Lecture 5: Micromachining

Transcription

1 MEMS: Fabrication Lecture 5: Micromachining Prasanna S. Gandhi Assistant Professor, Department of Mechanical Engineering, Indian Institute of Technology, Bombay,

2 Recap: Last Class E-beam lithography X-ray lithography Ion beam lithography Oxidation

3 Today s Class Si wafer preparation Clean room fundamentals Chemical etching process Anisotropic Etching Silicon micromachining Surface micromachining Bulk micromachining How to produce devices

4 Si-wafer preparation Czochralski process Cutting, CMP, cleaning

5 Clean Room Fundamentals Need Class of a clean environment Class X clean room not more than X particles (of size 0.5µm or larger) per cubic foot of air How this cleanliness is produced and maintained??

6 Clean Room Fundamentals Air conditioning plant HEPA filters air recirculation through these filters

7 Chemical Etching Without agitation (5) With agitation (20) Isotropic etching Etchant: HNA mixture. HNA can dissolve 550µm thick silicon wafer in about 20 min. HNA mixture removes silicon equally in all directions. SiO2 etch: 10-30nm/min

8 Chemical Etching Without agitation (5) Isotropic etching Undercut Etch bias Materials & etchants* Lift off process

9 Chemical Etching Mechanisms Anodic Dopant as etch stop Precautions Etch diagrams

10 Chemical Etching Choice of etchant: Ease of handling Toxicity Etch rate Topology of the surface to be etched Etch selectivity of mask material and other materials

11 <100> Chemical Etching <111> <001> <010> <100> surface wafer <110> surface wafer Anisotropic bulk etching Etchant: KOH, EDP (ethylendiamine, pyrocatechol and water), TMAH <111> direction has lower etching rates than <100> Can produce grooves, slanted/vertical walls

12 <100> Chemical Etching <111> <001> <010> Silicon crystal geometry* Examples of use of the crystal geometry in etching Fundes regarding etch shapes under different conditions*

13 <100> Anisotropic Etching <111> <001> <010> KOH, EDP and TMAH EDP etches oxide 100 times slower than KOH, KOH, TMAH dangerous to eye KOH less dangerous than EDP & TMAH Etch curves*: 5hrs to etch 300µm thick wafer H2 bubbles during KOH etching of Si EDP ages quickly in contact with oxygen producing red brown color, vapor is harmful HF dip is necessary for EDP: native oxide problem

14 Table 5.1 : Negative and Positive Resisits Lithography Name Type Sensitivity γ Optical Kodak 747 Negative 9 mj/cm AZ-1350J Positive 90 mj/cm PR102 Positive 140 mj/cm e-beam COP Negative 0.3 µc/cm GeSe Negative 80 µc/cm PBS Positive 1 µc/cm PMMA Positive 50 µc/cm X-ray COP Negative 175 mj/cm DCOPA Negative 10 mj/cm PBS Positive 95 mj/cm PMMA Positive 1000 mj/cm 2 1.0

15 (100) n-type Primary flat (100) p-type Primary flat Secondary flat Secondary flat (111) n-type Primary flat (111) p-type Primary flat Secondary flat Primary and Secondary wafer flats are used to identify orientation and type.

16 [110] A 54.7 o Top View A Cross Section A-A A square <110>-oriented mask feature results in a pyramidal pit.

17 [110] A [100] Masking Layer [100] Silicon A A Cross Section A-A Convex corners are rapidly undercut A A [100] 54.7 o Cross Section A-A A Convex corners where {111} planes meet are not stable. They are rapidly undercut. This permits creation of suspended structures.

18 [110] A Boundary of rectangular pit [100] Masking Layer Undercut regions Cross Section A-A A Any mask-layer feature, if etched long enough, will result in a rectangular V-groove pit beneath a rectangular that is tangent to the mask features, with edges oriented along <110> directions.

19 [110] Boundary of rectangular pit 5 o misalignment The effect of misalignment is to enlarge the etched region. This figure shows the effect of a 5 o misalignment for a rectangular feature.

20 Re-entrant resist profile Wafer with photoresist Directional evaporation (e-beam) Strip resist and lift off metal Illustrating the lift-off method for patterning evaporated metals.

21 Wafer with double-layer undercut masking Directional evaporation gradually closes opening At closure, a sharp tip is formed After lift-off The use of a modified lift-off process to create sharp tips.

22 Conclusions Chemical etching Isotropic Anisotropic Liftoff process Bulk micromachining

23 Next class Plasma based processes Plasma etching RIE Sputtering PE CVD

24 Wafer Cleaning Process Need CMP- Chemical mechanical polishing RCA cleaning

25 Chemical Vapor Deposition (CVD) Gases Wafer Temperature > 300 o C Chemical reaction in vacuum chamber High temperatures (>300 o C) Polysilicon, SiO 2, Si 3 N 4, tungston, titanium, copper etc. can be deposited Low pressure CVD (LPCVD) Plasma Enhanced CVD: low temperatures Pressure, temp, gas flow