EE 5344 Introduction to MEMS. CHAPTER 3 Conventional Si Processing

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1 3. Conventional licon Processing Micromachining, Microfabrication. EE 5344 Introduction to MEMS CHAPTER 3 Conventional Processing Why silicon? Abundant, cheap, easy to process. licon planar Integrated Circuit (IC) Fabrication: Crystal growth and epitaxy Oxidation and deposition Diffusion or implantation of dopants Lithography and etching Metallization and wire bonding Testing and encapsulation Fig 3.3 Typical n-mosfet fabrication technology. Movies: licon Run I and licon Run II (about 30 min. each) 3.1 Deposition: Thin s are essential building materials in semiconductor microsensors. Usually µm thick. Generally physical and chemical deposition means are used. 3.1.A Spin Casting: Thin material is in solution in a volatile liquid solvent. The dissolved material is poured on the wafer. Wafer is rotated at high speed The volatile solvent evaporates, leaving a uniform thin layer of solid material. Used for deposition of organic materials such as photosensitive resists polyimides and inorganic spin-on glasses. Thickness of the depends on: 1. Degree of solubility. Viscosity 3. Spin speed Advantages 1. Planarizes small irregularities on surface. mple 3. Inexpensive 1/ Processing

2 Disadvantages 1. Does not yield a continuos across steps higher than two to three times the thickness.. Suffers from shrinkage after bake, which causes a high-stress state. 3. Films tend to be less dense and therefore more susceptible to chemical attack. 3.1.B Evaporation: wafer holder Vacuum Bell Jar wafer Crucible heated by electron beam (can also be tungsten filament) shutter Material to be deposited pump Film thickness is determined by: 1. The time the shutter is opened.. Vapor pressure of the material which determines the evaporation rate. Advantages: 1. Relatively simple and inexpensive.. Works great for metal s with low melting point (aluminum, gold, copper). Disadvantages 1. Hard to deposit s with high melting point such as refractory metals (tungsten).. nce a point source is used, there might be shadow effects. / Processing

3 3. Coverage is determined by the mobility of the evaporated molecules on the surface. 4. Only thin layers can be obtained. Less than 1µ. 3.1.C Sputtering This overcomes many problems associated with thermal evaporation. Matching Network Target Forward Power Reflected Power Ion Sheath Plasma Vacuum Chamber rf generator Substrate Support Baseplate Schematic of a radio frequency sputtering system. First vacuum chamber is evacuated to Torr. Then Ar or He is let into the chamber. Then plasma is formed using dc or rf power supply. Target is cathode. Wafers (substrates) are anode. The ions of plasma take material of the target, which lands on the substrate coating a thin. Advantages: 1. Better step coverage than evaporation. Especially if magnetic fields are introduced into the plasma.. Almost all materials can be sputtered. 3. Can use more than one target: co-sputtering. 4. Can use multiple substrates: mass production. Disadvantages: 1. More complicated than evaporation. 3.1.D Reactive Growth In the previous methods, no chemical reaction occurs between the substrate and the deposited thin. In reactive growth, a chemically reactive species combines with the substrate to form a new. 3/ Processing

4 One example is oxidation: Formation of O on. Dry Oxidation + O O wafer surface gas Thin Film Wet Oxidation + H + H O O water vapor Advantages: 1. mple. Can be done in a furnace with reactive gases.. Excellent quality of thin. Disadvantages: 1. Due to diffusion rate limitation of the reactive species, only thin s are possible. Thin formation rate depends on: a. Reaction rate b. Diffusion rate of the reactive species For example for oxide: O O x x x Reaction occurs at the -O interface.. Limited to thin s. <1µm. reaction 4/ Processing

5 3.1.E Chemical Vapor Deposition: (CVD) In CVD, gas is broken down to its species, some of which nucleate on the substrate, forming a. It is done in a furnace. pressure sensor wafer boat wafers Exhaust to pump gas inlet 3-zone heater First, furnace is heated in inert gas (like N ). When the deposition temperature is reached, N is turned off, and the reactive gas is introduced. Many different thin s can be deposited using this method, including polycrystalline silicon, silicon nitride, silicon dioxide and refractory metals like tungsten. Most CVD s are amorphous or polycrystalline. However, a special CVD called epitaxial CVD grows crystalline s. 3.1.F Plasma Enhanced CVD (PECVD): The gas is decomposed into components with the aid of plasma which speeds up the process. While the gas containing the atomic components of the is introduced into the deposition chamber, the plasma is ignited with a RF source creating ionized species. Some of these species deposit on the substrate. Advantages: 1. Faster deposition rate.. Can be done at lower temperatures than regular CVD. Disadvantages: 1. Quality of thin is usually not good. Might contain cracks, voids.. Stoichiometry of the usually varies due to trapped by-products like H. This technique is used to deposit intermetal dielectric layers and organic layers where polymers can be obtained from monomers through plasma. 5/ Processing

6 3. Lithography: The way patterns are defined on thin s is called Lithography. If light is used to transfer patterns from a mask on to a wafer, then this special kind of lithography is called photolithography. This is analogous to the transferring images from a negative to a photosensitive paper to make photographs. The steps are: Positive Deposit Film (1) Negative (+) () Deposit Photoresist (-) mask (3) Pre-bake mask (+) (4) Exposure to UV light through mask (-) Thin (5) Develop Thin (6) Post Bake 6/ Processing

7 (+) Thin (7) Etch (-) Thin Thin (8) Remove Thin Alignment and Exposure: Most of the time, there are multiple lithographic and etching processes where the features of the devices have to be aligned. Alignment keys on the mask and the wafer are used for this. Steppers align and project the image of each die on the wafer one at a time. Contact aligners expose the wafer at the same time. In MEMS technology, there are times when lithography and patterning are required on both sides of the wafer. Then both masks have to be aligned to the wafer. top mask bottom mask First bottom and top masks are aligned wafer Then wafer is inserted in the middle and aligned to both masks. UV Light The masks are brought into contact with the wafer and exposed to UV Light 7/ Processing

8 4. Etching: A kind of patterning technique. Wet chemical etching or dry (plasma)etching can be used. 3.3.A Wet Chemical Etching: In wet chemical etching, wafer with patterned is immersed in an etchant, usually an acid. Agitation or heat can be used to speed up the etching. The etchant removes the thin not protected by the. The thin under remains. Most etchants are isotropic, (equal etch rate in all directions). Etchants will be described more in detail in relation to bulk and surface micromachining. The chemical etch has to be matched to the to be etched. isotropic etching Thin Film There are also some anisotropic wet etchants. 3.3B Dry Etching or Plasma Etching The chemical etchant is in plasma state. wafer plasma ~ rf power source gases Vacuum Pump 8/ Processing

9 Etchant gases are fed into a vacuum chamber at a moderately low pressure Torr. A rf voltage is used to create plasma that contains free radicals and ionized species. Free radicals react with thin to remove the thin. Plasma etching is anisotropic and therefore preferred in MEMS fabrication. Thin Film Parameters that affect the quality of plasma etch: 1. Pressure. Temperature 3. Frequency of RF source 4. Magnetic Field (if any) 5. Lift Off This is another patterning technique. Photoresist is deposited first and then the thin to be patterned. Mostly used for thin metal s. A sample set of steps: metal (1) Deposit Photoresist mask (5) Deposit thin (metal) () Prebake (3) Expose to UV light through mask Metal (4) Develop (6) Remove (acetone) lift-off 9/ Processing

10 Advantages: 1. No need to match the etchant to the specific thin. The process steps work for all thin metals.. Sharp, straight edges. Disadvantages: 1. Mechanical, does not work on elastic s. Works best only on thin metal s. 6. Diffusion and Ion Implantation To achieve required conductivity in silicon, controlled amounts of dopants are introduced and diffused into the wafer. p-type Boron n-type Arsenic or Phosphorus. 4. Thermal Diffusion Either the wafers are exposed to gaseous species containing the dopant, or solid sources are spun on wafers. The diffusion is performed at a controlled high temperature in a furnace. Examples for gaseous species: AsH 3 for As doping PH 3 for P doping B H 6 for B doping The diffusion is governed by Fick s diffusion equation. C ( x, t) t = D x C ( x, t) C(x,t)= dopant concentration at a distance x into the wafer at time t-(atoms/cm 3 ) D=diffusion coefficient (cm /s) D=D 0 e -Ea/kT D o =constant E a =activation energy for diffusion. Depends on the type of dopant and the host. K=Boltzmann s constant T=temperature 10/ Processing

11 D is independent of carrier concentration below intrinsic carrier concentration which depends exponentially on temperature. 18 n i ~ 5x10 at1000 C Above intrinsic carrier concentration, D becomes dependent on concentration. Fick s diffusion equation can be solved for D independent of doping concentration. Two Cases: Case 1: 1) C (x,0)=0 C (0,t)=C s =surface dopant conc. C (,t)=0 Solution: C x, t = Cserfc x / ( ) ( Dt ) erfc=complementary error function erfc=1-erf 1 erf = erfx = π x e 0 y dy Case : ) C(x,0)=0 C(,t)=0 C(x,t)dx=Q=amount of dopant Solution: Q C( x, t) = π Dt e ( x / Dt ) ( Gaussian) Case 1 corresponds to infinite supply of source, so the surface dopant concentration can be kept constant. Case corresponds to a finite supply of dopant. 5. Ion Implantation: Here charged ions are accelerated in vacuum and imbedded into the wafer. Usually the resultant dopant profile is Gaussian. 11/ Processing