The Thin Film
Thin film formation is an important step to provide solid material for micromachining.
There are a few methods and approaches for film formation, and we will cover three different ways.
Oxidation
One method to grow a film of $\text{SiO}_2$ from a polysilicon film or silicon wafer of single-crystal silicon is to heat the silicon in an oxygen-rich environment. This process is called oxidation.
The high temperature process is usually between $700$ to $1200^{\circ}C$.
The purpose of oxidation is to create both electrical and thermal isolation, biocompatibility and as the sacrificial layer.
Oxidation Equipment
Thermal oxidation is carried out in a high temperature furnace tube. The tubes are usually made of quartz or silicon carbide.
Oxidation Process
Oxidation is the process to grow a thin $\text{SiO}_2$ film. $$ \text{Si} + \text{O}_2 \rightarrow \text{SiO}_2 \ | \ \text{Dry Oxidation} \newline \text{Si} + 2\text{H}_2\text{O} \rightarrow \text{SiO}_2 + 2\text{H}_2 \ | \ \text{Wet Oxidation} $$
Important to note is that silicon is consumed during the oxidation process, and the thickness of the oxide layer is proportional to the consumed silicon.
Modeling Thermal Oxidation of Silicon
The oxidation occurs at the $\text{Si}-\text{SiO}_2$ interface. $\text{O}_2$ or $\text{H}_2\text{O}$ (gas) must diffuse through the grown oxide film.
The oxidation rate will decrease with time and the increased oxide thickness.
Let $N$ be the surface concentration of the oxidant.
The diffusion flux of oxidizing species, $$ J_D = -D \frac{\partial N}{\partial x} \cong \frac{D(N_0 - N_1)}{h}. $$
The flux of reaction rate, $$ J_R = kN_1 \ | \ [mol/m^2s]. $$
At steady state, $$ J_D = J_R = J = kN_1 \newline -D \frac{\partial N}{\partial x} \cong \frac{D(N_0 - N_1)}{h} = kN_1 \newline N_1 = \frac{DN_0}{kh + D} $$
Which means, $$ J = \frac{DN_0}{h + D / k}. $$
The film growth rate, $$ vJ = vkN_1 = \frac{dh}{dt}, $$
where $v$ (= $27.3 \text{ cm}^3/\text{mol}$) is the molar volume of the oxide.
$$ \frac{dh}{dt} = \frac{vDN_0}{h + D / k}. $$
If we integrate with respect to time, $$ t = \frac{h}{kN_0v} + \frac{h^2}{2DN_0v} $$
where $h(t = 0) = 0$.
For thin films, $$ h \approx kN_0vt \ | \ \text{reaction limit} $$
For thick films, $$ h \approx \sqrt{2DN_0vt} \ | \ \text{diffusion limit} $$
Oxidation Propety
Thin oxide is usually grown using dry oxidation. Thick oxide is usually grown using wet oxidation.
Dry oxidation results in slower growth rate, but high density, higher breakdown voltage.
- Advantages
- Easy and cost-effective.
- Good quality.
- Disadvantages
- High temperature → Thermal stress.
- Slow process.
- Silicon compounds only.
Chemical Vapor Deposition (CVD)
Compared to oxidation, deposition does not consume the substrate.
It is a chemical process that is driven by chemical reactions and decomposition of gases.
The slowest step dominates the growth rate, so it is a reaction-rate limited process and mass-transport limited process.
Film Conformity
When mass-transport is limited, the growth rate in each direction is dependent on the direction of gas molecule flux incident on the surface, which is a function of acceptance angle.
Poor thickness uniformity and conformity (step coverage). This also requires high tempurates.
Low-Pressure CVD (LPCVD)
LPCVD is a method to deposit thin films at low pressure.
This requires high temperature ($480$ to $1200^{\circ}C$), and a pressure level typically $100$ to $300$ m-torr (Long mean free path, high diffusivity and, good step coverage).
This is conducted in reaction-limited regime. This will yield better film uniformity, less concern about the wafer location and orientation, ergo higher productivity. Also less pin-hole problems, gas phase product is quickly removed.
This is commonly used for MEMS & IC fabrication.
Plasma-Enhanced CVD (PECVD)
Plasma combined with thermal energy to crack the molecules. This will lead to a faster reaction, also requires lower temperature, only in the $100$ to $400^{\circ}C$ range.
This is also conducted in reaction-limited regime.
However, the deposited film may be contaminated by the product, which will require periodic cleaning.
Comparison of CVD Methods
| Process | Advantages | Disadvantages | Applications |
|---|---|---|---|
| LPCVD | Excellent purity, uniformity, and conformity. Large productivity. | High temperature, low deposition | Oxide, Poly Silicon, Silicon Nitride |
| PECVD | Low temperature, fast deposition, good conformity. | Chemical and particulate contamination | Metal, Poly Silicon, Oxide, Silicon Nitride |
Physical Vapor Deposition (PVD)
PVD is a method to deposit thin films by physical means instead of chemical reactions.
In PVD process, the film is deposited via the following steps,
- The material to be deposited is physically converted to vapor
- The vapor is transported from the source to the substrate in a low-pressure environment (i.e. vacuum is required, also to prevent oxidation and to get better step coverage).
- The vapor condensed on the substrate to form a thin film.
Thermal Evaporation
Place wafers upside down to reduce particulates and face the source.
Using thermal energy to create vapor, however we might have possible contamination from the heater.
It is also hard to deposit thick films, and hard to control the thickness.
This way of evaporation is not for alloy deposition.
E-Beam Evaporation
E-beam evaporation is a method to deposit thin films by using an electron beam to heat the source material.
We have a focused electron beam to locally heat the source. This can be used to heat/evaporate even high-melting-point materials.
Note, centripetal acceleration causes electrons to emit x-rays, so we need to shield the chamber.
Physical Vapor Deposition (PVD) (cont.)
Film is deposited by the condensation of a vapor onto a cooler substrate.
Vapor generation methods include,
- Boiling vapor of a molten metal (thermal or e-beam evaporation)
- Material physically knocked, bombarded, or sputtered off (sputtering)
Thickness depends on angle between surface and the incoming source flux → this “conformity” issue.
Sputtering
Instead of using thermal energy, sputter deposition is a process to form films on the substrate from atoms that are generated by the bombardment of highly energized particles.
Ions generated by a plasma are accelerated towards the target material, and the momentum transfer causes the target material to be ejected.
Target ions are ejected by the ions, and then transported to the substrate. Atoms condense and form a thin film on the substrate, this can be used for alloys.
Plasmas and Glow Discharges
Plasma is partially ionized gas mixture with equal number of positive (gas+) and negative (e_) charges.
A glow discharge is a sustained plasma created by applying a voltage between two electrodes in a gas-filled chamber.
Non-Reactive Sputtering VS. Reactive Sputtering
In Non-reactive sputtering, the sputtering gas is not to be incorporated into the film. By far, Argon is the most popular gas.
In Reactive sputtering, the sputtering gas is to be incorporated into the film. This forms alloys such as $\text{Al}_2\text{O}_3, \text{Si}_x\text{N}_y$ and $\text{SiO}_2$.
Common choices of gas are $\text{O}_2, \text{N}_2, \text{NH}_3$.
Properties of Sputtering
- Advantages
- Better uniformity, adhesion and conformity (plane source).
- Control of alloy composition.
- Better control of film thickness.
- Disadvantages
- Ion bombardment is critical for some materials.
- Higher residual stress.
- Higher impurity due to the low vacuum.