Optical Lithiography1

7/28/2019 Optical Lithiography1

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Progress in microprocessors

~ 10, 000 transistors

Smallest feature ~ 5 microns

~ 2 cm


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Manufacturing costs

An AMD microprocessor has 233 million transistors.

Each transistor needs 3 electrical contacts.There are ~700 million contact holes per chip.

In AMDs factories these are made at the rate of ~35 billion per second.

Drilling one hole every

three seconds, it wouldtake over 3000 yearsto drill 35 billion holes.


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55 nm contacts using EUV lithography

Dense(aligned)

Dense(staggered)

Iso(aligned)

Iso(staggered)

Dense(aligned)

Dense(staggered)

Iso(aligned)

Iso(staggered)

EUV = Extreme Ultra Violet


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EUV Lithography

Absorber


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1. Si02 layer

2. resist

3. mask

4. UV light

5. acid

6. solvent


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1.) Contact: Resist is in contact with the mask: 1:1 magnificationAdvantages: Inexpensive equipment ($~50,000-150,000), moderately high resolution (~0.5 um or better butlimited by resist thickness- 0.1 um demonstrated)Disadvantages: Contact with the mask degrades the mask (pinholes and scratches are created on the metal-

oxidelayers of the mask, particles or dirt are directly imaged in the wafer, Wafer bowing or local loss ofplanarization results in non-uniform resolution due to mask-wafer gap variations.,

2.) Proximity: Resist is almost, but not in contact with the mask: 1:1 magnificationAdvantages: Inexpensive equipment, low resolution (~1-2 um or slightly better)Disadvantages: Diffraction effects limit accuracy of pattern transfer. Less repeatable than contact

3.) Projection: Mask image is projected a distance from the mask and de-magnified to a smaller image: 1:4 -1:10magnificationAdvantages: Can be very high resolution (~0.065 um or slightly better), No mask contact results in almost nomask wear (high production compatible), mask defects or particles on mask are reduced in size on the wafer.Disadvantages: Extremely expensive and complicated equipment, diffraction effects limit accuracy of pattern

transfer.


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1.) Resolution: How small of features can you make.(Current production state of the art is ~0.065 um)

2.) Registration: Can you repeatability align one layer toanother. (~1/3 of resolution or 0.06 um)

3.) Throughput: Can these be done in a cost effectivetime. (50-100 wafers an hour, down to 1 chip perhour).


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What is the shortest wavelength possible?

Photomasks today are made from fused silica.

Fused silica has a number of advantageous properties.

Chemical stability.

Transparency for ultraviolet light.

No intrinsic birefringence.A low coefficient of thermal expansion.


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A low coefficient of thermal expansion.

0.5 ppm/oC.

If a mask changes temperature by 0.1oC, then the distancebetween two features separated by 50 mm will change by2.5 nm.

This change in registration can be absorbed into overlaybudgets.

After reduction by 4.


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The transparency of fused silica falls off sharply for wavelengths < 157 nm.

An alternative material must be used.

CaF2.

The coefficient of thermal expansion of CaF2 is 19 ppm/oC.

Versus 0.5 ppm/oC for fused silica.

The 2.5 nm of mask registration error becomes nearly 100 nm.


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What goes wrong at < 157 nm?

There will be no optical lithography forwavelengths < 193 nm.


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Documents

Optical Lithiography1