Part 4 - Etching

Etching

Etching is a process to transfer the pattern of photolithography to the underneath layer. There are two types of etching: wet etching and dry etching.

Wet VS. Dry Etching

In wet etchants, the etch reactants are in liquid phase such that they could be dissolved.

In dry etchants, the etch reactants come from a gas or vapor phase source and are typically ionized (e.g., reactive ion ethcing, RIE).

Etching considerations

• Etch Rate
  • Rate of material removal [$\mu$m/min]
  • It is a function of concentration, mixing, temperature, $\ldots$
• Etch Selectivity
  • Relative (ratio) of the etch rate of the film to the mask, substrate, or another film.
• Etch Geometry
  • Anisotropic: etch rate is direction-dependent
  • Isotropic: etch rate is independent of direction
  • Sidewall slope (degree of anisotropy)

Dry Etching

To perform the dry ethcing process, we perform different steps. Let’s take a look.

Mechanisms in Dry Etching

1. Etchant Species Generation
2. Diffusion to Surface
3. Adsorption
4. Reaction
5. Desorption
6. Diffusion to Bulk

Similar to CVD (Chemical Vapor Deposition), the process include reaction and transporation (i.e., diffusion).

Gas Phase (Chemical) Dry Etching: XeF$_2$

When dry etching with a chemical gas, we would typically use XeF$_2$.

The XeF$_2$, during the adsorption step, reacts and will split to Xe and F.

Particular features of XeF$_2$ etching:

• Isotropic
• High ethcing rate, 3-10 $\mu$m/min for Si at around 1 torr (1 atm = 760 torr).
• High selectivity to SiO$_2$, Si$_3$N$_4$, Al, and photoresist (i.e. these could be used as etch masks).
• Very sensitive to moisture, forming HF which attacks many materials (e.g., SiO$_2$).

Plasma Etching: RF Plasma Physics

In plasma etching, we use a plasma to etch the material. The plasma is a partially ionized gas.

The frequencies are typically between 5 MHz - 13.56 MHz.

Electrons in gas molecules oscillating in the high-frequency region acquire enough energy to cause ionization.

When performed under low ion bombardment and caused mainly by chemical reactions with plasma species.

The wafer placed in the ground electrode is with low potential and with weak ion bombardment.

One important property is that physical etching is negligible, the process is also isotropic.

Sputtering Etching

Sputtering etching is a process to etch the substrate (at cathode) by the bombardment of high energized ions in the plasma.

This is a physical process by surface bombardment with high-energy ions of inert gas (e.g. Argon).

Reactive Ion Etching (RIE)

This is the most used dry etching technique, based on the combination of chemical activity of reactive species generated in the plasma with physical effects caused by ion bombardment.

Self bias is used to accelerate ions towards the wafer.

The high energy ion bombardment is in the range 300 - 700 V.

Lower operation pressure, typically 10 - 200 mtorr (usually lower than 0.1 tor).

Isotropic Wet Etching

Isotropic wet etching is a process where the etch rate is independent of the direction.

Etch at equal rates in all directions. This will round sharp edges or corners. It will also remove roughness and or damaged surfaces.

It is possible to achieve nearly vertical sidewall by long over-etching. However, it limits the minimal gaps between the structures.

Common Wet Etchants

• Silicon Dioxide (SiO$_2$) Wet Etching
  • SiO$_2$ + 6HF $\rightarrow$ H$_2$SiF$6{(aq)}$ + 2H$_2$O (HF Etchant)
  • Etches SiO$_2$ but no Si, may also attack AL, Si$_3$N$_4$, $\ldots$
• Silicon Nitride (Si$_x$N$_y$) Wet Etching
  • Using H$_3$PO$_4$ (Phosphoric Acid) or HF.
  • Does not etch Si, may attack Al, SiO$_2$ and other metals.
• Silver (Ag) Wet Etching
  • Can be etched in acidic or basic etchants, like NH$_4$OH + H$_2$O$_2$ + CH$_3$OH
  • Photoresist can be used as a mask.
• Aluminum (Al) Wet Etching
  • Typical Etchant: (H$_3$PO$_4$ : HNO$_3$ : CH$_3$COOH : H$_2$O = 4 : 1 : 4 : 1)
  • Nitride acid form aluminum oxide on surface and phosphoric acid and water dissolve the material.
  • Positive photoresist can be used as a mask.
• Titanium (Ti) Wet Etching
  • HF based solution or NH$_4$OH + H$_2$O$_2$ + H$_2$O.
• Gold (Au) Wet Etching
  • Etchant HCI : HNO$_3$ = 3 : 1 (will yield large undercut), KI or iodine in water (KI + I$_2$ + H$_2$O = 4 : 1 : 40, more practical, but opaque), or cyanide-based (toxic).
  • Photorisist can be used as a mask.
• Platinum and Palladium (Pt, Pd) Wet Etching
  • Pt: Etchant HCI : HNO$_3$ = 3 : 1 (aqua regia) above 25$^\circ$C.
  • Pd: HCI + HNO$_3$ + CH$_3$COOH = 1 : 10 : 10, or KI + I$_2$ + H$_2$O = 4 : 1 : 40.
• Isotropic Silicon Wet Etching
  • HNA: Mixture of nitric (HNO$_3$) and hydrofluoric (HF) acids.
  • HNO$_3$ oxidizes Si, HF removes SiO$_2$ and repeat.
  • High HNO$_3$ : HF ratio
    • Etch limited by oxide removal.
  • Low HNO$_3$ : HF ratio
    • Etch limited by oxide formation.

Anisotropic Wet Etching

Silicon ethcing depends hevily on the type of crystallization of silicon.

Etching of amorphous (No recongizable long-range order)/polycrystalline (completely orderded, in segments) silicon is isotropic.

It can be ansiotropic or isotropic (using HNA as just mentioned) for crystalline silicon (bulk) (Entire solid is made up of atoms in an orderly array).

Anisotropic (Crystalline) Silicon Etching

A process of preferential directional ethcing of material using liquid source etchants (a crystal orentiation dpendent etching).

The number of molecules per area is different in different facet.

The etching rate can be different at each crystal facet.

Wafer Bonding

For wafer bonding, we can use anodic bonding, fusion/direct bonding, or thermocompression bonding.

Fusion/Direct Bonding

Identical materials with ultra smooth (<1 nm roughness) are bonded without any intermediale layer.

Method:

1. Surface preparation: O$_2$ plasma, HF dip, hydration.
2. Wafers brought into contact to form weak bonds (van der Wall forces, hydrogen bonding).
3. Annealing at 600 - 1200$^\circ$C to convert physical forces to chemical bonds.

Anodic Bonding

Anodic bonding is also referred to as electrostatic bonding or field-assited thermal bonding.

This bonding process is assisted by applying an electric field. In this method we bond a conductive silicon substrate to a glass substrate which is rich in sodium.

Technique:

• Voltage applied 600 - 1000 V.
• Elevated temperature 200 - 500$^\circ$C.
• Positive ions in glass migrate, creating a depletion layer near Si surface. Voltage drop creates large electrical field pulling surfaces into contact.

Thermocompression Bonding

Thermocompression bonding is a technique for wafer packaging, using metals such as Au, Cu, and Al as the intermediate bonding layer between two silicon wafers.

Method:

• Metal deposited on substrates
• Bring them into intimate contact.
• Apply temperature and pressure.
• Diffusion of metal atoms between the surfaces, due to atmoic contact between both substrates.

For aluminum (Al):

• Bonding temperature 400 to 500$^\circ$C.
• Applied force above 70 MPa.

For Gold (Au):

• Bonding temperature 260 to 450$^\circ$C.
• Applied force above 20 MPa.

For Copper (Cu):

• Bonding temperature 380 to 450$^\circ$C.
• Applied force above 15 - 30 MPa.