Part 4 - Etching | rezarezvan.com

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 etching, RIE).

Etching considerations

Etch Rate
- Rate of material removal [ $\mu\mathrm{m/min}$ ]
- 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 etching process, we perform different steps. Let’s take a look.

Mechanisms in Dry Etching

Etchant Species Generation
Diffusion to Surface
Adsorption
Reaction
Desorption
Diffusion to Bulk

Similar to CVD (Chemical Vapor Deposition), the process includes reaction and transportation (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 etching rate, 3-10 $\mu\mathrm{m/min}$ 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.

Table 1: Materials, gases, and products in plasma etching.

Material	Etch Gases	Etch Products
Si, SiO $_2$ , Si $_3$ N $_4$	CF $_4$ , SF $_6$ , NF $_3$	SiF $_4$
Si	Cl $_2$ , CCl $_2$ F $_2$	SiCl $_2$ , SiCl $_4$
Al	BCl $_3$ , CCl $_4$ , SiCl $_4$ , Cl $_2$	AlCl $_3$ , Al $_2$ Cl $_6$
Organics	O $_2$ , O $_2$ + CF $_4$	CO, CO $_2$ , H $_2$ O, HF
Other (W, Ta, Mo, $\ldots$ )	CF $_4$	WF $_6$ , $\ldots$

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 torr).

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 $_2$ $_{2}$ ) Wet Etching
- SiO $_2$ + 6HF $\rightarrow$ H $_2$ SiF $_{6(aq)}$ + 2H $_2$ O (HF Etchant)
- Etches SiO $_2$ but not Si, may also attack Al, Si $_3$ N $_4$ , $\ldots$
Silicon Nitride (Si $_x$ $_{x}$ N $_y$ $_{y}$ ) 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)
- Nitric acid forms aluminum oxide on surface and phosphoric acid and water dissolves 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 HCl : 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).
- Photoresist can be used as a mask.
Platinum and Palladium (Pt, Pd) Wet Etching
- Pt: Etchant HCl : HNO $_3$ = 3 : 1 (aqua regia) above $25\,^\circ\mathrm{C}$ .
- Pd: HCl + 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 $_3$ $_{3}$ : HF ratio
  - Etch limited by oxide removal.
- Low HNO $_3$ $_{3}$ : HF ratio
  - Etch limited by oxide formation.

Anisotropic Wet Etching

Silicon etching depends heavily on the type of crystallization of silicon.

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

It can be anisotropic 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 etching of material using liquid source etchants (a crystal orientation dependent etching).

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

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 intermediate layer.

Method:

Algorithm 1: Fusion/direct bonding

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

Anodic Bonding

Anodic bonding is also referred to as electrostatic bonding or field-assisted 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 $200$ - $500\,^\circ\mathrm{C}$ .
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.