Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

Photolithography

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

• Photolithography Key Processing Area Driving Technological

Development

• Must Continually Improve Lithographic Resolution to Generate

Ever Smaller Features at Smaller Technology Nodes

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

• Basic Resolution Equation Given by

Where k1 Is a Measure of the Lithographic Aggressiveness, λ Is

the Illumination Wavelength, and NA Is the Numerical

Aperture of the Lens

• Resolution Was Continually Improved Over Numerous Years

by Continuously Decreasing k1 and λ and Increasing NA in

Order to Generate Ever Smaller Features

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

• Lithographic Aggressiveness k1 Has Been Continuously Improved

Over the Years

k1 Has Been Decreased from 0.80 to Around 0.25

Driven by Varied Mask Improvements and Exposure Techniques

- Optical-Proximity Correction (OPC) Adjusts for Various Diffraction Effects

- Phase-Shift Masking (PSM) Can Further Improve Resolution

- Off-Axis Illumination Also Improves Resolution

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

T-Shaped Feature Formed Using OPC (a), Featureless Blob Formed During

Normal Exposure (c), Serifs Commonly Used to Optimized Feature Using Optical-

Proximity Correction (b), and Final Optimized Exposed Feature (d).

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

Conventional Binary Mask (a) Versus Alternating Phase-Shift Mask

Used to Enhance Contrast and Improve Resolution (b) Showing

Resultant Mask Electric-Field, Amplitude, and Image-Intensity.

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

Off-Axis Illumination Showing Resultant Off-Centered Diffraction

Pattern in the Entrance Pupil of the Objective Lens (a) Which Allows Two

Beams to Be Collected During Exposure at a Larger Angle (b) and a

Smaller Pitch to Be Resolved Resulting in a Higher Resolution.

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

• These Varied Resolution Enhancement Techniques Reduce k1 and

Minimum Resolution by More Than 50%

• However, Mask Improvements Greatly Increase Mask

Complexity Substantially Increasing Mask Costs

Mask Costs Have Increased from Less Than $100,000 at 180 nm to

Approximately $2 Million at 28 nm

Mask Costs for 22/20 nm Has Further Risen to $5 to $8 Million

Mask Costs for Nodes Down to 7 nm Have Risen Even Further

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

• Wavelengths Have Also Decreased from 436 nm Down to 193 nm

Mercury-Arc Lamps Were Initially Used for the Illumination Source

- Lamps Emitted Strongly at Given Wavelengths Between 350 to 450 nm

- Most Common Wavelengths Were g-Line (436 nm) and i-Line (365 nm)

- g-Line Used Down to ~0.8 µm and i-Line Down to 0.4-0.8 µm

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

Typical Mercury-Arc Spectrum.

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

• Industry Then Transitioned to Excimer Lasers

First Transitioned to KrF Lasers Using a 248 nm Wavelength Between the 0.35 µm

and 0.18 µm Technology Nodes

Then Moved to ArF Excimer Lasers Using a 193 nm Wavelength at Smaller

Technology Nodes

Industry Attempted to Move to F2 Lasers at 157 nm

- Lenses and Reticles Must Be Made from CaF2 Instead of Fused Silicon (SiO2)

Which is Capable of Transmitting This Wavelength

- Encountered Both Severe Transparency and Manufacturability Issues

- Industry Could Not Ultimately Transition to 157 nm Wavelength

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

• NA Has Also Been Increased from 0.28 to 1.35

Increased NA from 0.28 to 0.80 by Continuously Increasing Size of Lens

- Lens Size Increased from Around 10 Pounds to Around 1 Ton

- Improves Capture Efficiency By Increasing Light Capture from the Side of the

Lens Improving Resolution

- NA Limited by Refractive Index of Air Which is 1.00

NA Can Be Further Increased by Increasing Refractive Index of Working Medium

- Transitioned to Immersion Lithography Using Highly Purified Water at 45 nm

- Increases Refractive Index of Working Medium to 1.44 at 193 nm Wavelength

- Increased NA to 1.35 Further Increasing Resolution

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

During Immersion Lithography Light Travels Downward

Through a System of Lenses and Then Through a Pool of Water

Before Reaching the Underlying Photoresist on the Wafer

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

Photolithography

Semiconductor Process Integration mtinker@utdallas.edu EE 6372 Summer Semester

• These Improvements More Than Enough to Generate the

Resolution Needed for Technology Used in the 1990s

• However, These Improvements Insufficient to Generate

Technology at 22 nm and Below

Could Still Generate Sufficient Resolution at 28 nm Half Node with Single Exposure

However, At 22 nm Required Multiple Exposures or Sidewall Spacer Image

Transfer to Generate Images with Sufficient Resolution

- Substantially Increased Cost of Process

- Ultimately Started Generating Severe Yield Issues Near 10 to 7 nm

- Driving Shift to Extreme Ultraviolet Radiation (EUV) Using 13.5 nm Radiation

at the 7 nm Technology Node

- EUV Effectively Soft X-Ray and a Fundamentally Different Technology Than

Optical Which Has Required Approximately 20 Years to Implement

Photolithography