dry etching - university of washingtonplasma etching • plasma etching involves physical...

R. B. Darling / EE-527 / Winter 2013

EE-527: Microfabrication

Dry Etching


Outline

• Stagnant gas phase etching (XeF2)• Plasma etching (sputter etching)• Ion milling• Reactive ion etching (RIE)• Deep reactive ion etching (DRIE)


The Need for Anisotropy

• For precise micromachining, it is highly desirable for the material removal (cutting) to be directional.

• Isotropic wet etches typically etch as fast laterally as vertically, which requires compensation in their mask and process designs.

• Isotropic wet etches are not capable of making features with aspect ratios (depth : width) greater than unity.

• Anisotropic etching can be achieved through the material (crystal or microstructure anisotropy) or through the process (ion bombardment directionality).

• Dry etching using plasmas is the most used and most versatile method for achieving the high anisotropy that is required for high-aspect ratio or small featured devices.


Etching Anisotropy

• Etch anisotropy determines the amount of masking layer undercut:

substrate wafer

masking layer

undercut undercut undercut

Isotropic EtchingAnisotropic Etchingfrom substrate microstructure

(single crystal silicon)

Anisotropic Etchingfrom direction of bombarding ion

(ion milling or RIE)


Other Reasons for Dry Etching

• Wet release of suspended structures can cause breakage and sticking due to surface tension of liquid pulling surfaces together upon removal.

• Dry etching is carried out at low pressures inside vacuum chambers, so particle contamination is greatly reduced.

• Dry etching is well suited for single wafer processing. • Dry etching allows for precision end point control.


XeF2 Stagnant Gas Phase Etching• Silicon is readily etched by noble gas halogens. • XeF2 is the most commonly used:

– 2XeF2 + Si → 2Xe + SiF4. – XeF2 is a solid at room temperature which can be sublimated

by low pressure (exposure to vacuum). – Typically used at a pressure of 1-2 Torr to give Si etch rates

of 1-3 µm/min. – Very high selectivity: virtually no etch rate for Al, SiO2,

Si3N4, and photoresist. – Leaves a very rough surface: ~10 µm granules. – Exothermic: ~1 W/cm2 of heat produced. – Samples must be thoroughly dehydrated before etching. – 2XeF2 + 2H2O → 2Xe + 4HF + 2O2. (HF etches SiO2!)


XeF2 Etching System

• Refer to Hoffman, et al., MEMS 1995 Conference.

Figure from Kovacs, MMTSB, 1998.


Commercial XeF2 Etching System

• XACTIX model X4 system:

Aluminum cantilevers released using XeF2 system. (XACTIX & Chad O’Neal, Louisianna Tech. Univ.)


Plasma Excitation

• Because of their low density, most gas phase etching chemistries have rates that are too slow at room temperature.

• Increasing the rate requires exciting the reacting species to form radicals and giving these radicals kinetic energy.

• There exist several ways to achieve this: – Direct heating. – Chemical energy from combustion. – Electromagnetically coupled energy from an electrical discharge and used

to create a plasma state.

• Advantages of plasmas: – Electrically controllable with high power efficiencies. – Creates free radicals through ionization. – Creates high kinetic particle energies without high substrate temperatures. – Performed in a vacuum chamber, giving good contamination control.


Plasma Etching

• Plasma etching involves physical bombardment of the substrate by an ion which is nominally inert.

– Ar is the most common gas used for this.

• The impact (momentum transfer) from accelerated Ar+ ions knocks loose substrate ions, called sputter etching or simply plasma etching.

• The etching rate is determined by the sputtering yield:

• The sputtering yield is a function of: – Substrate composition– Incident ion species– Incident ion kinetic energy– Incident ion angle

No. of ejected atomsSputtering YieldNo. of incident ions

S


Plasma (Sputtering) Etch Rates

Material Sputter Etch Rate, Å/minSi 200-380SiO2 260-400Si3N4 250Al2O3 80-130Cr2O3 50FeO 450-490Y2O3 75In2O3 / Sn2O3 80-200LiNbO3 390-420AZ1350 photoresist (novolac resin) 200-250

Rates are for Ar+ ions incident with an energy of 500 eV and with a flux of 1 mA/cm3.

Data from J. S. Logan, Handbook of Plasma Processing Technology, 1990.


Sputtering Yield: Ar+ → Al

Figure from D. N. Ruzic, Handbook of Plasma Processing Technology, 1990.


Sputtering Yield: Ar+ → Si



Sputtering Yield Versus Angle of Incidence


Ar+ ions at 1050 eV


Ion Milling

• Uses a broad-beam Kaufman-type ion source which can separate the energizing plasma from the substrates. – Can reduce plasma damage. Allows a more intense plasma

to be used as the source, e.g. an inductively coupled source. – Can increase the milling etch rate. Higher acceleration

energies can be achieved. – Requires a neutralization system to dissipate the charge that

is accumulated on the substrates. Substrates generally need to be conducting.

– High milling rates can produce significant heating of the substrate. The substrate holder is normally liquid cooled.

• Ion milling is fairly anisotropic and is not particularly sensitive to the type of substrate material.


Veeco Micro-Etch ME601 Ion Mill System

Figure from Veeco ME601 User’s Manual.


Veeco Micro-Etch ME601 Ion Mill System• Uses a 10 cm Kaufman ion source (Ar+); handles 4-inch wafers.

Figure from Veeco ME601 User’s Manual.


Reactive Ion Etching (RIE)• RIE combines chemical etching reactions with physical

ion bombardment. The ionizing gas is no longer inert. • It has been found that the combination of chemical

etching and physical bombardment produces etch rates that are much greater than just the sum of the two processes. The two processes accelerate each other.

• Etchant gases: – Halogens: F2, Cl2, Br2

– Halocarbons: CF4, CF2Cl2, CF3Cl, CCl4, C2F6, C3F8, CHF3, – Other halogen carriers: SF6, NF3, BCl3

– Oxidizer: O2

– Reducer: H2

– Inert: Ar, He


Silicon – Fluorine Chemistry• A fluorine source, such as SF6 or CF4 can be cracked by the

plasma to produce F− radicals. • The F− radicals will preferentially bind to exposed Si atoms,

displacing other atoms sitting on these sites. • Once 4 F− radicals have saturated the available bonds of a Si

atom, the SiF4 will desorb as a volatile species. • Bond energies: (ΔH°)

– Si-Si: 52 kcal/mole (energy to break a bond in single crystal Si) – F-F: 36.6 kcal/mole (energy to break a bond in F2) – S-F: 68 kcal/mole (energy to break a bond in SF6) – C-F: 116 kcal/mole (energy to break a bond in CF4) – Si-F: 135 kcal/mole (energy supplied by creating a bond in SiF4)

• F2 and SF6 will etch Si with no additional supplied energy. • CF4 will etch Si, but requires a little additional energy.


Silicon – Chlorine Chemistry

• Analogous to fluorocarbons, chlorocarbons can be cracked by the plasma, producing Cl− radicals, which can then combine with Si to form SiCl4, which is volatile and desorbs from the etched surface.

• Bond energies: (ΔH°)– Si-Si: 52 kcal/mole (energy to break a bond in single crystal Si) – Cl-Cl: 58 kcal/mole (energy to break a bond in Cl2) – C-Cl: 81 kcal/mole (energy to break a bond in CCl4) – Si-Cl: 90 kcal/mole (energy supplied by creating a bond in SiCl4)

• Chlorine etching always requires additional energy from the plasma, so it is always anisotropic.


Gas Feedstock Rules of Thumb

• For etching Si: – Fluorines are natively isotropic. Adding oxygen makes them increasingly

anisotropic. – Chlorines are natively anisotropic.

• Oxygen will ash most organic films, such as photoresist residue, producing CO2 and H2O. Don’t add oxygen if a fluorocarbon sidewall passivation is desired, as the O2 will remove it.

• Hydrogen will create HF with a fluorine chemistry and produce etching of SiO2, often preferentially to that of Si. This can be useful for sidewall passivation to achieve higher anisotropy, and for achieving greater Si/SiO2 etch selectivity.

• Argon will not affect the chemistry, but can be added when additional ion bombardment is needed. This makes most etches more anisotropic, but dilutes the reacting species.


Plasma Etching Effects – Loading• The etch rate decreases when the exposed area to be etched

increases. • This is usually caused by depletion of the feedstock gas and/or

build-up of reaction products. • R = R0 / (1 + kA)

– R0 = etch rate for nominally zero etch area (an asymptote). – K = a constant for a given reaction chamber. – A = exposed area to be etched.

• Plasma loading occurs for both the overall set of wafers in the chamber, as well as locally within the mask pattern of a given die.

• Design rules for mask layout are often developed to make all etch features identical to avoid local loading.

– Exact size vias and contacts are common. – Via and contact density rules are also common.


Plasma Etching Effects – Trenching

substrate

resist

Trenching of substrate occurs adjacent to resist edges.


Plasma Etching Effects – Redeposition

substrate

resist

Redeposition of sputtered resist occurs adjacent to resist edges.


Sidewall Polymerization – 1• Most RIE processes exhibit some degree of sidewall passivation. • Sidewalls receive less ion bombardment which can allow

passivation reactions to dominate over etching. • The bottom of a trench receives maximum ion bombardment

which prevents the passivation layer from building up. • Example: SF6 / O2: SF6 is normally almost isotropic, but

dilution with O2 allows SiO2 to form on sidewalls which passivates further sidewall etching. More O2 makes the etch increasingly anisotropic.

• Example: CF4 / H2: Fluorocarbons can etch or polymerize depending upon the F/C ratio: F/C > 3 gives etching, F/C < 2 gives polymerization. Adding H2 forms HF which can etch oxides, giving Si/SiO2 selectivity. Forming HF also reduces the available F−, so polymerization is enhanced. CHF3 is also used for achieving this.


Sidewall Polymerization – 2• Fluorocarbon RIE usually creates a –CF2– polymer on the

sidewalls, similar to Teflon® PTFE. • The chemical inertness of this polymer can be useful for

subsequent process steps, but this also makes its removal difficult. Oxygen plasma ashing is usually needed for removal.

• For low to moderate trench aspect ratios (depth : width), this polymerization can produce nearly vertical RIE sidewalls.

• For deeper trenches, it becomes increasingly difficult to keep the sidewalls shielded from bombardment from glancing angle ions, and the polymer layer is eroded as fast as it is created.

• Single chemistry RIE reaches a limiting aspect ratio of around unity for near vertical sidewall profiles.

• A solution is to use a two-chemistry / two-phase approach. – This is the basis for DRIE.


RIE Chemistries

Material RIE Gases RemarksSi CF4/O2, SF6, NF3 Nearly isotropicSi Cl2, BCl3, CCl4 Anisotropic, masked by SiO2

Si HBr, CF3Br AnisotropicSiO2 F2/H2, CHF3/C2F6, CHF3/CO2 Minimal etching of SiSi3N4 CF4, CHF3, SF6, NF3

TiSi2 CCl2F2, CCl4 Control of oxygen impurityWSi2 CF4/O2, SF6

W CF4/O2, SF6

Al Cl2, BCl3, CCl4, SiCl4 Removes native oxidesAl(Cu) Cl2, BCl3, CCl4, SiCl4 Removes native oxidesPolymers O2, O2/CF4 “Ashing”

Data from G. S. Oehrlein, Handbook of Plasma Processing Technology, 1990.


Example System: Drytek DRIE 100

• This was one of the first systems to handle multiple wafers for planar plasma processing, designed mainly for RIE processing.

• The system shown below is in the Stanford Center for Integrated Systems (CIS).

Although its model number is DRIE 100, this system is NOT a deep reactive ion etcher.

Photo from Stanford NanoFabrication Facility.


Deep Reactive Ion Etching (DRIE)• This is one of the few process tools that was developed

specifically for MEMS applications. • Lärmer and Schilp (Bosch) Deutsch patent of 1994:

– Alternate between etching and polymer deposition. (2 phases) – Etching phase removes the polymer on the bottom of the trench. – Polymerization phase protects the sidewalls from etching.

• Etching phase: – SF6 / Ar used with -5 to -30 V of substrate bias to produce nearly vertical

incident ions. This creates an anisotropic SF6 etch without needing O2.

• Polymerization phase: – CHF3 or C4H8 / SF6 used. The sidewall polymer is –CF2– , teflon-like.

• Can obtain nearly vertical sidewalls with ~30:1 aspect ratios. • Sidewalls have a characteristic scalloping that corresponds to

each cycle of the etching / polymerization phases.


DRIE Process

SF6 / Ar ETCH

CHF3 / H2 POLYMERIZATION

SF6 / Ar ETCH

Mask Layer

Mask Layer

Mask Layer

Mask Layer

Silicon Substrate

Silicon Substrate

Silicon Substrate

Silicon Substrate


DRIE Examples

• Commercial equipment is produced by STS, Plasma-Therm, Oxford Instruments, and Trion.

Klaassen et al. ‘95 (Stanford)

STS ‘99

20µm


Oxford Instruments PlasmaPro 100 DRIE System

Photo from Oxford Instruments.

dry etching - university of washingtonplasma etching • plasma etching involves physical...

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