EEE 496 MICROFABRICATION

Homework 3

1. What are the parameters that are used for evaluating an etching method? Please also explain

what each parameter means and write down what is considered good and bad in terms of

each parameter (e.g. lower fuel consumption in a car is good higher fuel consumption is

bad.).(20)

Answer

a) Selectivity: Selectivity is the difference in etch rate of different materials for a given

etching chemical. It is generally given as the ratio of etch rates for two layers for the

etchant. A good selectivity refers to a high ratio for etch layer:mask layer. Hence etch

layer is etched much faster than a mask layer.

b) Undercut: Undercut is the amount of lateral etching under a masking layer. İt is

measured as the distance from the edge of the mask to the furthermost etched point

horizontally. Good undercut would be small hence there would be small lateral etching.

c) Wall profile: It is the slope and roughness of vertical walls that are formed as a result of

etching. Ideally it should be 90o with no roughness at all. Hence a good value is as close

to vertical as possible and as smooth as possible.

d) Etch rate: It is the rate of removing material from the etch surface. It is measured in

depth/time. Good etch rate is a high etch rate (fast).

e) Controllability: It defines one’s ability to control etching parameters. If an etch process is

more controllable, we can stop it just in time to prevent over etching. Hence a more

controllable etching is better.

f) Uniformity and repeatability: They are basically consistent behavior of etching solution.

Uniformity is equality of etching parameters, especially etch rate, over a single wafer

surface. Repeatability is equality of these parameters between different wafers or

different batches. The more uniform and repeatable a process, the better it is.

2. An etching method is expected to be selective against which layers and what does selectivity

against each layer mean i.e. what are consequences of having low or high selectivity for each

layer?(6)

Answer

An etching method is expected to be selective against the mask layer and the underlying

layer or the layer below the etch layer. Selectivity against masking layer is required for its

protective function. A high selectivity means we can use a thin masking layer and the

masking layer would still be intact after etching has been completed. A low selectivity would

mean masking layer would be dissolved before etching is finished hence could not protect

surfaces from etching. Selectivity towards underlying layer is required for it’s etch stop

ability. A high selectivity means underlying layer would not be etched even if we continue

etching after the etch layer has been completely removed on a surface. This could be needed

if etching is a bit nonuniform and some surfaces finish before others and we have to wait

until all surfaces are finished.

3. If you were to fabricate very small features (i.e. high resolution), which type of etching would

you prefer isotropic or anisotropic and why?(4)

Answer

High resolution etching or etching of small features are done with anisotropic etching. Since

we would have very small tolerance towards undercut in a small feature, it would be advised

to use a method that produces near vertical walls with little undercut. Otherwise we would

lose much of the line width during etching and our line would be smaller or would be

completely etched. Hence we require anisotropic etching for small features.

4. Write down the two major defining properties of wet etching.(4)

Answer

Wet etching is chemical and wet etching is generally isotropic.

5. Explain the two modes of wet etching.(8)

Answer

Surface reaction limited mode: In this mode mass transport is adequate. There are sufficient

reactants on the reaction surface and etch rate is limited by the surface reaction rate.

Mass transport limited mode: In this mode mass transport is slow and inadequate. All

reactants arriving to the reaction surface are consumed very quickly. Limiting mechanism is

transport of reactant molecules to the reaction surface.

6. What does anisotropic wet etching generally refer to?(2)

Answer

Anisotropic wet etching generally refers to anisotropic wet etching of single crystal silicon

substrate.

7. Explain the working principle of anisotropic wet etching.(4)

Answer

Wet etchants such as KOH, EDP, or TMAH have different etch rates for different crystal

planes. Planes such as {100}, {010}, {001}, are etched very fast whereas {111} plane is etched

very slow. Hence there is directionality based on crystal orientation.

8. KOH, EDP, TMAH

a. Which one has the highest etch rate?(2)

b. Which one has the lowest selectivity?(2)

c. Which one is not compatible with CMOS electronic circuits and why?(4)

Answer

a) KOH>TMAH>EDP =>KOH

b) KOH>TMAH>EDP =>EDP

c) KOH is not compatible since it has K+ ions when decomposed, which diffuse in to silicon

and become mobile charges and cause problems for electronic circuitry.

9. What are the two categories for etch stop? Please also explain how they work.(8)

Answer

Time based etch stop: In this method etch rate is well known and controllable and the

finishing time is calculated and at that specific time etching is stopped by taking out wafers

from the etching solution.

Selectivity based etch stop: In this case there is an underlayer with a very low, ideally zero,

etch rate. Etching naturally stops when it reaches this underlayer.

10. Write down the two most important differences between physical etching and chemical

etching effects.(4)

Answer

Physical etching is anisotropic whereas chemical etching is isotropic. Chemical etching has

high selectivity whereas physical etching has low selectivity.

11. What are the methods for physical dry etching and what are their differences?(4)

Answer

Beam type etching (ion milling) and field type etching (sputter etching). Beam etching is a

serial process and does not need mask. Field etching is a parallel process and needs a mask

layer.

12. Draw an explanatory picture of a barrel plasma system.(4)

Answer

13. What is the most common purely chemical dry plasma etching application?(

Answer

Burning of photoresist or other organic films with

14. Why do we use a combination of physical and chemical etching?

Answer

Physical etching is directional but not

selective. Combining these two effects gives us a directional an

15. Please explain the three collision types in the RIE plasma system.(

Answer

Ionization: High energy electrons collide with neutral atoms and as a result of energy

exchange one electron from the outer shell of the atom is freed and the atom becomes an

ion.

Excitation: After collision an electron of the atom in one energy shell is excited to a higher

energy shell. These atoms are not

Dissociation: High energy electron collides with a molecule and causes the chemical bond to

break. This results in two ions from one molecule.

16. How does energy driven etching work?

Answer

Chemical reactants readily etch exposed surfaces at a cert

or excited sites on the horizontal surface

What is the most common purely chemical dry plasma etching application?(

Burning of photoresist or other organic films with oxygen plasma.

Why do we use a combination of physical and chemical etching?(4)

Physical etching is directional but not selective. Chemical etching is

selective. Combining these two effects gives us a directional and selective etching process.

the three collision types in the RIE plasma system.(12)

High energy electrons collide with neutral atoms and as a result of energy

exchange one electron from the outer shell of the atom is freed and the atom becomes an

sion an electron of the atom in one energy shell is excited to a higher

energy shell. These atoms are not ionized but have a higher energy electron in them.

High energy electron collides with a molecule and causes the chemical bond to

sults in two ions from one molecule.

How does energy driven etching work?(4)

Chemical reactants readily etch exposed surfaces at a certain rate. Plasma ions make crack

or excited sites on the horizontal surface upon impact. These sites or cracks become

What is the most common purely chemical dry plasma etching application?(2)

Chemical etching is isotropic but very

d selective etching process.

High energy electrons collide with neutral atoms and as a result of energy

exchange one electron from the outer shell of the atom is freed and the atom becomes an

sion an electron of the atom in one energy shell is excited to a higher

but have a higher energy electron in them.

High energy electron collides with a molecule and causes the chemical bond to

ain rate. Plasma ions make cracks

upon impact. These sites or cracks become

favorable locations for etching and etch rate on these surfaces become higher. Hence

horizontal surfaces, which are constantly bombarded by ions are etched faster compared to

vertical surfaces, which are not bombarded.

17. How does inhibitor driven etching work?(4)

Answer

An inhibitor film is formed on all surfaces either as a byproduct of etching reaction or a

byproduct of a separate inhibitor reaction. This film stops etching on all surfaces. However

horizontal surfaces are bombarded by plasma ions and these ions clear the film from

horizontal surfaces and etching continues on these surfaces.

18. Explain the working principles of end point detection systems.(8)

Answer

Laser interferometer system: A laser beam is sent to the surface of the wafer. If there is a

transparent film on the surface, this laser beam is reflected from the top and bottom surface

of the film. These two reflections form interference, whose intensity is related to the

thickness of the film and laser wavelength. As thickness change intensity of interference

changes and oscillates. When oscillations stop we reach at the end of the film hence end

point is detected. It is also possible to use reflections from the mask surface and film surface

to determine etch depth. Using this data we can also detect end point.

Optical emission spectrometer: every atom emits a specific wavelength of light when it is

recombined with an electron in its ion state. Using this principle we can say that plasma of

each atom has a different color. As long as etching continues we can see the color of etching

byproducts plasma. When this color diminishes, we know that etching has finished. This

point is the end point.