1. SAMPLE PREPARATION TECHNIQUES
DEPROCESSING - DRY ETCHING
EMT 361
SCHOOL OF MICROELECTRONICS
KUKUM

2. DRY ETCHING?
Dry = no wetting of IC packages / dies==> no liquid chemicals
==> dry chemicals?
gas / plasma phase chemicals
- mode of action = chemical + bombardment

3. MODE OF ACTION Chemical std stoichiometric reactions Bombardment
acceleration of ions by EF - collision
Combination

4. CONSEQUENCES… Selectivity : < wet etching
Anisotropy: > wet etching.
Etch Rate: < wet etching.
Etch Control: much easier to start / stop than wet etching.

5. ON THE OTHER HAND
IC’s sport smaller gates , thinner intervening layers
=> the challenge - remove layers of correct composition & thickness only
eg: typ. passivation, dielectric thickness = m

6. 2 MAJOR CLASSES
NON-PLASMA BASED
PLASMA BASED

7. NON-PLASMA BASED Isotropic etching of Si
Typically F-containing gases (fluorides or interhalogens) that readily etch Si
High selectivity to masking layers
No need for plasma processing equipment
Highly controllable via temperature and partial pressure of reactants

8. NON-PLASMA BASED I. Xenon Difluoride (XeF2) Etching:
· Isotropic etching of Si
· High selectivity for Al, SiO2, Si3N4, PR, PSG
2XeF2 + Si --> 2Xe + SiF4
· Typical etch rates of 1 to 3 mm/min
· Heat is generated during exothermic reaction
· XeF2 reacts with water (or vapor) to form HF
II. Interhalogen (BrF3 and ClF3) Etching:
· Nearly isotropic profile
· Gases react with Si to form SiF4
· Surface roughness: ~40 to 150 nm
· Masks: SiO2, Si3N4, PR, Al, Cu, Au, and Ni

9. XeF2 -white solid -Tr vapour pressure = 4T
-Si etch & Tr -vac. pump.
-etch rate m/min
-good S over Si & SiO2 , Si3N4 , Al, PR
But XeF2 + 2H2O --> H2O + 2HF ==>SiO2
-etch features granular structures~<10m ;
smoothened via interhalogens with Xe diluent

10. S of Si over xxx with BrF3 Si : SiO2 (LPCVD) = 3000:1
Si : Si3N4 = :1 ; Si conc.
Si : AZ4400&AZ1518 (hb) = 1000:1
Si : Al,Cu,Au,Ni = 1000:1

11. PLASMA BASED RF power is used to drive chemical reactions
Plasma takes place of elevated temperatures or very reactive chemicals
Plasma = partially ionized gas; “+” = “-“ and a different number of neutral molecules.
An ion-electron pair is continuously created by ionization and destroyed by recombination

12. PLASMA…???
Plasmas=conductive assemblies of charged particles, neutrals and fields that exhibit collective effects; carry electrical currents and generate magnetic fields; most common form of matter, >99% of the visible universe, and permeate the solar system, interstellar and intergalactic environments.
States of Matter Primary Natural Systems
solids condensed matter, compact (nuclear)
liquids,neutral gas fluid(Navier-Stokes) systems
plasmas electromagnetic(Maxwell-Boltzmann) systems

14. PLASMA..SEMICONDUCTOR INDUSTRY
an ionized gas with equal numbers of +ve & -ve charges.
e- come from neutral particles =>ionization produces charged particles ==># e- = # ions==> ne=ni
Ionization rate=ne/(ne+nn)
PECVD = %
Parallel plate etcher = 0.01%
ICP & ECR ~ 1-5%

15. PLASMA …COLLISIONS 1.IONIZATION-electron impact ionization
2.EXCITATION-insufficient energy transfer = de-excitation=energy release
3.DISSOCIATION-chemical bond breakage==free radical generation==highly reactive==chemical plasma process.
CVD Oxide etch: e-+CF4==>CF3+F+e-
PECVD silane: e-+CF4==>CF3+F+e-
PECVD oxide: e-+SiH4==>SiH2+2H+e-
e-+N2O ==>N2+O+e-

16. Various reactions and species
present in a plasma

17. PLASMA FORMATION 1. chamber evacuated 2. chamber filled with gas(es)
MHz RF E - electrode pair (>100kHz)
4. electrons accelerated = K.E up
5. Electrons-neutral gas molecules collisions= ions & more e’s
6. Steady state; ionization = recombination
Plasma discharge -central glow / bulk region and dark / sheath regions near electrodes
Bulk region = semi-neutral (nearly equal # of e’s and ions)
Sheath regions = nearly all of the potential drop; accelerates ions from bulk region which bombard the substrate
Maintained at 1 Pa (7.5 mTorr) to 750 Pa (5.6 Torr) with gas density of 2.7 x 1014 to 2 x 1017 molecules/cm3

18. AN e IN PLASMA Typical (KE) ~ 2-8 eV KE = 1/(2 mV2) = 3/(2 kT)
m = particle mass
V = particle mean velocity
k = Boltzmann constant
T = temperature (K)
=> 2 eV electron has T » 15,000 K and V » 6 x 107 cm/s = 1,342, mph

19. PLASMA BASED-3 TYPES
1) chemical reactions - chemically reactive gases or plasma = Plasma etching
2) physical removal - momentum transfer = Physical Sputtering and Ion Beam Milling
3) combination - physical removal and chemical reactions = Reactive Ion Etching

20. Common materials to dry etch Difficult materials to dry etch
Dry etching methods
Glow discharge methods
Dry physical etching (Sputter etching)
Plasma assisted etching
Dry chemical etching (Plasma etching)
Reactive ion etching (RIE)
Ion beam methods
Ion milling
Reactive ion beam etching
Chemical assisted ion milling
Common materials to dry etch
Si, SiO2, Si3N4, Al, W, Ti, TiN, TiSi2, Photoresist
Difficult materials to dry etch
Fe, Ni, Co, Cu, Al2O3, LiNbO3, etc.

21. PE
dry & anisotropic
RF energy is applied to a separate electrode with the substrates grounded.
Purely chemical etching
Glow discharge is used to produce chemically reactive species (atoms, radicals, or ions)

22. PE Take a molecular gas Establish a glow discharge
CF4
Establish a glow discharge
CF4+e  CF3 + F + e
Radicals react with solid films to form volatile product
Si + 4F  SiF4 
Pump away volatile product (SiF4 )

23. PE = chemical Due to their incomplete bonding,
free radicals are highly reactive
chemical species.

24. SPUTTERING / ION ETCHING / MILLING
Normal impinging - f(Emax)
Emax = cathode
Emax = anode
Since p transfer = billiard ball ; volatility not critical==> etch rate variance small
==>masking problem
Large Iion-- hi P==>lo mfp==redeposition

25. SPUTTER
Plasma energizes chemically inert projectile - moves at high v & strikes substrate
p transferred during collision
Substrate atoms dislodged IF projectile KE>Ebond
~ ion implantation, but low energy ions -to avoid implantation damage
Highly anisotropic
Etch rates for most materials are comparable (i.e., no masking)
Argon is the most commonly used ion source
Damage to underlying material => may change device properties
Rarely used in VLSI ; OK FOR FA

26. SPUTTER
Ions of sufficient E impinge vertically - p transfer - bond breakage - ballistic material ejection
<3 - 5eV - reflection / physisorbed
4 - 10eV - surface migration & damage
>10eV - subs. heating, surface damage & material ejection
>10keV - ion implantation

29. RIE
RF energy to the substrates; low pressure halogen-rich
material removed by combined action
Greater control over line widths and edge profiles is possible with oxides, nitrides, polysilicon and aluminum
==> bombardment of surface being etched with accelerated reactive ions.
==> accelerated ions sputter material off the substrate as they hit its surface, as well as react with the substrate material.
==> etching is accomplished by two processes: sputtering and chemical reaction
highly anisotropic
widely used in VLSI / FA

30. ---------------------------------~ F-
RIE - two electrodes - create an electric field - accelerate ions toward the surface of the samples.
plasma contains both positively and negatively charged ions in equal quantities.
Ions- generated from gas pumped into chamber.
O2 and CF4 gasses -plasma with many F- ions - accelerated in the EF- collide into the surface of the sample.
hard mask to protect certain areas from etching- exposing only the areas desired to be etched.

31. photoresist mask on silicon dioxide- etching ions accelerated into the etching region- combine with silicon dioxide and then are dispersed.
EF accelerates ions toward the surface, the etching caused by these ions is much more dominant than the etching of radicals - ions traveling in varied directions, so the etching is anisotropic.

32. RIE ETCH CHEMISTRIES Material Being Etched Etching Chemistry
Deep Si trench
Shallow Si trench
Poly Si Al
AlSiCu W
TiW
WSi2, TiSi2, CoSi2
Si02,
Si3N4
HBr/NF3/O2/SF6
HBr/Cl2/O2
HBr/Cl2/O2,HBr/O2,BCl3/Cl2,SF6
BCl3/Cl2,SiCl4/Cl2, HBr/Cl2
BCl3/Cl2/N2
SF6 only, NF3/Cl2
SF6 only
CCl2F2/NF3,CF4/Cl2,Cl2/N2/C2F6
CF4/CHF3/Ar,C2F6,C3F8,C4F8/CO, C 5F8,CH2F2
CHF3/02,CH 2F2,CH2CHF2

33. …more details Material Etch Gases Reactive Species By-product
Al Boron Trichloride and Chlorine Free chlorine AlC13
Polyimide Oxygen Monatomic oxygen and/or ozone CO and H20
Poly-Si CF4 / 8%ﾊ SF6 or Chlorine Free fluorine or free chlorine SiF4 or SiCl4
SiO2 Freon 14 and Freon 23 CF3 SiF4 and CO
Si3N4 Freon 14 (CF4) / 8% O2 Free fluorine SiF4 and N2
or Sulfur Hexafluoride (SF6)

34. RIE Highly anisotropic etching Less consumption of chemicals
Precise pattern transfer
High resolution
Less consumption of chemicals
Cost effective
Environmentally benign
Clean process
Vacuum
Ease of automation

35. Chem. vs Chem./Phys. Purely chemical etching (using only reactive
neutral species)
Isotropic etching
Chemical + physical etching
(using reactive neutral species
and ionic species)
Anisotropic etching

38. Anisotropic etching of Passivation (15,000Å Silicon Nitride) and intermetal dielectric (8,000Å Silicon Dioxide) Exposing both metal 1 and metal 2. 

39. Anisotropic etching of Passivation (15,000Å Silicon Nitride) and intermetal dielectric (8,000Å Silicon Dioxide) Exposing both metal 1 and metal 2

40. Material Etched: Si3N4 and SiO 2
Number of Metal Layers: 5
Chemistry : Fluorine Chemistry