photolithography ece/che 4752: microelectronics processing laboratory gary s. may january 22, 2004

Photolithography

ECE/ChE 4752: Microelectronics ECE/ChE 4752: Microelectronics Processing LaboratoryProcessing Laboratory

Gary S. May

January 22, 2004

Outline

IntroductionIntroduction Clean RoomsClean Rooms ExposureExposure MasksMasks PhotoresistPhotoresist Pattern TransferPattern Transfer E-Beam LithographyE-Beam Lithography

Definition Photo-imaging method by which geometric

patterns are transferred from a mask to the substrate (wafer).

Uses photosensitive polymer (called “photoresist”).

Features transferred to substrate surface by shining light through glass plates (called “masks”).

BasicProcess Flow

Process Sequence1) Clean wafer surface

bake (get rid of H2O) RCA clean apply adhesion promoter (HMDS = “hexi-methyl-di-

silizane”)2) Deposit photoresist (usually by spin-coating)3) Soft bake (or “pre-bake”) - removes solvents from liquid

photoresist4) Exposure (pattern transfer)5) Development - remove soluble photoresist6) Post bake (or “hard bake”) - desensitizes remaining

photoresist to light7) Resist removal (“stripping”)

Outline


The Need

Electronics fabrication requires a clean processing environment for lithography.

Goal: minimize dust particles that can settle on substrates or masks and cause DEFECTS.

Dust on a mask looks like an opaque feature; will get transferred to underlying layers; can lead to short circuits or open circuits.

Graphic Illustration Particle 1 may result

in formation of a pinhole in underlying layer.

Particle 2 may cause a constriction of current flow in a metal runner.

Particle 3 can lead to a short between the two conducting regions.

Class

)/(0

Dx

DCF

)/(0

Dx

DCF

)/(0

Dx

DCF

Numerical designation taken from maximum allowable number of particles 0.5 m and larger per ft3 (English system).

For IC fabrication, a class 100 clean room is required (about four orders of magnitude lower than ordinary room air).

For photolithography, class 10 or better is required.

Particle Size Distribution Curve

Sample ProblemA 300 x 300 mm square substrate is exposed for 1 minute under laminar flow at 30 m/min. How many dust particles will land on this substrate in a Class 1000 clean room?

SOLUTION:

1) Class 1000 => 35,000 particles/m3 (from graph)

2) Air flow volume over wafer/min = 30 m/min (0.3m x 0.3m) = 2.7 m3

3) # of particles = 35,000 x 2.7 = 94,500!!!

If each of these causes a defect, we are in serious trouble!

Outline


Performance Metrics ResolutionResolution: minimum feature dimension that : minimum feature dimension that

can be transferred with high fidelity to a can be transferred with high fidelity to a resist film. resist film.

RegistrationRegistration:: how accurately patterns on how accurately patterns on successive masks can be aligned (or overlaid) successive masks can be aligned (or overlaid) with respect to previously defined patterns. with respect to previously defined patterns.

ThroughputThroughput:: number of wafers that can be number of wafers that can be exposed/unit time for a given mask level.exposed/unit time for a given mask level.

Shadow Printing

Mask and wafer in direct contact (contact printing); or

Mask and wafer in close proximity (proximity printing).

Contact Printing*

Contact between the resist and mask provides a Contact between the resist and mask provides a resolution of ~1 resolution of ~1 m. m.

Drawback: dust particles on the wafer can be Drawback: dust particles on the wafer can be imbedded into mask where mask makes contact imbedded into mask where mask makes contact with the wafer. with the wafer.

Imbedded particles cause permanent damage to Imbedded particles cause permanent damage to mask and result in defects with each succeeding mask and result in defects with each succeeding exposure. exposure.

* * We use this in lab.We use this in lab.

Proximity Printing Small gap (10 – 50 m) between the wafer and the

mask. Minimizes mask damage, but … Gap results in optical diffraction at feature edges

that degrades resolution to 2–5 m. Minimum linewidth (or critical dimension):

gCD

when = wavelength and g = gap

Projection Printing

Wafer many centimeters from maskWafer many centimeters from mask To increase resolution, only small portion of the To increase resolution, only small portion of the

mask is exposed at a time. mask is exposed at a time. Small image area is scanned or stepped over the Small image area is scanned or stepped over the

wafer to cover the entire wafer surface. wafer to cover the entire wafer surface. After exposure of one site, wafer is moved to next After exposure of one site, wafer is moved to next

site and the process is repeated. site and the process is repeated. CalledCalled step-and-repeat projection, step-and-repeat projection, with a with a

demagnification ratio demagnification ratio MM:1 :1

Step and Repeat Projection

After exposuring After exposuring one site, wafer one site, wafer moved to next moved to next site and the site and the process repeats. process repeats.

Demagnification Demagnification ratio ratio MM:1 :1

Resolution

Given by:Given by:

where where kk11 is a process dependent factor and is a process dependent factor and

NA = numerical aperture, which isNA = numerical aperture, which is

NA1

klm

sinNA n

where is the index of refractionn

Depth of Focus

Expressed as:Expressed as:

where where kk22 is another process-dependent factor is another process-dependent factor

22 )NA(sin

2/

tan

2/DOF

kll mm

Outline


Making Masks CAD system used to describe the circuit patterns electrically. Digital data produced by CAD system drives a pattern generator that

transfers the patterns directly to electron-sensitized mask. Mask consists of a fused silica substrate covered with chrominum. Circuit pattern is first transferred to the electron-sensitized layer (electron

resist), which is transferred into the underlying chrominum layer for the finished mask.

Use of Masks

Patterns on a mask represent one level of an IC design.

Composite layout is broken into mask levels that correspond to the manufacturing process sequence.

15 – 20 different mask levels are typically required for a complete IC process.

Mask Composition

Fused silica plate 15 15 cm, 0.6 cm thick

Accommodates lens field sizes for 4:1 or 5:1 optical exposure tools

Outline


Definition

Photosensitive polymer compound that either gets more or less soluble when exposed to light.

Photolithography labs have yellow light because photoresist is sensitive to wavelenghts > 500 nm.

Types1. Positive: gets more soluble after exposure

2. Negative: gets less soluble after exposure.

Development

More exposure energy vs. Higher resolution

Contrast Ratio

where: where: EETT = sensitivity or “threshold” energy (where resist = sensitivity or “threshold” energy (where resist

becomes completely soluble)becomes completely soluble)

EE11 = energy to reach 100% resist thickness (50% for = energy to reach 100% resist thickness (50% for

negative resist)negative resist)

1

1

ln

E

ET

Larger => higher solubility of resist and sharper images

ET and E1 interchanged for negative resists

Outline


Steps

1. Apply adhesion promoter (HMDS)2. Spin coat photoresist at 1000 – 10,000 rpm3. “Soft bake” (90 – 120°C for 60 –120 sec) to

remove solvent4. Alignment5. Exposure 6. Development 7. “Post bake” (100 – 180°C) to increase adhesion8. Etch exposed regions 9. Strip resist

Illustration

Alignment Mask for each layer must be aligned to previous layer patterns For a minimum feature size ~ 1 m => alignment tolerance should be +/- 0.2 m To align, wafer is held on vacuum chuck and moved around using an xyz stage Alignment marks: special patterns on mask used to facilitate accurate alignment.

BEFORE

AFTER

Outline


Limitations of Optical Lithography

Resolution becoming a challenge for deep-Resolution becoming a challenge for deep-submicron IC process requirementssubmicron IC process requirements

Complexity of mask production and mask Complexity of mask production and mask inspection inspection

High cost of masksHigh cost of masks

Electron Beam Lithography

Involves direct exposure of the resist by a Involves direct exposure of the resist by a focused electron beam without a mask focused electron beam without a mask

Currently used to primarily produce Currently used to primarily produce photomasksphotomasks

Resolution as low as 10 – 25 nmResolution as low as 10 – 25 nm

Schematic Electron gun

generates beam of electrons

Condenser lenses focus the e-beam

Beam-blanking plates turn beam on and off

Advantages

Generation of submicron resist geometriesGeneration of submicron resist geometries Highly automated and precisely controlled Highly automated and precisely controlled

operationoperation Greater depth of focus than that available Greater depth of focus than that available

from optical lithographyfrom optical lithography Direct patterning on wafer without using a Direct patterning on wafer without using a

mask mask

Scanning RasterRaster: : beam scans sequentially over every beam scans sequentially over every

possible location on the mask and turned off possible location on the mask and turned off where no exposure is required where no exposure is required

VectorVector: : beam directed only to requested beam directed only to requested features, jumps from feature to featurefeatures, jumps from feature to feature

Disadvantages

Low throughputLow throughput Expensive resistsExpensive resists Proximity effect: backscattering of Proximity effect: backscattering of

electrons irradiates adjacent regions and electrons irradiates adjacent regions and limits minimum spacing between featureslimits minimum spacing between features

photolithography ece/che 4752: microelectronics processing laboratory gary s. may january 22, 2004

Documents