Etching and Lift-Off Processes Quiz

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Questions and Answers

What is a common fault associated with the lift-off process?

Incomplete etching of the substrate

Collapsing of the resist structure

Over-etching of the mask material

Formation of wing-like features due to metal adhesion (correct)

What is the significance of the ratio ER1/ER2 in etching?

It provides the selectivity of the target material to the underlying material (correct)

It determines the material composition of the substrate

It measures the overall success of the etching process

It indicates the speed of the mask material etching

Which angle is typically observed in a metal profile after lift-off?

70-75 degrees (correct)

90 degrees

45 degrees

60 degrees

Which of the following statements about etching is incorrect?

The mask material always etches at a faster rate than the target material Signup and view all the answers

What does the term 'critical dimension' refer to in the context of etching?

The finished dimension after etching is complete Signup and view all the answers

What is a characteristic disadvantage of wet etching?

It can lead to undercutting due to isotropic etching. Signup and view all the answers

What is the primary distinction between wet etching and plasma (dry) etching?

Wet etching occurs in a solution, while plasma etching occurs in a gas. Signup and view all the answers

Why is surface cleanliness critical in wet etching processes?

Any residual contamination can lead to rough etch surfaces. Signup and view all the answers

What typically requires safety precautions during the operation of plasma etching?

The use of dangerous gases in a vacuum environment. Signup and view all the answers

What is an example of a preparation step recommended before wet etching?

Performing an oxygen dry etch to remove resist residuals. Signup and view all the answers

What is a key advantage of wet etching?

Higher throughput due to batch processing capabilities. Signup and view all the answers

How do materials typically get removed during plasma (dry) etching?

Through chemical processes and physical bombardment (sputtering). Signup and view all the answers

What kind of etching is known to cause undercutting due to its isotropic nature?

Wet etching. Signup and view all the answers

What is the purpose of using a mixed-mode process in nanoscale etching?

To etch Si features while providing a fluorocarbon film. Signup and view all the answers

Which type of plasma etching is associated with the physical bombardment mechanism?

Reactive Ion Etching (RIE) Signup and view all the answers

What challenge is commonly encountered during plasma etching processes?

Accidental deposition of polymer on non-target areas. Signup and view all the answers

How does the ion velocity affect physical bombardment during plasma etching?

Vertical ion velocity minimizes bombardment on sidewalls. Signup and view all the answers

What ratio of SF6 to C4F8 is mentioned for the etching condition in the example provided?

20:90 sccm Signup and view all the answers

What is the purpose of the SF6 plasma in the pulsed mode process for deep Si etching?

Si etching Signup and view all the answers

At what temperature does the cryogenic etching process typically operate?

About –110 °C Signup and view all the answers

What is the reason for requiring ion bombardment in the pulsed mode process for deep Si etching?

Remove polymer from bottom surfaces Signup and view all the answers

Which characteristic is associated with the cryo-etched Si trench?

Smooth and clean sidewalls Signup and view all the answers

What is the maximum aspect ratio achievable with the cryogenic etching process mentioned?

10:1 Signup and view all the answers

What role does C4F8 play in the pulsed mode process for deep Si etching?

Contributes to sidewall passivation Signup and view all the answers

What is a benefit of the mixed mode process in deep Si etching?

Combines etching and passivation effectively Signup and view all the answers

What happens to the SiOxFy passivation layer when the silicon is warmed to room temperature?

It becomes volatile and leaves the surface Signup and view all the answers

Which of the following gases is least reactive in plasma etching processes?

H2 Signup and view all the answers

What effect does a higher F/C ratio have on the etching process?

Increases etching rate Signup and view all the answers

Which additive is specifically used to prevent metal corrosion in plasma etching?

O2 Signup and view all the answers

Which of the following gases is primarily used for etching silicon?

CF4 Signup and view all the answers

What is one of the key challenges faced in high aspect ratio (HAR) etching?

Electron shading effect Signup and view all the answers

Which method can help reduce or eliminate notching at the silicon-on-insulator (SOI) interface?

Utilizing low-frequency pulsed RF Signup and view all the answers

What happens to the etch rate when adding H2 to the plasma process?

Polymerization increases Signup and view all the answers

What role do gases that form involatile barrier layers play in the etching process?

Protecting etched sidewalls Signup and view all the answers

Which compound's volatility is specifically mentioned as challenging during gold etching?

Cl Signup and view all the answers

What effect does an insulating substrate have on charging during etching?

Increases charge buildup Signup and view all the answers

What is a common approach to minimize substrate heating during etching?

Using thermal conduction compounds Signup and view all the answers

What is a typical consequence of the electron shading effect during high aspect ratio etching?

Reduced etching resolution Signup and view all the answers

In plasma etching, what is the role of the passivating layer on sidewalls?

Preventing further etching Signup and view all the answers

Which interaction specifically affects etch species transport in high aspect ratio features?

Angle of ion incidence Signup and view all the answers

Study Notes

SEM of undercut profile

There are two methods to create an undercut profile “developer soak” and bi-layer LOR/S1818.
The bi-layer LOR/S1818 method is more reliable than the developer soak method.
The metal does not form a vertical profile after lift-off.
The angle of the metal profile is typically 70-75 degrees.
Metal gradually fills in the opening during the lift-off process.

Common Lift-Off Faults

Two common lift-off faults are flagging and non-removal of metal.
Flagging occurs when metal sticks to the edge of the resist, creating wing-like features.
Non-removal of metal can also occur during lift-off.

Etch as a means of pattern transfer

Etch is a process used to transfer a pattern into a substrate.
The process begins with lithography, where a resist is applied to the substrate.
The resist is then patterned, and the substrate is etched.
Finally, the resist is removed, leaving the etched pattern on the substrate.

Etching figures of merit

Target material etch rate (ER1)
Etch rate of underlying material (ER2)
Mask material etch rate (ER3)
The ratio of ER1/ER2 gives the selectivity of the target material to the underlying material.
The ratio of ER1/ER3 gives the selectivity of the target material to the mask material.
The critical dimension (CD) is the finished dimension after etching.
CD directly impacts transistor performance (threshold voltage).
Etch profile (taper angle j)

Taxonomy of etch profile

Generally, anisotropic etching is desired, but not always.
Different etch profiles include:
- Perfectly anisotropic, vertical
- Sloped or tapered
- Re-entrant
- Undercut, lateral etch
- Microtrenched
- Foot
- Notched
- Faceting of the mask

Wet and dry etching

Wet etching and plasma (dry) etching are two common etching methods.
Wet etching is performed in a solution, while dry etching is performed in a plasma.
Wet etching typically involves acids or alkalis and requires careful handling.
Dry etching involves a low-pressure ionised gas within a vacuum chamber, which requires safety precautions.
Dry etching removes material by chemical and physical bombardment (sputtering).

Wet etching characteristics

Advantages:
- Simple equipment
- High throughput (batch process)
- High selectivity.
Disadvantages:
- Isotropic etching results in undercutting.
- Small geometries are difficult to etch due to surface tension.
- The surface to be etched must be very pure.
- Critical etch time, dimensions change with etch time and bias develops.

Wet etching substrate preparation

Any residual resist can cause a rough etch due to high etch selectivity.
Remove residual resist before wet etching using an oxygen dry etch called an "ash".
Dedicated ashing tools can be used for this process.

Plasma etching gases

Plasma etching gases are usually halogen-based (F, Cl).
F and Cl are easily dissociated in plasmas and are highly reactive in radical or ion forms.
They create volatile byproducts with materials (Si, Ge, GaN, GaAs, SiO2, SiN, W, WSi, Ti, TiW, TiN, Al, but not Cu & Co).
Other gases (O2, N2, H2, NH3, H2O, CO, CH4) are used for specific effects:
- Ashing resist & polymers
- Removing or adding sidewall films
- Preventing metal corrosion
- Boosting selectivity

Plasma etching gases (2)

Control polymerization through F/C-Ratio.
F/C-Ratio is determined by the gases used (CF4, C2F6, C3F8, C4F8, CHF3, …).
Higher F/C-ratio leads to increased etching.
Lower F/C-ratio leads to increased polymerization.
Adding H2 consumes F and results in increased polymerization.
Adding O2 consumes C and results in increased etching.
Inert gases are stable and do not participate in chemical reactions.

Plasma etching chemistry choice

Chlorine and bromine are used to etch materials like SiN, SiO2, SiON, W, TiW, and WSi.
Fluorine is used to etch materials like GaAs, Si, Poly-Si, TiN, Ti, and Al.
Oxygen can be used to etch polymers and resists.

Volatility of compounds

Volatility curves for silicon etching (F typically used)
Volatility curves for gold etching (Cl used)
Chemical etching of Au is almost impossible at room temperature.

Etch reaction dynamics

To prevent isotropic behavior of etch gases and improve anisotropic etching for high-fidelity pattern transfer, all etched sidewalls must be protected from further etching. This is achieved by forming passivating or inhibiting layers on these sidewalls:
- Surface passivation:
  - Use gases that react with the wafer material and form non-volatile barrier layers (SiO2).
  - Freeze volatile reaction products (SiOxFy) at sidewalls by cryogenic wafer cooling.
- Inhibitor deposition:
  - Use polymer precursor gases to form physical barrier layers (e.g., C4F8).
  - Erode and redeposit inert mask materials.

Challenges – high aspect ratio etching

HAR causes three significant effects:
- Effects due to angular dispersion of ions and neutrals.
- Effects caused by transport of etch species and by-products.
- Effects caused by increased electron shading.

Challenges – high aspect ratio etching (2)

Etch rate depends on angle of incident ions.
HAR feature etched slowly leads to RIE-lag, causing faceting on top of the etched features.

Challenges – high aspect ratio etching (3)

Transport of etch species and by-products in and out is more difficult in HAR features.
Species are shadowed by corners of the trench and have more sidewall collisions.
Low sticking coefficient is desirable to transfer species in and out of HAR features.

Challenges – high aspect ratio etching (4)

Increased electron shading is due to nonuniform charging.
Electron charges build up on etched features.
Ions approach the surface almost vertically, but experience a lateral force.
These effects reduce the number of etch species reaching the bottom of the trench.

Challenges – charging effect

Controlling the notch at the SOI interface can be achieved by using low frequency pulsed RF on a low electrode.
This controls ion energy and charging at the SiO2 interface to reduce or eliminate the notch.

Challenges – substrate heating/resist reflow/burning (2)

Chilling fluid cools the wafer.
Helium gas fills the gap between the wafer and the substrate.

Challenges – minimising substrate heating

Thermal conduction compounds can be used to minimise substrate heating.
Examples include:
- Santovac 5 diffusion oil
- Crystal Bond 555 HMP wax
- Crystal Bond 509 wax
- Cool grease CGR7018

Challenges – dry etching damage

Ion and plasma damage can cause crystal defects, contamination, and gate oxide breakdown.

Conclusion

Three pattern transfer approaches were highlighted:
- Lift-off
- Wet etching
- Plasma (dry etching)
Plasma etching is discussed in detail, including its types (RIE & ICP), mechanisms, and challenges.

Acknowledgements

Dr. Haiping Zhou
Dr. Corrie Farmer
Dr. Muhammad Mirza
"Dry Etching Technology for Semiconductors" by Kazuo Nojiri (book on plasma processing)

Mixed process for nanoscale etch

A "mixed mode" process uses a plasma containing both etch (SF6) and deposition (C4F8) gases simultaneously.
C4F8 forms fluorocarbons, deposited on all surfaces (polymerization).
The deposit can only be removed by ion physical bombardment collisions.

Mixed process for nanoscale etch (2)

This process allows etching SiNWs with widths of 30 nm, 20 nm, and 10 nm.

Switched process for deep Si etching

A "pulsed mode" (Bosch) process uses an etching plasma (SF6) followed immediately by a deposition plasma (C4F8).
Short alternating SF6 and C4F8 process steps are used to provide deep Si etching.
SF6 provides Si etching.
C4F8 provides sidewall passivation by polymer deposition.
This process requires ICP, high density radicals, and low ion energy.
Ion bombardment is used to remove the polymer from bottom surfaces.
It achieves deep Si etching up to ~300 m deep with an aspect ratio of ~20:1.

Cryogenic process for deep Si etching

A "mixed mode" process combines the etch and passivation steps into a single continuous etch.
Cryogenic Si etch uses SF6 and O2 at ~ –110 °C.
SiOxFy passivation layer is formed on the Si surface at cryogenic temperatures.
This layer protects the vertical Si sidewalls while the horizontal Si is etched away.
The SiOxFy becomes volatile and leaves the Si surface when warmed up to room temperature.
The process offers high mask selectivity, a moderate etch rate, smooth sidewalls, profile control, and cleanliness.

Cryogenic process for deep Si etching (2)

Cryo-etched 2 µm wide and 10 µm deep Si trench:
- Smooth and clean sidewall
- ER 2-3 µm/min
- Si : SiO2 > 200:1
- Si : PR >100:1
Cryo-etched 50 nm trench:
- Etch rate ~ 0.5 µm/min
- Roughness < 10 nm
- Aspect Ratios ~ 10:1
- Controllable profile

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Test your knowledge on the lift-off process and etching significance with this quiz. Explore common faults, critical dimensions, and metal profile angles related to these techniques in material science. Perfect for students and professionals looking to refresh their understanding of these concepts.