Photolithography

Question 1

Which photoresist (neg pos) strengthens bonds in the photo resist when exposed to uv light?

Answer 1

Negative

Question 2

Which photoresist (neg pos) weakens bonds in the photoresist when exposed to uv light?

Answer 2

Positive

Question 3

oxigen interdiffusion occurs when and does what?

Answer 3

in negative photolithography and increases resolution, because oxygen moves to the exposed areas, limitting the spread of increased cross linking

Question 4

why do we use undercut photoresists?

Answer 4

for lift off, because this allows for a space for the contact of solvent to the photoresist.

Question 5

What causes broadening of the radiation profile?

Answer 5

scattering of photons at a reflective interfae

Question 6

How do you make the undercut for lift off?

Answer 6

overexpose the resist. negative photoresist is typically used. positive photoresist doesnt’ get these good angles.

Question 7

Aligning the mask: which is the most prefered method to align the mask? and why?

Answer 7

self-aligned or conformable masks. These have molecular contact with the machine under constructions, offeres superieor resolution, light has no change to diffract between layers.

Question 8

What is contact printing?

Answer 8

the photomask is pressed against the resist voverd wafer. this has good resolution, but contact between mask and wafer causes damage and contamination, so this is used in limitted cases.

Question 9

WHat is proximity printing?

Answer 9

this is when there is a small space between the mask and the photoresist. this takes care of some of the resolution factors that come with contact, but increase loss of resolution due to diffraction through an aperture.

Question 10

What limits your resolution in contact and proximity printing? (this question is from memory of first and second lecture)

Answer 10

the resolution is limited by the shortest wavelength that you have. … de Broglie Wavelength- the matter wave.

Question 11

What are the three photoresist profiles we discussed in class?

Answer 11

undercut, vertical (used for absolute accuracy), overcut (use when you want there to be connections). (watch out!!!! define which is under and over cut on an exam or assignment, because Madou changes which one he calls which from slides to his text book)

Question 12

What exposures are used to make: undercut, vertical, and overcut ?

Answer 12

undercut (lift-off)- high dose, vertical- medium, overcut - low

Question 13

What are some factors that effect resolution of photolithography?

Answer 13

diffraction of light at the edge of an opaque feature in the mask as light passes , alignment of wafer to mask, nonuniformities in wafer flatness, and debris between mask and wafer.

Question 14

What are the 10 steps (in order) of photolithography?

Answer 14

1)surface preparation, 2) Photoresist applicatoin, 3)Soft bake 4)Align and expose 5) Develop 6)hard bake 7)Inspection 8)Etch 9)Resist Strip 10)final inspection

Question 15

What is the first step of photolith? What does it entail?

Answer 15

dehydration bake, clean and dry the water, use hexamethyldisilazane (HDMS) to make the surface hydrophobic, larger contact angle

Question 16

What type of material is the mask made of?

Answer 16

quartz, this allows for UV to pass through (glass doesn’t)

Question 17

What is TG?

Answer 17

The glass transition temperature, where galss starts to melt.

Question 18

Since the photoresist is DISSOLVED away, this means the chemicals can attack the strengthened portion of the photoresist too. How do we prevernt the part we want to keep from going too?

Answer 18

We aim to have the exposed photoresist (positive photres) dissolve chemically at a rate 10 times faster than the unexposed.

Question 19

Which etching needs to be done in a vacuum?

Answer 19

electron and ion because they react with gas– i.e. most expensive

Question 20

Which etching (ion, electron, xray, photons) can write the narrowest?

Answer 20

Ion- because a heavier particle has the shortest wavelength, i.e. better resolution

Question 21

Classify: beam, parallel exposure— ion, xray, photon, electron

Answer 21

beam, parallel, parallel, beam

Question 22

How is the photoresist applied?

Answer 22

dropped onto spinning wafer (vacuum suction holds it still), or other ways like layer films (not common)

Question 23

What does the resist spinning thickness rely on?

Answer 23

spin speed, solution concentration, and molecular weight

Question 24

Why do we soft bake? which step is this?

Answer 24

this is step 3. we want partial evaportaion of the photoresist solvents, this improves adhesion, uniformity and etch resistance, and optimizes light absorbance characterisitcs of photoresist

Question 25

What is projection printing?

Answer 25

This is when there is a larger space than proximity printing between the mask and photoresist, and between them are imaging optic systems.

Question 26

What is the difference between developing and etching? Which steps are these?

Answer 26

These are steps 5 and 8. Developing means dissolve off the unwanted photoresist. this exposes the substrate layer to be etched, which is done via wet acid etch or dry plasma etch.

Question 27

What kind of light is used in a clean room? why?

Answer 27

Yellow light, because UV light is part of the process and must be controlled

Question 28

What is happens in photoresist strip?

Answer 28

This removes the remaining photoresist after etching, using wet acid strip or dry plasma strip. this is followed by wet clean to remove any residual

Question 29

What is the relationship between the lense size in projection printing and resolution?

Answer 29

the bigger the lense, the better resolutoin. aperture of this lense is very important. The writing of your mask can me more liberal here, because the image you project is smaller than your mask.

Question 30

What happens in submersion lithography?

Answer 30

the refractive index increases from 1 in air to 1.46 in water, thus increasing resoltuion

Question 31

What is CD?

Answer 31

It is the critical dimension, the smallest feature made consistently in a certain process ~4nm

Question 32

When are diffraction effects dominant?

Answer 32

at smaller dimensions, so they’re important in MEMs

Question 33

What is Moore’s law?

Answer 33

States that the number of transistors per square centimeter roughly doubles every two years witout an increase in cost. this hits a snag when CDs are 30 nm, and quantum mechanics comes into play

Question 34

What is numerical aperture? what does it represent?

Answer 34

It represent the ability of the lens to collect diffacted light, and is given by NA= n*sin(alpha) wher n is index of refraction and alpha is the acceptance angle of the lense.

Question 35

how does resolution relate to numerical aperture?

Answer 35

R is inversly proportional to NA: R=0.61*wavelegnth/ NA

Question 36

The way to _______ resolution is to decrease the wavelength at which the machine can operate,and to _____ the numerical aperature of the lens.

Answer 36

increase, increase

Question 37

What is the relationship of DOF and R?

Answer 37

depth of focus is inversely proportional to resolution

Question 38

What is MTF? What process is this important in?

Answer 38

the modulation transfer function. the MTF of an exposure system is defined as the ratio of the modulation in the image plane to that in the object or mask plan ( M image/ M mask) as a function of spatial frequency. MTF in other words is the ratio between image intensity modulation over object intenstiy modulation… The MTF makes a difference in Projection Printing.

Question 39

What is M? what is the ideal M value?

Answer 39

M is the Modulation: (I_max-I_min)/(I_max+I_min): measure the degree to which diffraction effects cause incident radiation to fall on the resis between the images of two slits in a mask. Ideal optics M=1;

Question 40

Where is the MTF perfect/ideal where it looks like a rectangle? WHat happens after this point?

Answer 40

At the mask aperture. Intensity bends and curves.

Question 41

MTF should generally be greater than. ____ for the resist to resolve features.

Answer 41

0.5

Question 42

How does MTF look as a function of (1/feature size)?

Answer 42

As features get larger, MTF approaches 1. min is 0.

Question 43

What is OPC?

Answer 43

Optical Proximity Correction: used to compensate somewhat for diffraction effects, it helps sharpen the edges of the features created. This improves resolution by decreasing k1.

Question 44

Phase Shift masks: explain them (how they work and what they do)

Answer 44

This process changes the phase of light in adjacent patterns by 180 degrees, which looks like the amplitude flipped, and leads to destructive interference instead of constructive interference. This results in improved MTF of aerial image on wafer, making k1 smaller. This takes advantage of the wave nature of light, and extends resolution capability of currect optical lithography. Imporoves resoltuoin and ODF

Question 45

is the photoresist sensitive to intensity or to the sign of the light wave?

Answer 45

sensitive to the intensity, not to the sign.

Question 46

Objective lense does what?

Answer 46

reduces the image, i.e. demagnifies and projects mask image

Question 47

Ther condenser lense does what?

Answer 47

It filters out the desired wavelength.

Question 48

What is farfield?

Answer 48

These are planar waves far from a distrubance

Question 49

what is nearfield?

Answer 49

These are turbulant waves near the optical disturbance

Question 50

What happens as the aperture for an image projection becomes smaller?

Answer 50

The beam, or cone, or alpha becomes larger and larger. smaller hole, we need bigger lense, so NA increases

Question 51

What is k1 in the resolution equations?

Answer 51

It is an experimentally determined parameter that depends on resist parameters, process conditions, and mask aligner optics. ranges from 0.3-0.5

Question 52

Match the three: proximity, contact, & projection —> nearfield, farfield, intermediate

Answer 52

proximity is intermediate (nearfield or fresnel), projection is farfield, contact is nearfield

Question 53

In projection printing, what is the equation for resolution?

Answer 53

R=k1*lamda/NA, k1 is emperical constant, Lambda is wavelength, NA is numerical aperture.

Question 54

What is a typical value of NA in air?

Answer 54

0.93

Question 55

What is the shortest wavelength possible? give a its numerical value

Answer 55

the de Broglie wavelength, or matter wave, which is about 1 Å.

Question 56

Define coherence:

Answer 56

the measure of the degree to which the light emitted from a source is in phase at all points along the emitted wave fronts. A point source would be all coherent light exposure, but in reality we deal with light sources of finite size (not just a point) so the degree of coherence of the light on the resist plane is dependent on that size and the resulting light is usually only partially coherent. varies from 0 to infinitiy (from cohereent to incoherent radiation.)