Time-depending yield degradation at microchip manufacturing
Dwelling time before the final electrical test results in the dies failing
Manage the process flow to achieve the lowest dwelling time
The resistivity of the bumps increases during the dwelling
Eliminate the dry etch operation. Replace it with the relevant wet etch process.
Some stuff that has high resistivity is formed on the surface of the bump.
The best way to eliminate high resistivity on the surface of a bump is to use a chemical etching process.
Previous process affected the bump surface that results in the formation of dielectric stuff on the surface of the bump.
One way to eliminate this dielectric stuff on the surface of the bump is to clean the bump surface with a solvent or chemical solution. This will help to remove any residue or contaminants that may have been left behind from the previous process. Additionally, it is important to ensure that the surface of the bump is dry before any further processing takes place. If necessary, the bump can also be polished to remove any remaining dielectric material.
The dry etch process contaminates the bump with fluorine that converts slowly to the CuF2 that covers the surface of the bump
One way to eliminate this contamination is to use a wet etch process instead of a dry etch process. Wet etching is a chemical process that uses an aqueous solution to remove material from the surface of a substrate. This process does not involve fluorine, so it will not cause the formation of CuF2 on the surface of the bump. Additionally, wet etching can be more precise and can produce better results than dry etching. As a temporary solution - wash the wafers with water prior to the final test.
The dry etch operation is performed with the fluorine compound
The best way to eliminate the use of fluorine compounds is in a dry etch operation. Eliminate the dry etch operation at all.
To create and analyze the process flow of Cu bumps disconnection
1 operation - Cu removal using H2SO4 + H2O2 solution (Chem 1)
2 operation - Ti removal between the bumps using dry (plasma) etch with NF3 gas
3 operation - Ti residue removal with diluted HF solution (Chem 2)
1. Cu removal with H2SO4+H2O2
OE 3.75
2. Ti removal between bumps with NF3 plasma
OE 1.51
3. Ti residue removal with HF solution
OE Ideal operation
Productive operations effectiveness
Effective
Ineffective
Operation types breakdown
Productive
Providing
Corrective
Metrology
| Operation Type | Does it increase cost? | Does it increase product value? | Recommendation |
|---|---|---|---|
Productive | Yes | Yes | Improve |
Providing | Yes | No | Eliminate |
Corrective | Yes | No | Eliminate |
Metrology | Yes | No | Eliminate |
| Operation | Type | Merit | Recommendation |
|---|---|---|---|
| 1. Cu removal with H2SO4+H2O2 | Productive | OE 3.75 | Consider improving |
| 2. Ti removal between bumps with NF3 plasma | Productive | OE 1.51 | Consider improving |
| 3. Ti residue removal with HF solution | Corrective | OE Ideal operation | Do nothing and eliminate it when possible |
The dry (plasma) etch operation is the most harmful and should eliminated. A wet etch should be used for Ti removal instead of the dry (plasma) etch.
Create task - to try the perform process in two steps:
- Wet etch with Chem 1 - to remove Cu layer
- Wet etch with Chem 2 - to remove Ti layer between the bumps
The operation aims to remove Cu from the surface of the wafer.
The process - Wet etch with H2SO4 + H2O2 solution that dissolves Cu and does not dissolve Ti
Product - Dissolved Cu
H2O2 solution | 10 |
H2SO4 Solution | 7.5 10 |
Cu thin layer | 7.5 |
Cu Bumps | 5 |
Chem 1 | 5 |
Ti thin layer | 2.5 |
The process of the Cu removal seems OK. The solution Chem 1 also removes Cu from bumbs that is not currently useful, but could be useful in the whole process improvement
The operation aims to remove Ti on the open area between the bumps.
The process - Dry etch (plasma) with NF3 gas. Fluorine generated from NF3 in plasma interacts with Ti and converts it to TiF4 with low melting and boiling points. At the process conditions, TiF4 presents in the form of gas that is removed by the exhaust of the dry etch chamber.
Product - TiF4 - gas
Ionized Fluorine | 10 10 |
Plasma | 8.3 4.3 |
Ti - thin layer - open | 6.7 |
Dielectric layer | 5 |
NF3 | 3.3 |
Ti - thin layer - covered by bumps | 3.3 7.1 |
Cu - Bumps | 2.9 |
The process seems to be very problematic. Fluorine removes the open part of the Ti layer but also interacts with Cu bumps and penetrates the bump. The Cu bump "contaminated" with Fluorine leads to the time-dependence dies degradation.
The model is as follows:
- F penetrates the Cu bump
- F diffuses to the bump surface
- F interacts with Cu and creates a CuF2 thin layer on the top of the bump
- CuF2 is a dielectric material that leads to the bump electrical conductivity reduction and the final test failing
- F diffusion and CuF2 formation rate are low; therefore, the failures have a time-dependence occurrence
The main conclusion:
Fluorine plasma does not etch Cu, but affects Cu.
To solve the problem - this operation should be eliminated.
The operation aims to remove the residue of Ti remains between the bumps after the dry (plasma) etch operation.
The process is performed by wet cleaning using diluted HF solution (Chem 2). HF solution dissolves rare refractory metals including Ti. HF converts the Ti to soluble acid H2TiF6. Dissolved Ti is removed from the wafer with liquids
Product - Dissolved Ti
HF - solution | 10 |
Ti residue | 6.7 |
Dielectric layer | 3.3 |
Ti thin layer under bumps | No impact |
Cu - bumps | No impact |
A wet clean process based on diluted HF can be principally used for Ti removal and replace the dry (plasma) etch operation. The main point of such a wet process is to ensure Ti removal between the bumps and not affect Ti under the bumps
If | The interaction rate will be reduced. |
|---|---|
Then | The undercut will not be deep. |
But | The process will take a very long time. |
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