Q4IC Technology

Photoresist is a light-sensitive organic polymer material that is spin-coated onto the wafer surface as a thin, uniform film and then selectively exposed to light (typically ultraviolet light) through a patterned photomask, causing a chemical change in the exposed regions that alters their solubility in a subsequent developer solution; the pattern remaining after development then serves as a protective masking layer during the following etching or ion implantation process step, allowing the underlying pattern (previously present only on the mask) to be transferred into the actual device layer on the wafer.

Positive versus Negative Photoresist

Positive photoresist undergoes a chemical change upon light exposure that increases the solubility of the exposed regions in the developer solution, so that after development, the exposed areas are dissolved and washed away while the unexposed areas remain on the wafer; the resulting resist pattern on the wafer is therefore an exact positive-tone reproduction of the mask pattern (wherever the mask was transparent and light passed through, the resist is removed). A common example is the diazonaphthoquinone-novolac (DNQ-novolac) resist system widely used in conventional (g-line and i-line) lithography, in which the DNQ photoactive compound acts as a dissolution inhibitor in its unexposed state, but photochemically converts upon UV exposure into a carboxylic acid that is readily soluble in an alkaline developer, dramatically increasing the exposed region's dissolution rate relative to the unexposed novolac resin matrix. Negative photoresist, by contrast, undergoes a chemical change upon light exposure that decreases the solubility of the exposed regions (typically through a light-induced crosslinking reaction that forms an insoluble polymer network), so that after development, the exposed areas remain on the wafer while the unexposed areas are dissolved and washed away; the resulting pattern is therefore a negative-tone reproduction of the mask pattern. A common example is the older KTFR (Kodak Thin Film Resist, based on cyclized polyisoprene rubber with a bis-azide crosslinking agent) resist historically used in early IC fabrication, though negative resists generally suffer from resist swelling during development (since the crosslinked but still partially solvent-permeable network absorbs some developer solution), which degrades resolution compared to modern positive resists, and consequently most advanced, fine-geometry lithography today uses positive-tone chemically amplified resists instead.

Applications of Dry Etching

• Gate electrode patterning: dry (plasma) etching is essential for defining polysilicon or metal gate electrodes in MOS transistors with the highly anisotropic (vertical sidewall), precisely controlled critical dimensions required for modern sub-micron and nanometer-scale gate lengths, which wet etching's inherently isotropic undercutting cannot achieve.
• Contact and via etching: dry etching creates the high-aspect-ratio vertical holes through interlevel dielectric layers needed to make electrical contact between different metal interconnect layers or between metal and the underlying silicon, where the vertical sidewalls are essential to avoid excessive lateral space consumption in densely packed circuits.
• Metal interconnect patterning: dry etching (particularly reactive ion etching using chlorine-based chemistries for aluminum) is used to pattern fine-pitch metal interconnect lines with the vertical sidewall profile needed for reliable, densely spaced wiring.
• Shallow trench isolation (STI) formation: dry etching creates the narrow, deep trenches in silicon used to electrically isolate adjacent devices in modern CMOS technology, again requiring the anisotropic profile that only dry etching can reliably provide at fine pitch.
• Selective removal of thin films: dry etching is used throughout the process flow to selectively remove exposed regions of oxide, nitride, polysilicon, or metal films as defined by the overlying patterned photoresist, wherever anisotropic, well-controlled etch profiles and minimal undercut are required.

Proximity, Contact, and Projection Printing

These three techniques differ in how the photomask pattern is optically transferred onto the resist-coated wafer during exposure. In contact printing, the photomask is placed in direct physical contact with the resist-coated wafer during exposure, giving essentially 1:1 image transfer with minimal diffraction-induced resolution loss since there is effectively no gap for the light to diffract across; however, direct physical contact between mask and wafer causes mask damage and particle contamination transfer with repeated use, severely limiting mask lifetime and yield, which made contact printing impractical for high-volume production despite its excellent resolution. In proximity printing, a small, deliberately maintained gap (typically 10-50 micrometers) is left between the mask and the wafer during exposure, avoiding the physical contact damage problem of contact printing, but at the cost of somewhat degraded resolution due to diffraction of light across this gap, with resolution limited approximately by the relation related to the square root of the product of wavelength and gap distance; proximity printing therefore trades away some resolution for substantially improved mask lifetime compared to contact printing. In projection printing, the photomask is held at a substantial physical distance from the wafer, and a projection lens system (typically providing 4x or 5x demagnification in modern step-and-repeat or step-and-scan tools) images the mask pattern onto the wafer, completely eliminating any physical contact between mask and wafer (essentially unlimited mask lifetime), and additionally allowing the mask pattern itself to be several times larger than the final printed feature size (since the projection optics demagnify the image), substantially easing the mask-making tolerance requirements; projection printing, particularly in step-and-repeat or step-and-scan form, is the standard technique used in essentially all modern high-volume IC manufacturing due to this combination of excellent resolution, mask longevity, and eased mask fabrication tolerances.

It is further worth noting that the historical progression from contact to proximity to projection printing was driven directly by the increasingly stringent mask lifetime and defect density requirements of growing wafer sizes and shrinking feature geometries: as wafer diameters grew from a few tens of millimetres to the 300 millimetre standard used in modern fabs, and as minimum feature sizes shrank from tens of micrometres to the nanometre scale, the yield loss caused by even a single mask-damaging contact event in contact printing became economically unacceptable, directly motivating the industry-wide transition first to proximity printing and ultimately to projection printing as the sole technique used in current high-volume, fine-geometry semiconductor manufacturing.

It is also worth noting that the choice of photoresist tone (positive or negative) in a given lithography step is often driven by which tone gives the more favorable pattern for the specific feature being defined: since positive resist retains resist in unexposed areas, isolated dark features on the mask (such as an isolated contact hole pattern, where most of the mask area is opaque with only small transparent openings) print naturally using a positive resist and clear-field mask combination, while certain other pattern types are more naturally suited to a dark-field mask and negative resist combination, meaning the historical shift toward almost universal use of positive resist in modern IC lithography reflects both positive resist's superior resolution (due to its immunity to the swelling effects that afflict negative resist during development) and the prevalence of mask and pattern styles that are naturally suited to the positive-tone process.