Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
37 Cards in this Set
- Front
- Back
|
What does wet etching use and what is it used for?
|
It uses acids, bases or solvents to remove undesired material
|
|
|
What is selectivity?
|
It is the ratio of etch rates of various materials (ie. selectivity 100 for SiO2 vs. Si means that the oxide etches 100 times faster than silicon)
|
|
|
What are the advantages of wet etching?
|
- simple technology with detailed history
- inexpensive compared to other etch processes -lots of tools available: acid, bases, solvents, surfactants, buffers... - some etchants have very high selectivities |
|
|
What are the disadvantages of wet etching?
|
- Etch is often Isotropic
- difficult for small features -large amounts of chemical waste - etchant rate and endpoint can be difficult to control/measure |
|
|
What is Isotropic Etching?
|
Etching proceeds in all directions at the same rate, thus under the mask as well as downwards
|
|
|
What is Etch Bias?
|
(average) undercut for a particular process
|
|
|
Etching Trenches and Vias? What's the equation for Trench width?
|
- feature will end up wider than on the mask
- Trench width ~ Mask Width + 2(Etch Depth) -ie. 3 um lithography and a 1 um thick Al thin film -> 5 um wide structure |
|
|
Etching Mesas or Line? What's the equation for Line width?
|
- feature will end up smaller than on the mask
- line width ~ mask width - 2(Etch depth) - ie. 3 um lithography and a 1 um thick Al thin film = 1 um wide Mesa or line |
|
|
What is the speed of etching determined by?
|
determined by the 'rate determining step'
|
|
|
The rate determining step can be...?
|
- diffusion of etchant species to the surface
- chemical reaction at the surface (dependent on etchant concentration and temperature) -diffusion of products from the surface |
|
|
Etch rate can be limited by what? Discuss.
|
Etch rate can be limited by topography: in small geometry features (1 to 3 um) reactants have limited diffusivity and poor mass transfer in small confined regions
|
|
|
Resist Scumming?
|
- a thin layer of photoresist may remain on the wafer surface (incomplete developing) and block etching
|
|
|
"Bucket Etching"?
|
- wafers are placed in a beaker/container
- cheap and simple - uniformity is an issue (when mass transfer/diffusion issues dominate) - diffusion of etchant and byproducts is a key issue - skill + agitation techniques affect etch rate |
|
|
Spinner/Sprayer? What is a major concern?
|
- wafer spinner is used to achieve high uniformity in etches dominated by mass transfer, new etchant is constantly sprayed over the wafer and byproducts are swept away
- major concern: safety + significant volume of waste chemicals |
|
|
Wet Etch uniformity?
|
- agitation is used to move reactants in and products out from the surface (when mass transfer/diffusion issues affect etch uniformity)
|
|
|
Etch rate uniformity?
|
+ or - 10% across a 4 "substrate"
|
|
|
Agitation?
|
- often manual motion of the beaker containing the wafer and the etchant
- can't be used with HF, BOE, piranha, etc (safety issue) |
|
|
Selective SiO2 etch?
|
BOE etch ( HF based, extremely selective [almost 100% over Si)
SiO2 + 6HF --> H2 + SiF6 + 2H2O |
|
|
What are the 2 problems?
|
The reaction consumes HF, and thus the rate will decrease with time AND HF will attack photoresist somewhat.
|
|
|
Problem 1: The reaction consumes HF, and thus the rate will decrease with time.
|
Solution: add a buffer of NH4F, which can maintain a constant concentration of HF by the reaction:
NH4F <--> NH3 + HF |
|
|
Problem 2: HF will attack photoresist somewhat.
|
Solution: The ammonium fluoride buffer also modifies the pH to a value that limits photoresist removal
|
|
|
Final Trick?
|
sometimes a surfactant is added to reduce the surface tension of of the BOE, making it easier to etch small features
|
|
|
Equation for HNA? HNA is commonly used as what?
|
- HNA = HF + HNO3 + Acetic Acid
- commonly used as an isotropic Silicon etchant |
|
|
List the 3 Selective Si Etch mechanism.
|
- Nitric acid is a strong oxidant that will chemically oxidize the Si to SiO2
- HF then etches the SiO2 - Acetic acid is used as a dilutant |
|
|
the etch rate can reach nearly how many um/min, and what does it depend on?
|
- 500 um/min, depending on concentrations
|
|
|
Acetic acid is used as a dilutant, thus...?
|
one can etch a hole right through a typical Si wafer in about 1 min
|
|
|
Overall reaction?
|
Si + HNO3 + 6HF --> H2SiF6 + HNO2 + H2 + H2O
|
|
|
A common etch of anisotropic etching of Si?
|
KOH, isopropyl alcohol and water in a 23:14:63 mixture
|
|
|
This mixture's etch rate?
|
etch rate 100 times faster downward than sideways.
|
|
|
Characteristic sidewall features at ___ degree to the wafer normal.
|
54.7 degree
|
|
|
Other common isotropic etches are...?
|
EDP (ethylenediamine pyrocatechol) and TMAH (Ch3)4NOH
|
|
|
Surface passivation of Si?
|
- can prevent oxidation of an Si wafer surface after it has been cleaned
- Si is bound to H, preventing oxidation - hydrogenated surface is stable in air for up to a few days |
|
|
The hydride layer desorbs at about ____ degree Celsius.
|
500.
|
|
|
Chemical Mechanical Polishing?
|
- chemically corrosive abrasive is used to polish or planarize the surface of a wafer
- slurry of nanoparticles is rubbed over the wafer with a polishing pad - both chemical and mechanical processes etch the surface - Eg. 200nm alumina slurry particles --> W removed by oxidation and abrasion - alumina close in hardness to W so scratching is lessened |
|
|
Ion Milling?
|
- uses an energetic (~500eV) beam of Ar ions to sputter away the surface of the wafer
- physical, not chemical, etching - can be used for almost any material, even those with no good etchants or volatile etch products |
|
|
Normally uses a _________ ___ gun. Discuss.
|
Kauffman ion
- Ar ions created in a plasma inside the gun accelerated out by electrodes |
|
|
What are the problems of Ion Milling?
|
- mask taper? transfer
- redeposition from the mask, trenching |
