1. Etching Chapters 11, 20, 21 (we will return to this topic in MEMS)

2. Lithography and etching
<Si>
1) photoresist patterning 2) Etching with reactive chemicals (acids, bases, plasmas)
Etching thin film
Same procedure applies both to etching thin films and to etching silicon wafer itself. Thicknesses vary !
Photolithography can be redone if problems detected, but after etching no repair is available.
<Si>
Etching bulk silicon

3. Main methods of etching
Wet etching
solid + liquid etchant  soluble products
Si (s) + 2 OH H2O  Si(OH)2(O-)2 (aq) + 2 H2 (g)
Plasma etching
solid + gaseous etchant  volatile products
SiO2 (s) + CF4 (g)  SiF4 (g) + CO2 (g)
Ion beam etching/ ion milling
solid + energetic ion  energetic atom + reflected ion
Au + Ar+  Au* + Ar+

4. Isotropic etching Proceeds as a spherical wave
Undercuts structures (proceeds under mask)
Most wet etching processes are isotropic
e.g. HF etching of oxide, H3PO4 etching of Al

5. Isotropy: good and bad
If you want closely spaced lines (as in comb-drive capacitor), undercutting is bad, but if...

6. Undercutting in action: dome resonator
RIE etching a small hole in polysilicon
Isotropic HF wet etching of oxide under polysilicon
 Membrane can move
H. G. Craighead

7. Ion beam etching
Ar+ ions will hit the wafer, and sputter away surface atoms.
Applicable to all materials.
Very slow process.
Poor selectivity.

8. Plasma etcher (= RIE) RIE = Reactive Ion Etching
RF power applied to bottom electrode
Gas is excited and partly ionized, ion density 1010 ions/cm3
Electric field accelerates ions towards bottom electrode and wafer
directional ion bombardment
Ions assist in etching by
-supplying energy to surface
-breaking bonds
Most etching is done by reative neutrals (their density is 1015 cm-3)
Gases thru top electrode
plasma
Wafer on lower electrode
Pumping system
RIE = Reactive Ion Etching

9. Plasma etched profiles
Franssila: Microfabrication

10. Anisotropic etching of silicon
(100)
(111)
Anisotropic wet etched profiles in <100> wafer. The sloped sidewalls are the slow etching (111) planes; the horizontal planes are (100). Etching will terminate if the slow etching (111) planes meet.

11. Anisotropic wet etched profiles

12. Terminology: two cases of anisotropic etching
Anisotropy in plasma etching (and ion milling) is due to directionality of ion bombardment
In anisotropic wet etching crystal planes etch at different rates
54.7o
(100)
(111)
Franssila: Microfabrication

13. Etch mask Protective layer that is very slowly attacked by etchant
Resist is the simplest etch mask to use  always consider resist mask first
Aggressive etchants will prevent use of resist  hard mask required
Quiz: what is a photomask ? How does it relate to etch mask ?

14. Photoresist as an etch mask
Most simple to use
Tolerates RIE: selectivity 10:1 at best
Does not tolerate long RIE
Does not tolerate most wet etchants such as KOH
photomask
photoresist
Lithography: Photoresist spinning
Lithography: UV-exposure
Photoresist development
Etching with resist mask

15. Selectivity Selectivity is defined as etch rate ratio:
S= rate 1/rate 2
Silicon etch rate 500 nm/min
Oxide etch rate 15 nm/min
Selectivity 33:1
Resist etch rate 200 nm/min
Selectivity 2.5:1
S ≠ ∞ There is always some unintentional loss of material

16. Hard mask photomask photoresist SiO2 <Si> Cleaned silicon wafer
Thermal 1100 °C
Lithography: Photoresist spinning
Lithography: UV-exposure
Undercut
Photoresist development
Photoresist removal
HF etching of SiO2
KOH etching

17. Wet etching vs. plasma etching
Wet etching: chemical reaction; simple wet bench and acids or bases needed
Plasma etching: chemical and physical processes; requires RF-generator, vacuum system and gas lines

18. Etching two-layer films
Ideal case
Real RIE profile
Isotropic, example
<Si>
If two layers are perfectly aligned, they were made in the same etch step.
Otherwise alignment error would be visible.

19. Etching two-layer films, problems
<Si>
One film can be etched anisotropically but the other is etched isotropically
(there is no guarantee that plasma etch profile will be anisotropic)
Both layers isotropic, but different rates

20. Equipment: 1- or 2-sided processing
Beam processes 1-sided Immersion processes 2-sided
-photon beams (=lithography) -liquids (=wet etching)
-atom beams (=evaporation) -liquids (=cleaning)
-ion beams (=implantation) -gases (= oxidation, diffusion)
-mixture of these (=plasmas) -gases (=CVD)

21. Wet etch vs. plasma etch Oxide wet etch in HF Undercut, isotropic
Oxide plasma etch in CHF3
Vertical walls, no undercut
Film removed from backside
Film remains on backside

22. Typical wet etchants SiO2 HF <Si> KOH (10-50%) anisotropic etch
<Si> HNO3:HF:CH3COOH isotropic etch
poly-Si HNO3:HF: H2O
Al H3PO4:HNO3:H2O
W, TiW H2O2:H2O
Cu HNO3:H2O (1:1)
Ni HNO3:CH3COOH:H2SO4
Au KI:I2:H2O
Pt, Au HNO3:HCl (1:3) “aqua regia”

23. Plasma/RIE etching Three mechanisms at work simultaneously:
Chemical etching:
spontaneous etching (thermodynamics)
Deposition of films ( nCF2*  (CF2)n
Physical etching:
damage creation, broken bonds
extra energy supplied (desorption)
sputtering (of (CF2)n)
Vertical walls possible, but not guaranteed

24. Flows and reactions in etching
1. etchant flow
2. ionization
3. diffusion
4. adsorption
5. reaction
6. desorption
7. diffusion
8. pump out 1 2 3 4 5 6 7 8

25. Plasma/RIE etch gases Silicon SF6 (or Cl2) SiF4, SiCl4
Material Etch gas Product gases
Silicon SF6 (or Cl2) SiF4, SiCl4
Oxide CHF3 (or C4F8) SiF4, CO2
Nitride SF6 (or CF4) SiF4, N2
Aluminum Cl2 AlCl3
Tungsten SF6 WF6
Copper no plasma etching practical

26. Mechanism of anisotropy in RIE
Fig. 11.5: All surfaces are passivated by a thin film from CF-gases, but directional ion bombardment will clear films from horizontal surface while leaving passivation film on the sidewalls, enabling etching to proceed vertically only.

27. Aspect ratio The ratio of height to width Pillar array AR 5:1
Nanopillar 15:1
Lauri Sainiemi
Nikolai Tsekurov

28. DRIE: Deep Reactive ion etchingb c
Etch pulse of SF6
Passivation pulse of C4F8
Etch pulse of SF6: remove CF-polymer from bottom, then etch silicon
Sidewall is vertical,
but undulating (scalloping)

29. DRIE
of silicon

30. DRIE thru-wafer Things to consider:
-mask material (hard mask needed !)
-alignment of top and bottom structures
-which side to etch first
-what is the aspect ratio that can be etched
-what wall thickness is strong enough a b c

31. Anisotropic wet etching of silicon
In ANISOTROPIC wet etching some atomic planes etch faster than others
(100) planes are typically fast etching planes
(111) planes are typically slow etching planes
V-grooves etched on (100) silicon wafer

32. Alkaline anisotropic etchants: some main features
Etchant KOH
Rate (at 80oC) 1 µm/min
Selectivity (100):(111) 200:1
Selectivity Si:SiO2 200:1
Selectivity Si:Si3N4 2000:1

33. Membrane formation Nitride membrane; no timing needed
Timed silicon membrane; thickness depends on etch rate and wafer thickness control. Thin membrane thickness control bad.
SOI wafer, membrane thickness determined by SOI device layer thickness

34. Hot plate sensor Things to consider: Silicon etching in the -beginning
-middle
-end of the process ?
Wafer becomes weak when lots of silicon removed.
The thin films may not tolerate KOH/TMAH etching.
Could the top side be protected by something ?
Pt heater
nitride
membrane
Pt measurement electrodes
sensor material
oxide

35. Silicon anisotropic wet etch strengths
Extremely simple and reliable
Batch process: 25 wafers at a time
Smooth surfaces (RMS << 50 nm)
Exactly defined sidewall angles
Franssila: Microfabrication

36. Silicon DRIE strengths
Any shape
Any size
Any crystal orientation
High aspect ratio

37. RIE + isotropic+ anisotropic wet: ink jet nozzle
Shin, S.J. et al: Firing frequency improvement of back shooting inkjet printhead by thermal management, Transducers ’03, p. 380

38. Nanofluidic filter