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Direct pattern writing by a scanned electron beam in an electron-sensitive photoresist.

Radiant energy per exposed area. For steady intensity $I$ and exposure duration $\tau$, $D=I\tau$; the developed photoresist profile also depends on chemical processing.

A photoresist whose exposed regions become less soluble in developer over the relevant process window, so adequately exposed regions remain after development.

Oxygen-plasma ashing, preferably a remote or downstream arrangement when low damage matters, removes organic photoresist chemically. Feed oxygen through a plasma source energized by radiofrequency or microwave power. Electron-impact dissociation generates reactive oxygen species; neutral radicals reach the wafer in a separate downstream region. Keep the wafer off a strongly biased driven electrode so energetic ion bombardment is much smaller than in directional reactive-ion etching. A barrel asher with shielded wafers is another chemical-stripping implementation.These equations represent radical production and an ideal complete-oxidation balance; actual exhaust can also contain CO and fragments. Pumping removes volatile products. Directional shape fidelity is unnecessary for blanket stripping, while access around surface relief and high chemical removal selectivity are useful. Monitor endpoint and wafer temperature, and verify removal with a witness surface. oxygen can oxidize exposed materials, charging can still damage sensitive structures, and inorganic or metal-containing residues may require a compatible separate clean. Ashing removes organic contamination; it is not a universal residue-removal step.^[ref]

Finite numerical aperture and diffraction remove high spatial frequencies from a mask image. Neighboring features modify the aerial intensity, and the photoresist threshold turns this blurred intensity into biased edges, shortened lines and rounded corners. optical proximity correction deliberately predistorts the mask: line-end extensions and small serifs can compensate for local loss of exposure, while other edges are moved back where exposure spreads too far. The correction is calibrated for illumination, resist and focus; it does not restore arbitrarily high lost spatial frequencies.

double patterning separates a dense target pattern into two sparser patterns, each printable within the single-exposure process window. Two exposures with an intervening pattern-fixing or transfer step combine their features on the wafer. For regular lines, alternating line assignment can halve the final pitch while each exposure retains a larger pitch. optical proximity correction corrects local image bias; double patterning divides the density burden between exposures. Overlay error and the minimum printable isolated feature still constrain the result. Simply exposing the same unmodified resist twice without an appropriate process separation does not give the same independent thresholding.

First, use a phase-shift mask: neighbouring openings transmit appropriately phased light, increasing the intensity slope and improving separation of neighbouring bright regions. The benefit is pattern dependent and phase conflicts or mask defects must be controlled. Second, use off-axis illumination matched to the grating orientation, so useful diffracted orders fall inside the projection pupil and contribute interference contrast at smaller pitch. Check orientation dependence and the reduced process window; it does not enlarge the geometrical pupil.

Third, use optical proximity correction and subresolution assist features to compensate the pattern-dependent aerial-image distortion at edges and corners. Verify the printed critical dimensions using a focus–exposure matrix and scanning electron microscopy. These are three adaptations of the existing exposure process that lower effective $k_1$; none promises arbitrary subwavelength structure. A thinner, higher-contrast photoresist and an antireflection underlayer can further improve pattern transfer, but do not change the wavelength.

One workable route uses two resist coatings and a metal-defined optical aperture. The following are eleven fabrication operations; exposure/development is grouped as one lithographic operation, and rear metallization includes its contact activation if required. Detailed cleaning, rinsing and inspection occur between compatible operations.

1. Spin-coat a first photoresist on the front surface to define a mesa mask.
2. Expose and develop that mask, retaining resist over the complete active-cell mesa, including its future front contacts.
3. Etch the exposed semiconductor through the p-type layers and slightly into the n-type base. This removes the unwanted junction around the cell and produces the lateral mesa step.
4. Strip the first photoresist, leaving a clean mesa with its cap intact.
5. Spin-coat a second photoresist suitable for the lift-off process.
6. Expose and develop openings at the intended front grid or contact pads, positioned safely inside the mesa edge.
7. Evaporate a p-contact metal stack onto the exposed gallium arsenide and resist. Au/Zn-based contact metallization is a conventional candidate, subject to doping and contact-resistance qualification.
8. Dissolve the second photoresist to lift off unwanted metal, leaving the front contacts on the cap.
9. Selectively remove the cap from the light-receiving aperture, using the front metal as a hard mask and the aluminium gallium arsenide window as the etch stop.
10. Deposit the rear n-contact metallization, for example a qualified AuGe/Ni/Au system, with an appropriate contact-activation anneal if required and compatible with the front contact.
11. Deposit an antireflection dielectric over the optical aperture through a mechanical shadow mask that keeps the contact pads accessible. Its refractive index and thickness are chosen for the target optical band.

The essential material changes are junction isolation at the mesa edge, metal retained on the front cap, cap removed only from the optical aperture, and a conducting rear contact. The metal systems and anneal are a process design, not compositions specified by the drawing. A workable design must also qualify the passivation of etched sidewalls and the front-contact surface preparation.

A fully clear opening requires the higher clearing threshold, not merely the onset of thinning. At $\tau =20\mathrm{s}$,The intensity curve crosses this level at about $x=0.25$ and $3.75\mu \mathrm{m}$. Therefore the clear opening is approximatelyThe longer exposure widens the opening because even the relatively dim image tails now accumulate enough exposure dose to clear the positive photoresist. The edge readings are approximate; interpolating a plotted curve does not support nanometre precision.

For a positive photoresist, let $D_r$ be the largest dose retaining the nominal film and $D_c$ the clearing dose. With the conventional photoresist contrast definition,For a sinusoidal aerial image, the bright-to-dark dose ratio is $(1+C)/(1-C)$, where $C$ is the Michelson contrast. A dose window that clears bright regions while retaining dark regions requiresThis assumes a fully modulated input pattern and that the plotted modulation transfer function gives the aerial-image contrast. It is a dose-window criterion, not a statement that the resist creates optical information absent from the image.^[ref]

The two thresholds are approximatelyReading their intersections with the supplied curve gives, to graphical accuracy,for the lower- and higher-contrast resists respectively. These are the results independent of the dimensional frequency convention. Their minimum full periods are $P=1/\nu$, and equal line/space widths are half those periods:An ArF excimer laser uses $\lambda \simeq 193\mathrm{nm}$. A numerical length requires specifying what the caption's “Rayleigh limit” means. Taking it literally as the reciprocal of the Rayleigh criterion point separation givesIf instead the graph uses the common incoherent-cutoff normalization ${\nu }_0=2\mathrm{NA}/\lambda$, the widths are $302\mathrm{nm}$ and $201\mathrm{nm}$, with periods twice those values. The caption does not explicitly provide an equation for ${\nu }_0$ or define whether “feature size” means period or half-pitch, so these conventions must not be silently mixed. The higher-contrast resist resolves the higher normalized frequency in either convention.

MIT 6.774, Fall 2004, institutional lecture transcript. The discussion distinguishes largely chemical photoresist ashing in barrel/plasma systems from more strongly directional reactive-ion etching. The source is a teaching lecture, not an experimental paper.

Chemical stripping of organic material, commonly photoresist, by reactive species produced in a plasma. oxygen radicals convert suitable organic material into volatile oxidation products. A remote source reduces direct energetic ion bombardment, but exposed materials can still oxidize and inorganic residue can remain.

A photoresist whose exposed regions dissolve faster in the developer than unexposed regions.