DUV Excimer Laser, UV bulbs, Stepper, and Step/Scan Technology:
I. Stepper: This is the tool that shoot a whole field image and step to the next field.
----Advantages:
-----1) It's faster than the scan and step tool
-----2) Field to field exposure is more uniform
----Disadvantages:
-----1) Uses expensive lens system (cost over $1M)
-----2) Excimer laser can densify the quartz and change its --------characteristics.
-----3) Limited number of systems can be manufactured due to limited
--------number of companies that can grind the lenses to high --------precision.
-----4) Possibility of worse spherical abberation compared to the scan
II. Scan/Step: A small portion of the lens is used to focus and a
small part of the field is exposed. The beam is scanned across the entire field. The stage then move the wafer to the next field and the process is repeated.
-----Adavantages:
-----1) Mirrors are used heavily to focus the beam. Mirrors don't
--------get densified by the excimer laser. Thus doesn't have to --------change the lens out often.
-----2) Cheaper because less lenses are used. Lenses are more
--------expensive than mirror.
-----3) Better spherical aberration (for focusing)
-----4) More systems can be built because mirrors are easy to produce
-----Disadvantages:
-----1) Slower throughput.
-----2) Possible stitching across the field
III. UV bulbs: DUV wavelength source for the stepper or scanners.
-----Advantages:
-----1) Cheap
-----Disadvantages:
-----1) Intensity is not constant and decreases as approaching bulb
--------life. Thus inconsistent exposure dosage.
-----2) Spectrum is broad even with filter. This will lead to
--------focusing problem due to spherical arberration and variation
--------in same feature size across the field.
IV. Excimer Laser:
-----Advantages:
-----1) Monochromatic. Consistent dosage from field to field.
-----2) No drift in wavelength and dosage as the life of the laser
--------approaches its PM frequency.
-----3) Higher throughput
-----Disadvantages:
-----1) It is expensive.
-----2) It densify the lenses.
So much for the background. What leading fabs who have used DUV excimer laser steppers and scanners throughout the industry found out is that the yield is higher, the throughput is increased, and the speed is running at higher speed compared to same chips that were manufactured using DUV bulbs. Higher speed and higher yield has to do with variations in the polygate CDs. The tigher the distribution of the poly CDs the better the chip performs since all transistors are running at same frequency. The tighter the distribution the easier it is to drop the gate width to get higher speed without going beyond the designed gate width. If you go beyond the designed gate width then the transistor will be leaky and will not switch. Had the distribution of the gate width been large from using the UV bulbs it will be impossible to drop the gate width further without getting some transistors with gate widhts beyond the forbidden region.
Due to this result CPU companies such as Intel, IBM, AMD,TXN, etc. will use DUV excimer laser to make fast microprocessors. DRAM/SRAM makers will use DUV excimer laser to achieve higher throughput. lower cost of ownership, and higher yield.
ASM will introduce the scan and step tool soon. The reason they resort to this technique is that ZEISS can only supply a limited number of lenses and the demand for the steppers is much greater than ZEISS can satisfy.
Maxwell |
