EtchingChapters 11

(20,21 too, but we will return to this topic)

sami.franssila@aalto.fi

Lithography and etching

1) photoresist patterning

2) Etching with reactive chemicals (acids, bases, plasmas)

Etching thin film

Etching bulk silicon

<Si>

<Si>

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.

Isotropic profile

Anisotropic plasma etched profile

Both are needed !

Anisotropic plasma etching is used when profile control and small linewidths are important

Isotropic etching is used when we want to release structures (so that they can bend/move/vibrate)

Etching basics

Photoresist

Thin film 1Thin film 2

Silicon wafer

Initial state After etching

Photoresist consumed

Profile not 90o

Underlying film 2 loss

Selectivity

Selectivity is defined as etch rate ratio:

Silicon etch rate 500 nm/minOxide etch rate 15 nm/minSelectivity 33:1

Silicon etch rate 500 nm/minResist etch rate 200 nm/minSelectivity 2.5:1

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

Wet etchingsolid + liquid etchant soluble products Si (s) + 2 OH- + 2 H2O Si(OH)2(O-)2 (aq) + 2 H2 (g)

Plasma etchingsolid + gaseous etchant volatile products SiO2 (s) + CF4 (g) SiF4 (g) + CO2 (g)

Ion beam etching/ ion millingsolid + energetic ion energetic atom + reflected ionAu + Ar+ Au* + Ar+

Main methods of etching

Isotropic wet etching

• Proceeds as a spherical wave• Undercuts the masking layer• Most wet etching processes are isotropice.g. HF etching of oxide, H3PO4 etching of Al

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 (5:5:2)• Au KI:I2:H2O• Nitride H3PO4 180oC, CVD oxide mask• Pt, Au HNO3:HCl (1:3) “aqua regia”

Terminology: two cases of anisotropic etching

54.7o (100)

(111)

(111)

Anisotropy in plasma etching (and ion milling) is due to directionality of ion bombardment

Wet etched anisotropic only possible in crystalline materials (silicon on this course)

Plasma etcher (= RIE)

Gases thru top electrode

Pumping system

Wafer on lower electrode

plasma

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 waferdirectional 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)

RIE = Reactive Ion Etching

Plasma/RIE etching

Three mechanisms at work simultaneously:

1) Chemical etching (reactive neutrals):spontaneous etching (thermodynamics)

2) Physical etching (ion-assisted):damage creation, broken bondsextra energy supplied (desorption)sputtering (of (CF2)n)

3) Deposition of films ( nCF2* (CF2)n

Flows and reactions in etching

1. etchant flow2. ionization3. diffusion4. adsorption5. reaction6. desorption7. diffusion8. pump out

1 2

3

4

5

6

7

8

3

Plasma/RIE etch 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

Material Etch gas Product gases

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.

Aspect ratio The ratio of height to width

Pillar array AR 5:1

Nanopillar 15:1

Lauri Sainiemi

Nikolai Tsekurov

DRIE: Deep Reactive ion etching

a

b

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)

DRIEof silicon

Photoresist as an etch mask

– Most simple to use– Tolerates RIE: selectivity around 10:1 (=silicon

etches 10 times faster than the resist)– Does not tolerate long RIE– Does not tolerate most wet etchants such as KOH

<Si>

Cleaned silicon wafer

Lithography: Photoresist

spinning

photoresist

Photomask

Lithography: UV-exposure

Photoresist development

Hard mask

<Si>

Cleaned silicon wafer

Thermal oxidation @ 1100 °C

Lithography: Photoresist

spinning

SiO2

photoresist

Photomask

Lithography: UV-exposure

Photoresist development HF etching of SiO2

Photoresist removal

Undercut

KOH etching

DRIE thru-wafer

a

b

c

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

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

Alkaline anisotropic etchants: some main features

Etchant KOHRate (at 80oC) 1 µm/minSelectivity (100):(111) 200:1Selectivity Si:SiO2 200:1

Selectivity Si:Si3N4 2000:1

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

Hot plate sensor

Pt heaternitridemembrane

Pt measurement electrodessensor material

oxide

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 ?

Thermal pressure sensor

heat sink heater resistor thermopile nitride

p0

p0

p1

Silicon DRIE strengths

•Any shape•Any size•Any crystal orientation•High aspect ratio

Silicon anisotropic wet etch strengths

•Extremely simple and reliable

•Smooth surfaces (RMS << 50 nm)

•Exactly defined sidewall angles

•P++ etch stop easy

•Batch process: 25 wafers at a time

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

Microfabrication 31

Polishing (CMP for Chemical Mechanical Polishing)

Smoothing Planarization DamasceneSiO2 Cu or WAl

Rotary CMP tool

Microfabrication 32

60-90s per wafer

Microfabrication 33

CMP: Chemical-Mechanical Polishing

Chemical Mechanical Polishing (CMP) combines

(1) chemical action with (2) mechanical abrasion

to achieve selective material removal.

Polishing in action

Microfabrication 35

Results of CMP

SiC wafer before and after CMPCMP of SiO2

Microfabrication 36

Erosion and dishing in CMP

Size dependent Pattern density dependent

Microfabrication 37

CMP tool input variables

• -platen rotation 10-100 rpm• -velocity 10-100 cm/s• -applied pressure (load) 10-50 kPa• -slurry supply rate 50-500 ml/min• -slurry chemicals (pH, conc.,

viscosity,...)• -pad (material, porosity,

hardness,...)• -abrasives (size,type, hardness,

conc.,...)• -wafer (curvature,

mounting)• -patterns (size, pattern density) • -films (hardness, µ-

structure, stress, ...)

Microfabrication 38

CMP outputs resemble etching

• -polish rate, 100-500 nm/min • -selectivity 1:1 – 100:1

(blind polishing and stopped polishing)• -overpolish time• -pattern density effects• -uniformity across wafer, 10%• -wafer-to-wafer repeatability, 10%

Microfabrication 39

Planarity

Conformal deposition

Surface smoothing

Local: spin-on film

Global: CMP

Photonic crystals by CMP

Log pile photonic crystal fabrication

Si wafer

Poly-Si

oxide

CMP of oxideCVD of oxide CVD of poly-Si

Logpile (2)

Poly-Si litho & etching

CVD of oxide CMP of oxide

CVD of poly-Si

Logpile (3)

1. Layer 1 polysilicon etched2. Deposit CVD oxide 3. CMP of oxide flat surface4. Continue for layers 2-65. Etch all CVD oxide away with HF

Microfabrication 44

CMP vs. etching

• Similar rates 100 – 1000 nm/min

• Similar selectivities 1:1 to 100:1

• Both chemical and physical effects (as in

RIE)

• Need post-processing: Post-CMP cleaning is

chanllenging particle removal, resist

stripping is simpler