1. Surface Preparation and Wet Processing
Thomas S. Roche, Ph.D. Motorola Corporation  1999 Arizona Board of Regents for The University of Arizona
These slides were put together by me for the purpose of introducing the new engineers in our fab to the chemistry and processes involved in the wet processes used in typical silicon semiconductor manufacturing areas like ours. I do not go into great detail on most of the topics but try to give them an overview of what is going on.
If you have any comments, corrections or criticisms, please contact me at I would be very interested to know if anyone uses these slides.
I have added notes since I do not like to fill the slides with facts. I would rather have something to talk about.

2. Outline Why wet processing? Etching and cleaning
What chemicals are used?
Chemistry of the processes

3. Why Wet Processing? Isotropic Non-damaging Selective
Clean - particle and metal contaminant removal
Well understood

4. Isotropic Isotropic = the same in all directions
Photoresist
Oxide
Silicon
Isotropic Etch
Anisotropic Etch
This slide is here to note the isotropic nature of wet etching and cleaning processes. But it is also meant to introduce the idea that anisotropic processes are not always what is needed in semiconductor manufacturing. We could not etch small spaces without anisotropic processes but we would also have problems if we only used anisotropic processes.
Is anisotropic etching always better?

5. Anisotropic Etch Spacer Etch
Spacer etch relies on the fact that the coating film (e.g. Si3N4)deposited is conformal to the surface and thus the deposits are thicker in corners as measured from the wafer surface to the surface of the deposition film.
Thus when anisotropically etched to the thickness of the original deposition, a residual spacer will be left in the corners as shown in the picture.
An isotropic etchant would attack from all sides and remove the thicker portion of the film much more effectively.
Spacer etch is a common practice used to protect gates and it does not require a mask.
This method of forming spacers uses the anisotropic nature of the reactive ion etching (RIE) process with no masking steps. This process could not be performed with wet etchants.

6. Anisotropic Etch Stringer Formation
This slide is meant to describe one reason why some etches are still done wet.
When a film is deposited over a patterned surface, small areas of thicker deposition can be found due to the geometry of the pattern on the surface. If a straight anisotropic etch is used, these thicker areas may not be removed and may form what are called “stringers” which may come off unexpectedly in later processing, contaminating the surface.
Anisotropic etch can cause problems with strips of deposited layers.
Like the spacer formation, small “stringers” can be left if the strip is
done anisotropically.

7. Non-Damaging
Reactive ion etching and plasma etching can damage silicon surfaces.
These techniques also have some etching effect on the silicon surface.
HF solutions show better selectivity of oxide to silicon than plasma.
An HF solution has fewer chemical species than a fluorine based plasma. It is a better controlled reactant than a plasma.
RIE is a physical/chemical process that does not stop absolutely when the material to be etched is removed and RIE, like plasma etch, can also damage the surface electrically. Thus, it is not used when critical silicon is exposed such as in a pre-gate clean.

8. Wet Etch Solutions HF and Buffered Oxide Etchant - oxide etch
Phosphoric Acid - nitride etch
Nitric Acid and HF - polysilicon etch
Selective metal etches
Al etch - nitric acid, phosphoric acid, acetic acid Ti etch - ammonium hydroxide and hydrogen peroxide W etch - hydrogen peroxide and ammonium hydroxide
Potassium hydroxide solution - silicon etch

9. HF Etches
HF based solutions (Dilute HF or BOE) etch silicon dioxide with a rate dependent on the concentration of HF in the solution.
The actual etchant in more concentrated fluoride solutions is HF2, which is a result of the equilibria:
Very selective
No attack on silicon or silicon nitride
However, photoresist or thin polysilicon are not always adequate masks.
The chemistry of HF solutions, especially BOE, is more complex than indicated here and recent studies suggest that one of the most active etching species in solution is actually H2F2.
No attack on Si or Si3N4 is not an absolute. HF will slowly attack Si especially on the presence of oxygen but this attack is so slow that it can usually be neglected. Concentrated HF will etch Si3N4 but in the concentrations typically used in manufacturing, little etch occurs.
HF solutions, especially at higher concentrations can migrate through photoresists and can infiltrate through polysilicon by moving through the open spaces between the crystals of the polysilicon. This depends on the quality of the polysilicon deposited.

10. BOE Solution
BOE is a solution made by mixing 40% NH4F solution with 49% HF solution.
Typically used for removal of large amounts of oxide
Can be used with photoresist
Higher capacity for Si, since it can tie up the etch product as SiF62-
BOE is what we typically call this solution but it is also called BHF (buffered HF). The original form is as described and thus is a very high ionic strength solution. Different companies use diluted forms of this solution by adding water to the solution.
BOE etches faster than HF because the added fluoride in solution pushes the equilibria toward the faster etching species such as HF2-. So 10:1 BOE etches faster than 10:1 HF
BOE contains very little undissociated HF which is the species that will move through photoresist.
Again the chemistry of silicon dissolved in fluoride solution is much more complex than indicated here. Other Si(F)x(OH)y species are involved. The capacity of BOE for dissolved oxide is much greater than HF solution because of the excess fluoride present which complexes with the silicon dissolved in solution. So the equations do not fully capture that fact but are meant to indicate that not as much HF is consumed for each silicon atom dissolved into solution.
In HF Solution:
Si HF
2 H2O + H2SiF6
In BOE:
SiO HF + 2 NH4F
2 H2O + (NH4)2SiF6

11. Nitride Etch Concentrated (85%) Phosphoric acid
High temperature (> 145 °C)
Etches oxide at a slow rate (selectivity > 30)
Can also etch polysilicon
Difficult to operate
Influenced by the water content
Water is injected or dripped in
Bath is covered and fitted with a condenser
Difficult to rinse
Nitride etch is one of the most difficult processes to control because phosphoric acid is not a stable solution at these high temperatures, tending to dehydrate to P2O5 and other polymeric species and also losing the water present in the bath. The issues of keeping water in the bath are somewhat difficult to deal with since it involves adding water to the solution which is at temperatures above the boiling point.
Rinsing difficulty is related to the fact that phosphoric acid is a fairly high viscosity solution and does not mix easily with water. So when the wafers coated with phosphoric are put into a cold water rinse, the phosphoric can cool and become more viscous before it is completely removed.

12. Polysilicon Etch Solutions of nitric acid and HF
The higher the nitric:HF ratio, the slower and more selective the etch
Typical use - 300:1 ratio for removal of poly on product wafers and 5:1 ratio for removal of poly on dummy wafers
With the more dilute and selective etches, it is important that no oxide be present on top of the poly since that will inhibit the etch of the polysilicon.

13. Cleans
Most wet processes in modern fabs involve cleaning of surfaces. Some of these cleans also involve etching.
The number of cleans in a process flow is typically 20-25% of the total steps in the process and involve all parts of the flow.
Cleans typically precede all diffusion steps and film deposition steps, and they follow all RIE or plasma etch steps and plasma strip processes.
Previous slides described etching, i.e. removal of films from surfaces or other films. This introduces the idea that a more frequently used function of wet processes is cleaning, the removal of contaminants which are things that are not necessarily deposited purposely on the surface but that would interfere with further processing or electrical performance.

14. Silicon Surfaces Bare silicon surfaces - unstable Hydrogen terminated
React with oxygen to form “native” oxide
Hydrophobic
Native oxide - stable
Not equivalent to thermal oxide
Thickness = about 10Å
Self limiting
Grown by exposure to air or oxidizing solution
Hydrophilic
Native oxide is the oxide formed by atmospheric oxidation of silicon. Due to the nature of this process, the thickness is limited to about 10A. I also sometimes use the term chemical oxide to describe the thin native oxide-like material that is formed when a bare surface is oxidized by an oxidizing solution like sulfuric-peroxide or SC1. It probably has the same properties as native oxide. Both are not as “perfect” as an oxide thermally grown at high temperatures.

15. Surface Characteristics
Silicon surfaces etched in HF become hydrophobic if no oxide is left on the surface.
All other surfaces should be hydrophilic.
I usually use the example of water beading up on a freshly polished car as an example of a hydrophobic surface as opposed to water coating the surface of my not polished car.
Hydrophilic
Hydrophobic

16. Surface Characteristics
Surfaces with oxygen termination are hydrophilic since they interact with water molecules
Surfaces with hydrogen termination are similar to organic surfaces and do not interact with water. They are hydrophobic.
O O O O O O O O O
H H H H H H H H H
SI SI SI SI SI SI SI SI SI
SI SI SI SI SI SI SI SI SI
HYDROPHILIC
HYDROPHOBIC

17. Particle Attachment Hydrophobic wafers tend to pick up any
particles on the surface
of the solution as they
are withdrawn through
the liquid-air interface.
HF processed bare silicon surfaces are the typical hydrophobic surface and are a typical problem in a fab.
Many of the particles in the process or rinse tanks are present at the air-solution interface (because they are insoluble in solution) so recirculating baths are designed to overflow so that the surface is continually renewed and those particles are hopefully swept over the sides of the tank.
Hydrophobic
Surface
Hydrophilic
Surface

18. Oxide Surfaces Oxide surfaces can be formed:
by room temperature oxidation of “bare” silicon surfaces (native oxide)
by heating silicon in an oxygen atmosphere (thermal oxide)
by deposition in a low pressure furnace (LPCVD oxide)
by deposition from decomposition of TEOS in a plasma reactor (plasma TEOS or plasma oxide)
All oxide surfaces are hydrophilic, but they etch at different rates in HF.
The different methods of forming oxides (in typical manufacturing, what is referred to as oxide is almost always silicon dioxide) all result in something called oxide but the perfection of the oxide is the determinant of the etch in a wet solution. Thermally grown oxide is always the slowest etching film among these oxides because it is closest to the perfect oxide. Other oxides contain species which disrupt the oxide lattice and make the material easier to etch (and less reproducible in etching properties). These other oxides become more like thermal oxide when heated to high temperatures.
All oxides can become easier to wet etch when they are exposed to processes such as RIE or especially implant.

19. Cleaning Solutions
Sulfuric Acid-Hydrogen Peroxide (aka SPM or Piranha) solution
Organic removal
Ammonium Hydroxide-Hydrogen Peroxide (aka SC1 or APM)
Particle removal
Hydrochloric Acid-Hydrogen Peroxide (aka SC2 or HPM)
Metallic contamination removal
Dilute HF solutions
Metallic contamination removal with oxide removal

20. Sulfuric Acid-Hydrogen Peroxide (SPM/Piranha)
Operated at > 100°C
Useful for its oxidizing (organic destruction) capabilities
Will grow an oxide on bare silicon but will not grow oxide on oxidized surfaces
Peroxide has a limited lifetime, replenished by spiking with added peroxide
Most of the solutions used in cleaning have oxidation capabilities and SPM is probably the best oxidant we use. It was originally introduced to remove photoresist from surfaces but in most cases that is done by ashing (plasma) now. The only times that SPM is still used for resist removal is when the structure on the surface is believed or proven to be susceptible to plasma damage. Those cases do occur in advanced products. Many people still use SPM as a first clean in cleaning processes unnecessarily.
The actual oxidizing species present is not established but some people still refer to this solution as Caro’s acid, H2SO5 (peroxymonosulfuric acid) but that species is not stable at these temperatures.
Despite the fact that most photoresist is removed by ashing, SPM is still used after ashing partially because ashing processes are not perfect and can leave residual resist or carbonized resist, especially if the resist has been used as a mask for implantation. The other reason is that, despite the fact that photoresist has improved in purity, it still contains metallic contaminants that will be concentrated on the surface when the resist is ashed off. Some type of wet process is needed.

21. Sulfuric Acid-Hydrogen Peroxide (cont’d)
Sulfuric acid disadvantages
Difficult to remove from surfaces
Water used for rinsing is usually very hot
Amount of sulfur left on surfaces is very high.
Removed easily with a brief HF dip
Solutions also effective in removal of some metal contamination
Sulfuric acid is rather difficult to remove completely from surfaces and appears to form some type of addition compound or associated complex with the oxide surface. If you rinse sulfuric incompletely from the surface, you can come back a day later and find particles have developed on the otherwise pristine surface. These are believed to be ammonium sulfate (there is always a bit of ammonia in the atmosphere in a manufacturing facility) and can be removed by rinsing but they prove that the sulfuric was not rinsed completely.

22. Sulfuric Acid-Ozone
Some facilities use sulfuric acid with ozone bubbled through it
Effective solution
Long bath life - no water added
Easily overwhelmed by organics
Another variant - ozone in cold water
Good oxidant
Rate of resist removal is fairly slow
Sulfuric-ozone is a solution that has been used for many years in some facilities. The solution is a very good oxidant but the oxidant is present in such small concentration that photoresist coated wafers easily overwhelm the oxidant present.
Ozone in water is another more recently developed solution for this type of process. The problem again is the relatively low concentration of ozone in solution. One way to increase the concentration is by using cold water, but that then slows the oxidation reaction. Work is continuing on the use of ozone-water and it appears that the optimum method for doing this may be to spray water on the wafer surface maintaining a thin layer of water on the surface and have ozone present in the water and in the atmosphere of the spray chamber.The ozone concentration can be more easily replenished by diffusion into the water from the gas phase as it is destroyed at the wafer surface.

23. NH4OH-H2O2 (SC1/APM)
Typically used with a Megasonic unit for particle removal
Temperature range of 50-80°C
Standard solution is 5:1:1 of water:ammonia:peroxide
Problems
Peroxide is unstable in basic solution, especially in the presence of metallic impurities.
Ammonia has high volatility.
Metallic impurities readily adsorb onto surfaces.
Presence of the oxidant is critical!
If lost, solution will etch silicon
Maintained by spiking
SC1 is one of the most important solutions used in semiconductor processing since it is the best method of removing particles. The megasonic transducer typically used in these processes improves the efficiency of particle removal. The exact mechanism of particle removal by SC1 is not well established but I believe much of it has to do with the fact that this is the only basic solution we use. This results in a change in the charge on the surface of the oxide which probably results in repulsion of the particles from the surface. Another possible mechanism is the etch of oxide lifting off the particles but we have no evidence to suggest that this is a main effect since processes which etch less than 5Angstroms are quite effective in removing particles and particle removal is more effective for larger particles than smaller.
Loss of oxidant from solution will cause etching of the silicon. This was more of a problem when lower grades of chemical were used since they could contain high levels of transition metal ions that catalyze the decomposition of peroxide.

24. NH4OH-H2O2 Details
Can grow a native (chemical) oxide on a bare silicon
Also etches oxides at a low rate (~1Å/min), therefore can etch silicon
Dilutions down to 50:1:1 probably as effective
May have problems the current solutions do not
Al and Fe contamination - typical problem
Other transition metals can also deposit
Can also result in surface roughening
SC1 especially at high temperatures and long exposures can cause surface roughening. This is due to the fact that the solution both etches oxide and grows oxide. Thus, it can, in effect, etch silicon.
Metals are a big problem with this solution. Metals which are soluble in solution such as Cu, Al and Fe tend to come out of solution onto the surface. They may be incorporated into the chemical oxide that SC1 grows on the silicon surface. The tendency for metal deposition is so great that Al can be deposited on the surface in significant quantities when it is present at undetectable ppt levels in the solution.

25. NH4OH-H2O2 Details (cont’d)
Lower concentrations
More likely to deposit metals
Control of the concentration is critical
Tradeoffs to be considered with lower concentrations:
higher changeout frequency/shorter bath life
lower particle removal efficiency
metallic contamination
ammonia evaporation and peroxide decomposition
Lower concentrations of SC1 may result in higher proportion of metal deposition because ammonia is effective at tying up metals in solution by forming complexes. At lower concentrations of ammonia, the metals may more readily be deposited on the surface.

26. HCl-H2O2 (SC2/HPM) Temperature range of 50-70°C
Standard dilution is 5:1:1 of water:HCl:peroxide
Mechanism for operation - chlorides of most metallic contaminants are soluble in an acidic solution
Peroxide may not be needed at low metallic contamination levels.
SC2 is really used to compensate for the metallic contamination that SC1 can introduce. It is really a “safety valve” to make sure that any metallic contamination introduced anywhere along the line will not get into the next process which is typically a furnace.

27. Solvent Based Cleaning Solutions
Resist strippers - proprietary mixtures of organic solvents and active ingredients
The solvents are typically NMP (N-methyl pyrrolidone)
NMP is water soluble and neutral in aqueous solution
“Active ingredients” are typically amines
Attack the resist and dissolve into the solvent media.
New strippers - “semi-aqueous”, containing both organic solvents and water along with “active ingredients” and corrosion inhibitors
Solvent based cleaning solutions are only used after metal deposition because they show no advantages over SPM solutions and SPM cannot be used in the presence of metal.
Amines are effective for dissolving positive resists and NMP is a good solvent for the dissolved polymers.
New strippers have been developed taking into account the fact that in many cases, the stripper is not really removing much resist. The backend etching processes like metal and via etch frequently leave little resist on the surface. What needs to be removed is the residues of the etch processes, often complex materials containing metal oxides, organic residues and teflon-like polymers.

28. Solvent Based Cleaning Solutions: Problems
Corrosion of metal lines due to the transition of the surface from the organic solution to rinse water
Any “active ingredients” left on the surface attack the metal in water
Solution- use an intermediate rinse solvent
Metallic contamination (especially sodium)
Solution - use only low sodium strippers
Less of a problem with “semi-aqueous” strippers
The problem with metal corrosion with these processes is often due to the fact that amines will not cause corrosion in a solvent media but when they come in contact with water in the rinse process, they result in a basic solution which corrodes the surface. Thus the intermediate rinse is critical with these types of strippers. In addition, at this point in the process, the surface has lots of places that are difficult to rinse like the vias which are essentially deep holes down to the underlying metal.
The concern with metal contamination at this point in the process is less with the typical transition metals or aluminum (since that is part of the surface now) but with sodium. Sodium, Li an K are described as “mobile ions” since they can move through oxides (under the influence of an electrical field) into the underlying pattern where it can cause electrical problems.
Mobile ion contamination is less of an issue with the semi-aqueous strippers because these ions are more soluble in the solutions that contain water. At the same level of metal ion contamination, the ions are much more likely to deposit from the organic solvent than the water containing solution.

29. Solvent Modified Cleans e.g. EG-BOE
Residues after metal etch processes often contain compounds of the metal etched, along with other species.
Cleaners contain fluoride and a polar solvent such as ethylene glycol to dissolve these metallic compounds.
Involves a balance of acidity and etching of the fluoride with the inhibition of metal attack by ethylene glycol
The solvent cleans noted on previous slides are used in cleans after both metal etch and via etch. These cleans are used only after metal etch because they do not remove resist but are designed to remove metal etch residue. Also, after metal etch, the surface is particularly susceptible to corrosion since the etch gasses contain chlorine. Thus, the resist that may be present after the etch is often ashed in situ in the metal etch chamber since the resist seems to trap the chlorine species.
These solutions will etch some oxide.

30. Water Most heavily used chemical - 2000 gal/wafer
Used mainly for removing other chemicals
Rinse times
until solution reaches high resistivity
given time period
Problems
Bacteria
TOC
Metallic contamination
Water usage is probably typically lower than the number quoted but there are facilities where this level has been reached and exceeded.
Rinsing is done either to a resistivity level or for a period of time. Usually rinse baths are fitted with a resistivity monitor but this does not always trigger the end of the rinse.
Control of TOC in water is believed to be important because these organic compounds that might be present can absorb on surfaces interfering with other processes.
Metal contamination is very important in water because even low levels can absorb on oxide surfaces which have some ion exchange capability. Since so much water is used, the metal levels need to be very low. Sodium is one of the most important contaminants.

31. Wafer Rinsing Continuous flow of water past wafers
Completion of rinse is determined by measuring the resistivity of the water
(Resistivity = very sensitive method of determining the presence of ions in solution)
Dump rinsing - used for wafers that are hydrophilic
Hot water - used after some processes (but not HF treated surfaces)
It is difficult to determine how much rinsing is enough so processes have always been designed to “overkill”. This can have the unintended effect of introducing metal contamination to the surface depending on the metal contamination levels of the water.
Hot water is used after some cleans but must not be used after an HF etch which reveals bare silicon since hot water will etch bare silicon.

32. Wafer Rinsing, cont’d
Resistivity of the water flowing through a rinse bath has always been used to measure the completeness of the rinse. However, it should be recognized that this measurement really only determines the concentration of ions in the water. If a chemical absorbed on the surface and did not rinse off quickly for some reason(such as sulfuric acid), you might be under rinsing if you rely on resistivity. More likely, you are over rinsing because the chemical you can measure in the rinse water is no longer on the surface.
Resistivity measurements describe the purity of the water in the rinse bath - not the amount of chemical left on the wafer.

33. Bacterial Contamination
All DI systems contain bacteria, which can grow and contaminate systems if nutrients are provided.
Ozonation or hydrogen peroxide (oxidizing agent) is typically employed to keep down bacterial levels.
Once bacterial contamination is established, it is difficult to remove since the bacteria form colonies which are difficult to completely destroy with oxidant solutions.
Bacteria can live even in ultrapure water despite the small amount of nutrients present. They will thrive in areas where they are not in the main flow of the solution, so called “dead legs”, and thus the design of the water plumbing is critical.
Bacteria can act as particulate contamination on wafer surfaces especially after HF etch where the surface is hydrophobic.

34. Drying Critical step in cleaning
Objective - remove as much of the water as possible before it can dry on the surface and leave residues from any impurities in the water
Two types of dryers
Spin rinse dryer (SRD) - throw off as much water as possible
IPA vapor - displace water with IPA
IPA vapor dryer styles
Tank
Vapor jet (VJD)
Marangoni-type
The drying process is quite critical because this is a way of concentrating any metal ions in the DI used in the facility and transferring them onto the surface. The methods used can be described as spin drying and displacement of the water by another chemical(IPA). The displacement method is preferred in advanced manufacturing.
The older and original IPA dryer (tank) involves boiling a pool of IPA in a tank and then placing the wafers to be dried in the vapor phase above this pool. The IPA vapor cloud at first collapses as the cold wafers enter the vapor cloud then the vapor re-builds and finally envelops the wafers over time as the temperature of the wafers increases. The wafers are then withdrawn from the vapor and the IPA flashes off. One of the problems with this method is that the vapor moves up from the bottom and this occurs over a period of time. Thus the water is relatively slowly displaced.

35. IPA “Vapor Jet” Dryer
This slide takes a bit of explanation but is an interesting processor for IPA drying. The top view shows the placement of the wafer boat in the processor. The boat is in a box within the processor which is open only at the top and has a drain at the bottom of it (not shown). The IPA liquid is injected into the space below the box at the bottom of the processor where there is a plate held at a constant temperature just below the boiling point of IPA. After injection, a flow of N2 is begun which causes the IPA to evaporate. The vapor then moves around the outside of the box, condensing on the wafers from above. This displaces the water from the top of the wafer down and dries the wafer in a nitrogen stream.

36. Marangoni Drying A low concentration of IPA vapor condenses on the
water surface.
As the wafers are withdrawn
slowly from the water, the IPA layer displaces the water.
The IPA evaporates from the wafers and they are dried.
Marangoni drying is a process where, as the wafers are withdrawn from the water, a stream of nitrogen containing a low concentration of IPA is diffused over the surface of the water forming an IPA layer on the surface (despite the fact that IPA is soluble in water). The IPA displaces the water and the surface dries in the nitrogen stream. A number of patent disputes have arisen about this process in its application in wafer drying.

37. Types of Cleans Resist Strips - post etch and post implant
Etch residue removal
Pre-Metal deposition cleans
Pre-diffusion cleans

38. Resist Strip
Resist removed by ashing - a plasma process which “burns” the resist off the surface
(Note - Resist contains measurable amounts of impurities such as sodium.)
Ashing concentrates impurities on the surface.
Must be followed by a wet process to remove contaminants
Sulfuric acid-hydrogen peroxide used before metal is present on the wafer
Solvent strippers used after metal deposition
Resist strip is probably the most common wet process used in semiconductor manufacturing since it follows all photolithography steps. The difficulty of removing the resist residue is related to the process for which the resist was applied with implant resist strips being typically the most difficult since the implant often removes hydrogen from the hydrocarbon, producing a carbonized residue which is more difficult to ash.

39. Post-Etch Cleans
Reactive ion etching (RIE) = a physical-chemical process which can leave residues
Difficult to remove (can be fluorinated polymers) so both ash and wet clean are usually used
Residues are created to inhibit attack of the plasma on the sidewall of the etched film
Changes in the etch process or the material being etched can require changes in the clean

40. Residues from RIE Processes
Post Etch Residue
This slide describes what happens in many RIE processes. A residual material is formed in the etch process sometimes on top of the resist but often on the sidewalls in the area being etched. This sidewall material is formed purposely in the etch process to make the process more directional (anisotropic) and can be a complex material containing fluoropolymers from the etch gas and metallic or inorganic species from the etch products. These complex materials can be difficult to remove. An example would be the materials formed on the sidewall of a via (the connection between one metal layer with another) and, if not removed, can make filling of the via with metal impossible.
Post Ash Residue

41. Metal Etch Veils Post Etch Post Clean
Residual material can be seen on the top edges of metal lines in the pictures on the right.

42. Post Implant Resist Strip
Implantation sometimes uses resist masks
High dose implants can convert photoresist to a coal-type material
Can be ashed if it is done carefully, but problems can result
Residue can be more complex since it can contain residue from the implant
May require more than a sulfuric-peroxide clean

43. Pre-Metal Contact Clean
Clean before metal is deposited
Connects the silicon circuit to the metal lines on top of the die
Nature of the silicon surface is critical to the metal contact resistance
Performed with HF containing etchant
Important consideration - selectivity of the etchant to the films present (e.g. oxide, BPSG, TEOS)

44. Metal Contact
This slide is a TEM of one type metal contact formed when a metal is deposited on silicon and heated. The material on the left is the silicon and on the right is the metal. The thin 5.5nm layer is the silicide formed by reaction of the metal with the silicon which makes the actual contact between the silicon and the metal. There are more details to this slide which I will not go into.

45. Pre-Diffusion Clean Cleans before furnace operations are critical
Defects or contamination will be incorporated into the wafer if not removed prior to furnace
Most important steps - those where bare silicon is present
Pre-initial
Pre-sac gate
Pre-gate
Typically full cleans (Piranha+BOE+SC1+SC2)

46. Gate Oxide Stack Polysilicon Gate Oxide Silicon
This slide is a TEM of a gate oxide with silicon below and polysilicon which is the gate electrode above it. All surfaces here are critical and can affect the electrical operation. The lower Si-SiO2 interface can be impacted by the roughness and cleanliness of the pre-gate clean and the SiO2-Polysilicon can also be impacted since some metals that might be left on the surface after the pre-gate clean can remain at the top surface of the oxide as it grows. Certain metals at this interface can result in poor deposition of polysilicon by catalyzing the deposition resulting in a rough surface for the polysilicon. The deposition of polysilicon on this oxide is usually accomplished by direct transfer from the oxide growth furnace and is done in minimum time.

47. Silicon Oxide Interface
I put this TEM in to show what the gate oxide-Si interface really looks like. The silicon crystal structure can be seen and the interface shows that this is not exactly a perfect surface where silicon abruptly ends and oxide begins but there is apparently some intermixing of the silicon and the oxide. I also use this to point out to the new engineers that the silicon is crystalline and the oxide is not.