MEMS Class 2 Materials for MEMS Mohammad Kilani

1. MEMS Class 2 Materials for MEMS Mohammad Kilani

2. MEMS Materials Silicon-Compatible Material System • Silicon • Silicon Oxide and Nitride • Thin Metal Films • Polymers Other Materials and Substrates • Glass and Fused Quartz Substrates • Silicon Carbide and Diamond • Gallium Arsenide and Other Group III-V Compound Semiconductors • Polymers • Shape-Memory Alloys

3. Silicon • The second most abundant element in the Earth's crust coming after oxygen, making up 25.7% of it by weight. • In its crystalline form, it has a dark gray color and a metallic luster. • Occurs in clay, granite, quartz and sand, mainly in the form of silicon dioxide (also known as silica) and silicates (compounds containing silicon, oxygen and metals). • The principal component of glass, cement, ceramics, most semiconductor devices, and silicones, the latter a plastic substance often confused with silicon. • Reacts with halogens and dilute alkalis, but most acids do not affect it (except for a combination of nitric acid and hydrofluoric acid). • One of very few materials that is economically manufactured in single crystal substrates. • Tremendous wealth of information accumulated on silicon and its compounds over the last few decades.

4. Silicon Oxides • The surface of silicon oxidizes immediately upon exposure to the oxygen in air (referred to as native oxide). The oxide thickness self-limits at a few nanometers at room temperature. As silicon dioxide is very inert, it acts as a protective layer that prevents chemical reactions with the underlying silicon. • Silicon Oxide is stable and is electrically and thermally insulating. • Can be preferentially etched in hydrofluoric acid (HF) with high selectivity to silicon. • Silicon dioxide (SiO2) is thermally grown by oxidizing silicon at temperatures above 800°C, • A drawback of silicon oxides is their relatively large intrinsic stresses, which are difficult to control. This has limited their use as materials for large suspended beams or membranes • Silicon nitride (SixNy) is also a widely used insulating thin film and is effective as a barrier against mobile ion diffusion—in particular, sodium and potassium ions found in biological environments.

5. Silicon in the Periodic Table Has the symbol Si and atomic number 14

6. The silicon atom The Silicon atom has 4 electrons in its outer shell A silicon unit cell consists of eight silicon atoms. Silicon and germanium have a diamond crystal structure. The silicon structure belongs to the class of face center cubic unit cells.

7. Silicon Crystallography The structure can be seen as two interpenetrating face centered crystal sublattices with one sublattice displaced from the other by one quarter of the distance along the body diagonal of the cube.

8. Miller Convention As a crystal is periodic, there exist families of equivalent directions and planes. Notation allows for distinction between a specific direction or plane and families of such. Use the [ ] notation to identify a specific direction (ie [1,0,-1]). Use the < > notation to identify a family of equivalent directions (ie <110>). Use the ( ) notation to identify a specific plane (ie (113)). Use the { } notation to identify a family of equivalent planes (ie {311}). A bar above a index is equivalent to a minus sign.

9. Vectors in the Silicon Lattice 3 x 2 = 6 equivalent <100>vectors 3 x 22 = 12 equivalent <110>vectors 1 x 23 = 8 equivalent <111>vectors

10. Crystallographic Planes 6 (100) planes, corresponding to 6 faces. Each two opposite faces results in the same plane 3 {100} planes 12 (110) planes, corresponding to 12 edges. Each two opposite edges result in the same plane 6 {110} planes 8 (111) planes, corresponding to 8 vertices. Each two opposite vertices result in the same plane 4 {111} planes

11. The Four {111} Planes

12. Angles between crystallographic planes Example: Angle between (001) and (111) planes

13. {111} planes has slow etch rate in KOH solution perspective view of a {100} wafer and a KOH-etched pit bounded by {111} planes

14. ODE etching with concave corners in the mask openings Square and rectangular openings in mask with the silicon etched for short or long time. Anisotropic etching or Orientation Dependent Etching (ODE).

15. ODE etching with convex corners in the mask openings Etching at convex corners and the formation of suspended beams of a material that is not etched (e.g., silicon nitride, p++ silicon). The {411} planes are frequently the slowest etching and appear at convex corners.

16. Etching through arbitrarily shaped opening in mask.

17. Flat assignments Flats at 180 deg for n-type and 90 deg for p-type Flats at 45 deg for n-type, no secondary for p-type.

18. Wafer Processes

19. Silicon Production Silicon is commercially prepared by the heating of high-purity silica in an electric arc furnace using carbon electrodes. At temperatures over 1900 °C, the carbon reduces the silica to silicon according to the chemical equation SiO2 + C → Si + CO2 Liquid silicon collects in the bottom of the furnace, and is then drained and cooled. The silicon produced via this process is called metallurgical grade silicon and is at least 99% pure. Using this method, silicon carbide, SiC, can form. However, provided the amount of SiO2 is kept high, silicon carbide may be eliminated, as explained by this equation: 2SiC + SiO2 → 3Si + 2CO In 2000, metallurgical grade silicon cost about $ 0.56 per pound ($1.23/kg)

20. Silicon Purificattion (Zone Melting Method) The first silicon purification method to be widely used industrially is the zone method. Rods of metallurgical grade silicon are heated to melt at one end. Then, the heater is slowly moved down the length of the rod, keeping a small length of the rod molten as the silicon cools and resolidifies behind it. Since most impurities tend to remain in the molten region rather than resolidify, when the process is complete, most of the impurities in the rod will have been moved into the end that was the last to be melted. This end is then cut off and discarded, and the process repeated if a still higher purity was desired

21. Silicon Purificattion (Siemens Process) Today, silicon is purified by first converting it to a liquid silicon compound that can be purified by successive fractional distillation, and converting the compound back to Silicon. In the Siemens process, Trichlorosilane (HSiCl3) is the silicon compound used as an intermediate. Trichlorosilane, which has a boiling point of 318 °C is formed via a reaction with HCl as follows: Si + 3HCl → SiHCl3 + H2 at 600 K High-purity silicon rods are exposed to trichlorosilane gas at 1150 °C. At this high temperature, the trichlorosilane gas decomposes and deposits additional silicon onto the rods, enlarging them according to chemical reactions like 2 HSiCl3 (g) → Si + 2 HCl + SiCl4 at 1400 K Silicon produced from this and similar processes is called polycrystalline silicon. Polycrystalline silicon typically has impurity levels of 1 part per billion or less.

22. Silicon Crystallization Czochralski process The majority of silicon crystals grown for device production are produced by the Czochralski process, since it is the cheapest method available. A small seed crystal of the material to be grown is lowered to the surface of the melt and then drawn upwards, slowly. As the seed crystal is pulled from the melt, it draws with it a layer of molten material. This material cools gradually, taking on the same crystalline structure as the seed crystal.

23. Silicon Crystallization Czochralski process Pellets of dopant material are added to the melt if extrinsic semiconductor material is required. The process is named after Jan Czochralski, who discovered the method in 1916 while investigating the crystallization rates of metals.

24. Silicon Crystallization Float-Zone process When silicon is grown by the Czochralski method the melt is contained in a silica (quartz) crucible. During growth the walls of the crucible dissolve into the melt and Czochralski silicon therefore contains oxygen impurities with a typical concentration of 1018cm − 3. For certain electronic devices, particularly those required for high power applications, silicon grown by the Czochralski method is not pure enough. For these applications, float-zone silicon (FZ-Si) can be used instead. Up to 99.99999999999% (11 nines) purity may be obtained

25. Wafer Preperation •Size the boule •X-ray orient and grind one or more flats •slice •lapping •etching •polishing and cleaning

26. Fabrication Processes