Etching Etching Terminology Etching Considerations or ICs Wet Etching Reactie Ion Etching (plasma etching) 1
Etch Process - Figures o Merit Etch rate Etch rate uniormity Selectiity Anisotropy 2
(1) Bias d m etching mask h ilm substrate d Bias B d d m B can be > 0 or < 0. d m substrate d 3
Complete Isotropic Etching Vertical Etching = Lateral Etching Rate B = 2 h Complete Anisotropic Etching Lateral Etching rate = 0 B = 0 4
(2) Degree o Anisotropy A 1 B 2 h 0 A 1 isotropic anisotropic B = 2 B = 0 h 5
Etching o Steps with a Slope θ t θ t start inal ilm x = t 1 x = t 2 l cotθ substrate Let etching time x 1 x 2 = ertical etch rate l = lateral etch rate To minimize x make θ large x= x + x 1 2 = cotθ + t ( ) = t l cot θ = 1/ tan θ = b / a 6
Example θ = 90 o l t t ilm start inal sub θ = 0 o ilm sub 7
Worst-Case Design Considerations or Etching step step height ariation ariation o ilm thickness across waer Mask ilm etching mask can be eroded during ilm etching Substrate (a) Film thickness ariation: h h ( ) ( ) = 1+δ max target thickness alue ariation actor 8
(b) Film etching rate ariation ( ) ( min) = 1 φ Worst case etching time required to etch the ilm ilm h ( max) h = = ( min) ( 1 + δ ) ( 1 φ ) (c) Oeretching around step ariation step h 2 h 1 h2 Oeretch time (raction) = h + h h 1 2 2 9
total ( 1 + δ ) h tt = 1 ( ) ( + ) 1 φ = h h 1 2 (d) With Mask Erosion beore θ W/2 ater h ilm ilm Let m, m// be the ertical and lateral etching rates o the mask. Let be the ertical etching rate o the ilm. (ignoring lateral ilm rate or simplicity) 10
W = ( ) cotθ + // t 2 m m T m = h 1+ δ 1+ ( )( ) ( 1 φ ) cotθ + m// m Goal : Minimize W Small W θ 90 >> ν h small o m 11
Etching Selectiity S m between ilm material and mask material S m V V m ( ertical components only) Wet Etching S m is controlled by: chemicals, concentration, temp. RIE S m is controlled by: plasma parameters, plasma chemistry, gas pressure, low rate & temperature. 12
Examples HF solution mask SiO 2 /Si etched by HF solution SiO 2 Si S SiO2, Si Selectiity is ery large ( ~ ininity) SiO 2 /Si etched by RIE (e.g. CF 4 ) S SiO2, Si Selectiity is inite ( ~ 10 ) 13
For a gien allowable W/2, what is the minimum selectiity required? S m h ( ) ( ) ( 1+ δ )( 1+ ) = min W 2 ( ) cotθ + m// 1 φ m [Note] I m aries rom run-to-run, m S ( ) = ( 1 + φ ) max m (min) = m h W 2 m ( + δ)( + )( + φ ) 1 1 1 φ Um = uniormity 1 actor m cot θ + m // m 14
Wet Etching 1 3 2 1 2 3 Reactant transport to surace Selectie and controlled reaction o etchant with the ilm to be etched Transport o by-products away rom surace 15
Wet Etching (cont.) Wet etch processes are generally isotropic Wet etch processes can be highly selectie Acids are commonly used or etching: HNO 3 <=> H + + NO - 3 HF <=> H + + F - H + is a strong oxidizing agent => high reactiity o acids 16
Si wet etching mechanisms 17
Wet Etch Processes (1) Silicon Dioxide To etch SiO 2 ilm on Si, use HF + H 2 O Etch rate (A/min) 6:1 BOE 1200 650 18 26 T( o C) SiO 2 + 6HF H 2 + SiF 6 + 2H 2 O Note: HF is usually buered with NH 4 F to maintain [H + ] at a constant leel (or constant etch rate) NH 4 F NH 3 + HF 18
Wet Etch Processes (cont.) (2) Silicon Nitride To etch Si 3 N 4 ilm on SiO 2, use H 3 PO 4 (phosphoric acid) (180 o C: ~100 A/min etch rate) Typical selectiities: 10:1 or nitride oer oxide 30:1 or nitride oer Si 19
Wet Etch Processes (cont.) (3) Aluminum To etch Al ilm on Si or SiO 2, use H 3 PO 4 + CH 3 COOH + HNO 3 + H 2 O (phosphoric acid) (acetic acid) (nitric acid) (~30 o C) 6H + + 2Al 3H 2 + 2Al 3+ (Al 3+ is water-soluble) 20
Wet Etch Processes (cont.) (4) Silicon (i) Isotropic etching Use HF + HNO 3 + H 2 O 3Si + 4HNO 3 3SiO 2 + 4NO + 2H 2 O 3SiO 2 + 18HF 3H 2 SiF 6 + 6H 2 O (ii) Anisotropic etching (e.g. KOH, EDP) 21
Eect o Slow {111} Etching with KOH or EDP Mask opening aligned in <110> direction => {111} sidewalls 22
[110]-Oriented Silicon {111} planes oriented perpendicular to the (110) surace => possible to etch pits with ertical sidewalls! Bottom o pits are lat ({110} plane) i KOH is used {100} etches slower than {110} V-shaped ({100} planes) i EDP is used {110} etches slower than {100} 23
(3) Field-Emission Tips 24
Drawbacks o Wet Etching Lack o anisotropy Poor process control Excessie particulate contamination => Wet etching used or noncritical eature sizes 25