Electron beam machines etch patterns onto a glass mask.
Photoresist is spun on wafers. UV exposes resist through the mask. Photolithography is similar to photography.
Lithography areas are illuminated with yellow light. The photoresist is insensitive to wavelengths longer than yellow. Normal "darkrooms" must use "safelight" that is dark red.
Exposure, mask positioning, and environment (humidity, temperature) must be tightly controlled.
Common equipment includes I-LINE steppers and DUV eximer scanners.
Photoresist is designed for particular wavelengths. GLINE and ILINE are near UV Helium spectral lines. DUV (deep UV) is a recent process.
Photoresist is composed of a resin polymer base with photosensitive material, solvents to make the resist fluid and dyes to tailor its spectral characteristics.
Photoresist is spun at 2000-4500 rpm. Adding solvent changes the viscosity, which affects the thickness of the final layer.
Factors affecting lithography
Resolution depens on the aspect ratio (AR) - thickness/critical dimension (linewidth). Typical values for AR = 3.5.
Focus latitude refers to the depth of focus.
Focus range determines linewidth as a function of exposure dose.
Selectivity is the rate at which photoresist etches.
Wavelengths (g-line = 436 nm and i-line = 365 nm) are spectral lines from a high-pressure Hg lamp.
After spinning the resist, the wafers pass through a track system where they undergo a dehydration bake at 150-200 C. HMDS (?) is used to stabilize the photoresist. Spining the wafers at 300-400 rpm produces a lip on the edge. Solvent is squirted on to eliminate the edge lip. Then the wafers are soft baked 80-100 C to evaporate the solvent and improve adhesion.
Exposure occurs through a 5 to 1 reduction lens. Alignment targets on the mask are generated to align the masks that follow. Laser interferometric systems are used to adjust tilt during the registration process.
Exposures require mJ energy levels. Reflected light from the substrate contributes to the exposure.
Wafers are developed in a acquous solution of TMAH at a PH of about 0.262 Normal (basic). The basic solutions neutralizes the acids on exposed photoresist. Most semiconductor processes use a positive photoresist.
After a water rinse, the wafers are baked at 105-130 C to drive out water and harden the resist. Then the wafers are inspected visually to track defects.
R = k1 l/NA where NA = numerical aperture
Depth of focus = k2 l/(NA)
DUV (deep UV) uses 248 nm light for 0.18-0.25 micro processes.
Anti-reflection coats are often needed on shiny materials to reduce standing waves and interference effects. Silicon-oxynitride is used as an AR coat.
DUV resist includes an active chemical to make the resist more transparent to DUV wavelengths.
DUV process uses 4:1 reduction. An initial mask 100x132 mm is reduced to 25x33 mm.
DUV processes used laser-produced light a krypton-fluorine gas laser at 248 nm. A scanned light slit is used rather than steppers.
Inspection makes use of high-resolution bright-field microscopy.
Future systems will use DUV-F2 157 nm light or non-optical techniques such as projection E-beam and x-ray.
Maintained by John Loomis, last updated 15 Dec 2003