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

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

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

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

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 = 1 - 20m

2 MAJOR CLASSES NON-PLASMA BASED PLASMA BASED

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

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

XeF2 -white solid -Tr vapour pressure = 4T -Si etch exo @1-4T & Tr -vac. pump. -etch rate 10 - 1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

S of Si over xxx with BrF3 Si : SiO2 (LPCVD) = 3000:1 Si : Si3N4 = 400-800:1 ; Si conc. Si : AZ4400&AZ1518 (hb) = 1000:1 Si : Al,Cu,Au,Ni = 1000:1

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

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

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 = 0.0001% Parallel plate etcher = 0.01% ICP & ECR ~ 1-5%

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-

Various reactions and species present in a plasma

PLASMA FORMATION 1. chamber evacuated 2. chamber filled with gas(es) 3. 13.56MHz 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

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 = 1,342,161.76 mph

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

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.

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)

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 )

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

SPUTTERING / ION ETCHING / MILLING Normal impinging - f(Emax) Emax = eVT=e(|VDC|+VP) @ cathode Emax = eVP @ anode Since p transfer = billiard ball ; volatility not critical==> etch rate variance small ==>masking problem Large Iion-- hi P==>lo mfp==redeposition

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

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

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

--------------------------------- ~ 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.

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.

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

…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)

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

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

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

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

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