF3 when reacted with water turns into HF at room temperature. • Both are isotropic etchants Chang Liu MASS UIUC Organic Sacrificial Layer • Photoresist – etching by plasma etching (limited lateral etch extent) – or by organic solvents (acetone or alcohol) • Polyimide – etching by organic solvents • Advantage – extremely low temperature process – easy to find structural solutions with good selectivity • Disadvantage – many structural layers such as LPCVD are not compatible. Structure material must be deposited under low temperatures. – Metal evaporation is also associated with high temperature metal particles, so it is not completely compatible and caution must be used. Chang Liu MASS UIUC Criteria for Selecting Materials and Etching Solutions • Selectivity – etch rate on structural layer/etch rate on sacrificial layer must be high. • Etch rate – rapid etching rate on sacrificial layer to reduce etching time • Deposition temperature – in certain applications, it is required that the overall processing temperature be low (e.g. integration with CMOS, integration with biological materials) • Intrinsic stress of structural layer – to remain flat after release, the structural layer must have low stress • Surface smoothness – important for optical applications • Long term stability Chang Liu MASS UIUC Stiction = Sticking and Friction Chang Liu MASS UIUC Origin of Stiction • As the liquid solution gradually vaporizes, the trapped liquid exert surface tension force on the microstructure, pulling the device down. • Surfaces can form permanent bond by molecule forces when they are close. Chang Liu MASS UIUC Antistiction Method I - Active Actuation Method • Use magnetic actuation to pull structures away form the surface – reduced surface tension length of arm • Limitations – only works for structures with magnetic material. Chang Liu MASS UIUC Antistiction Method II Organic Pillar • Use organic pillar to support the structure during the liquid removal. • The organic pillar is removed by oxygen plasma etching. Chang Liu MASS UIUC Antistiction Drying Method III - Phase Change Release Method Supercritical CO2 Drying • • • • • • • Avoid surface tension by relaying on phase change with less surface tension than watervapor. * p. 128-129 Supercritical state: temp > 31.1 oC and pressure > 72.8 atm. Step 1: change water with methanol Step 2: change methanol with liquid carbon dioxide (room temperature and 1200 psi) Step 3: content heated to 35 oC and the carbon dioxide is vented. Free-standing cantilever beams upto 850 m can stay released. Chang Liu MASS UIUC Super Critical Drying • When a substance in the liquid phase at a pressure greater than the critical pressure is heated, it undergoes a transition from a liquid to a supercritical fluid at the critical temperature. • This transition does not involve interfaces. • Criteria – chemically inert, non-toxic – low critical temperature • CO2 – critical temperature 31.1 oC – critical pressure 72.8 atm.(or 1073 psi) Chang Liu • • • Exchange methanol with liquid CO2 at 25oC and 1200 psi closeoff vessel and heated to 35 oC, no interface is formed. Vent vessel at a constant temperature above critical temperature. MASS UIUC Antistiction Method III - Self-assembled Monolayer • Forming low stiction, chemically stable surface coating using self-assembly monolayer (SAM) • SAM file is comprised of close packed array of alkyl chains which spontaneously form on oxidized silicon surface, and can remain stable after 18 months in air. • OTS: octadecyltrichlorosilane (forming C18H37SiCl3) Chang Liu MASS UIUC Result of SAM Assembly • Surface oxidation: H2O2 soak • SAM formation – – – – Chang Liu isopropanol alcohol rinse CCl4 rinse OTS solution CCl4 rinse MASS UIUC Structural-Sacrificial Compatibility Chang Liu MASS UIUC Chang Liu MASS UIUC Chang Liu MASS UIUC Chang Liu MASS UIUC Foundry Process • Why: – Reduce the cost of development by providing standard and unusual processes at reasonable cost. • How: – Wafer sharing: many processes are performed on one wafer with many users sharing the mask. • Drawback: limited process materials and steps – Machine sharing: a user’s wafer is dedicated and ships back-andforth among several vendors. • Drawback: long development and transport time – Dedicated foundry: a user’s wafer is handled at one site by dedicated personnel. • Drawback: highest cost among all forms of foundry process. Chang Liu MASS UIUC The Importance of Design Rules Chang Liu MASS UIUC Example: MUMPS Process Multi User MEMS Process Chang Liu MASS UIUC The Versatility of MUMPS Chang Liu MASS UIUC Compatibility Table Polysilicon Metal No Silicon oxide Photoresist (cured) Yes, slower Yes, slow speed Yes No, avoid long soak No Yes No No Yes No No No Yes No Dry plasma etching HF wet etching Uncured photoresist Photoresist developer Organic rinse Baking Yes No No No No Metal etchant No No No Yes Chang Liu No No. Sputtering is possible No, avoid long contact No MASS UIUC Case 4.1, Electrostatic Actuators • Curved beam due to intrinsic stress in the cantilever. • Helps: – Release • Hinders: – Capacitance calculation Chang Liu MASS UIUC How good is the design and process? • Design: – Advantage: • Direct integration of mechanical cantilever with FET transistors – Low noise sensor • Materials – Relatively difficult material – Exotic wafer • Processes – Difficulties: • Cantilever release using web silicon etchant may be a problem – Requires foundry process and new process development if industrialized Chang Liu MASS UIUC Case 4.2: Torsional Capacitive Accelerometer Chang Liu MASS UIUC How good is this design? • Design: – Simple – No electronics integration • Greater noise • Material: – Simple – Readily available • Fabrication process – Does not require exotic materials or processes – Sacrificial release may be a problem, like the previous case Chang Liu MASS UIUC Case 4.3: Membrane Parallel Plate Pressure Sensor Chang Liu MASS UIUC Evaluation • Design: – Results in hermetically sealed structures – Result in large gap distance to reduce damping • Materials: – Silicon materials – Doped silicon • Fabrication: – Length steps – Delicate bonding and handling – Process development is lengthy Chang Liu MASS UIUC
