Silicon 012

8/8/2019 Silicon 012

http://slidepdf.com/reader/full/silicon-012 1/71

SILICON VLSI TECHNOLOGYSILICON VLSI TECHNOLOGYSILICON VLSI TECHNOLOGYFundamentals, Practice and Modeling

CHAPTER 12CHAPTER 12CHAPTER 12

EX0250

8/8/2019 Silicon 012


8/8/2019 Silicon 012


References

• HNA

• Schwartz and Robbins, “Chemical Etching of Silicon, IV

Etching Technology,” Journal of Electrochemical Society:Solid-State Science and Technology, Vol. 123, No. 12,Dec. 1976, pp.1903-1909.

• EDP• Reisman, Berkenblit, Chan, Kaufman and Green, “The

Controlled Etching of Silicon in CatalyzedEthylenediamine-Pyrocathechol-Water Solution,” Journalof Electrochemical Society: Solid-State Science andTechnology, Vol. 126, No. 8, Aug. 1979, pp.1406-1415.

8/8/2019 Silicon 012


Continue• KOH

• Seidel, “The Mechanism of Anisotropic Silicon Etchingand Its Relevance for Micromachining,” Tranducers’87,pp.120-125.

• Don L. Kendall and G R. deGuel, “Orientation of the ThirdKind: The Coming of the Age of (110) Silicon,” inMicromachining and Micropackaging of Transducers, ed.By C. D. Fung, P. W. Cheung, W. H. Ko and D. G.

Fleming, Elsevier Science Publishers, B. V. Amsterdam,1985, pp.107-113.

• TMAH

• Schnakenberg, Benecke and Lange, “TMAHW Etchantsfor Silicon Micromachining,” Transducers’91, pp.815-818.

• Tabata, Asahi, Funabashi and Sugiyama, “AnisotropicEtching of Silicon in (CH3)4NOH Solution,”

Transducers’91, pp.811-814.

8/8/2019 Silicon 012


Wet Etchants

• Isotropic: HNA or HNW• Anisotropic: KOH, EDP, TMAH

• Etching stop: silicon doped with boron• Note: selectivity and anisotropy of wet

chemical etch is far superior to that

attainable with dry etching. Emphasis is on

chemical etching

8/8/2019 Silicon 012


8/8/2019 Silicon 012


Chemical Etching Process

• Reactants transport from bulk solution tosurface.

• Surface reaction

• Reaction products transport from surface to

bulk solution.

8/8/2019 Silicon 012


Example: HNA

KOH, TMAH, etc.

8/8/2019 Silicon 012


8/8/2019 Silicon 012


Dangerous, avoid

Etch rate changes drastically

small variation of compositio

avoid

Most useful,

smooth surface

8/8/2019 Silicon 012


More Explanations• In the 49.25% HF – 69.51% HNO3 - CH3COOH (or water)

system, in the region around HF vertex, the reaction isprimarily oxidation reduction controlled. One has rough,pitted surfaces, and sharply peaked corners and edges. In

the region around HNO3 vertex, the reaction is diffusioncontrolled. Different crystallographically oriented surfacesshould etch at the same rate. The etch rate on both (111),the slowest etching plane, and the (110), one of the fastest

etching planes, has the same etch rate. One has specularsurface and rounded corners. In the region between HF andHNO3 vertex, it has rectangular corner.

8/8/2019 Silicon 012


8/8/2019 Silicon 012


and consistent etch rate for H2

O more

than 25% can not be obtained.

8/8/2019 Silicon 012


More Descriptions

• A: fast etch (chemical reaction), difficult to

control because of the extreme sensitivity of therate to small fluctuations in composition. producesheat and many bubbles of H2,

• B: etching rate slow down by lack of oxide, coverthe largest range of compositions. The etch rate isalmost identical in this region. The boundary

between B and F is located where the etch ratesbecome too low to be generally practical fordevice fabrication.

8/8/2019 Silicon 012


Continue

• C: the entire surface of Si is covered with a sheath of SiO2.The more reactive centers may begin to etch first, but theysoon become covered with an oxide layer. The oxideprotects the reactive center from excessive oxidation whilethe remainder of the surface catches up. The net effect isthat the hydrofluoric acid, which is the rate-limiting factor,

acts on a uniform layer of oxide. The result is a smooth,specular surface with rounded corners and edges.

• In summary: nonselective, producing specular surfaces and

rounded geometry. The surface quality is mediocre at theupper end of the region, but it improves a short distancefrom region A.

C i

8/8/2019 Silicon 012


Continue

• Line between region D and E corresponds to a

transition of mechanism from HF-limited process

to an HNO3-limited process. The geometry of specimens etched in regions D and E is square as

the result of the transition to a catalyst coupling

mechanism. In D, the reaction is relatively moreHNO3 dependent, and vestigial evidence of

selective etching is apparent. This leads to rough

surface. In E, the reaction is more HF dependent.

This leads to highly lustrous, specular surfaces.

8/8/2019 Silicon 012


Example Problems

• A suitable sizing etch for thinning a silicon chip. Toremove a rather large amount of material, control theresultant thickness within narrow tolerances, retain

square edges and corners and preferably end up with asmooth surface for subsequent operation.

• Area B is not ideal because of the catalyst coupling factor;rates are too erratic and they are also too much too

sensitive to small composition variations. The surfaces aretoo rough, although the geometry is favorable. In C, therate varies slowly with composition and surfaces are good,but the geometry is unfavorable. Possible solution is in thelower region of C where the geometry turns square, orpreferably in area E. Two step etch is used. First, etch in Bwhere the rates are moderate but the catalyst coupling isassured, and then finished up with a composition fromregions C or E.

8/8/2019 Silicon 012


8/8/2019 Silicon 012


• Fresh EDP is light brown. When oxidized, it becomes dark and stain everything. It smells very much and has to be

done in enclosed space.• As etching goes on, the EDP decreases with increase in

water content. This will increase the etching rate.

• The significant reduction in etch rate in heavily doped Si(with boron) is due to the depletion of ion caused by boron,which reduces the reaction rate.

• Light emission can cause ion, which can increase thereaction rate. <100> is faster than <110> by 20%.

8/8/2019 Silicon 012


Note: do not etch less than 100o

C for B or F etch since thiswill form insoluble residues (white powder).

8/8/2019 Silicon 012


8/8/2019 Silicon 012


• Concentration at 20~50 wt% is good. At

higher concentration, it is dangerous.

• KOH is transparent like water. No smell,

can be treated like water.

• Below 20%, residues (white powder) can be

formed.

• K is bad for IC, people uses NH3OH.

8/8/2019 Silicon 012


8/8/2019 Silicon 012


1. If IPA is added, then R100 >

R110. However, IPAvaporizes at high

temperature.

2. During etching, theconcentration of KOH

should be monitored. More

KOH is added if it is

consumed.

3. Rule of thumb: lower

etching rate, more uniform

surface can be obtained.

4. In lab., one can not obtain

a high etch ratio. This may

be due to the alignment

problem.

8/8/2019 Silicon 012


8/8/2019 Silicon 012


Typically uses 50~90

o

C and 40% wt concentration.Possible temperature can reach 110oC. For 50%

concentration boilin tem erature of KOH is 120oC.

8/8/2019 Silicon 012


8/8/2019 Silicon 012


Other Problems• Undercut

• Corner compensation (refers to convex

corner)

• Alignment

8/8/2019 Silicon 012


8/8/2019 Silicon 012


Wafer Bonding

8/8/2019 Silicon 012


8/8/2019 Silicon 012


Ref: C. Harendt, W. Appel, H. G. Graf, B. Hofflinger and E. Penteker,J of Micromechanics and Microeng. Vol. 1, 1991, pp.145-151.

8/8/2019 Silicon 012


8/8/2019 Silicon 012


Ref: A. Hanneborg, M. Nesse and P. Ohlckers, J. Micromech. Microeng., Vol.

1, 1991, pp.139-144.

8/8/2019 Silicon 012


Silicon 012

Documents

Transcript of Silicon 012