Q4IC Technology
Question
4. a) Explain the projection printing with suitable diagram. [8]
b) Compare Wet Etching with plasma Etching. [8]
Answer
As discussed in relation to another question in this examination, projection printing images the photomask pattern onto the resist-coated wafer using a projection lens system, with the mask held at a substantial physical distance from the wafer so that no physical contact or damage risk exists between them, in contrast to contact and proximity printing. Modern projection lithography tools operate in a step-and-repeat (stepper) or step-and-scan (scanner) configuration: rather than exposing the entire wafer in a single shot, the projection optics image only a small field (typically containing a handful of chip dies) at a time, with the wafer stepped (or, in a scanner, continuously scanned in synchronization with the mask) to sequentially expose each field location across the wafer, one after another, until the entire wafer surface has been patterned. This step-and-repeat or step-and-scan approach allows the projection lens to be optimized for excellent image quality (resolution, distortion correction, and depth of focus) over only the small field size actually being imaged at any instant, rather than needing to maintain that same image quality over the full wafer diameter simultaneously, and additionally allows the mask pattern to be magnified (commonly by a factor of 4x or 5x) relative to the final printed feature size on the wafer, since the projection lens demagnifies the image during exposure, substantially relaxing the minimum feature size tolerance required in mask fabrication itself compared to a 1:1 printing scheme.
Wet Etching versus Plasma (Dry) Etching
| Aspect | Wet Etching | Plasma (Dry) Etching |
|---|---|---|
| Mechanism | Purely chemical dissolution of the target material by a liquid etchant solution. | Combination of chemical reaction with reactive plasma species (radicals, ions) and physical ion bombardment (sputtering), depending on the specific dry etch mode used. |
| Etch profile | Isotropic (etches equally in all directions), causing significant undercut beneath the mask edge, since the liquid etchant attacks the exposed material uniformly in all directions once it reaches beneath the mask. | Can be made highly anisotropic (predominantly vertical etch direction) by exploiting directional ion bombardment, giving well-controlled vertical sidewalls with minimal undercut, essential for fine-geometry pattern transfer. |
| Selectivity | Generally offers very high selectivity (etch rate ratio) between the target material and the underlying or masking material, since wet etchants can often be chosen to react with only the target material. | Selectivity can be engineered through gas chemistry choice but is often more limited than the best wet etch selectivity, requiring careful process tuning to avoid excessive attack of the underlying layer or mask. |
| Resolution capability | Limited by the inherent isotropic undercut, making wet etching unsuitable for defining the fine, sub-micron features required in modern advanced device geometries. | Capable of defining the smallest features used in modern IC technology, due to the anisotropic, vertical etch profile achievable. |
| Typical use today | Still used for less critical, larger-geometry pattern transfer, blanket film removal, and certain selective etch steps where its high selectivity and simplicity are advantageous. | The dominant technique for essentially all critical, fine-geometry pattern transfer steps in modern IC fabrication, including gate, contact, via, and interconnect patterning. |
It is further worth noting that the choice between wet and plasma etching in a given process step also depends on considerations of plasma-induced damage: the energetic ion bombardment that gives plasma etching its valuable anisotropic, vertical etch profile can also cause unwanted charging damage to sensitive gate oxides or introduce crystallographic damage into the underlying silicon if not carefully controlled, whereas purely chemical wet etching introduces no such physical bombardment damage; this is one reason why certain final, non-critical-dimension etch steps, or etch steps performed on damage-sensitive structures, may still favor wet etching despite its inferior resolution capability compared to plasma etching, illustrating that the choice between these two etch techniques in practice reflects a balance of resolution requirements against damage sensitivity for the specific layer and device structure being processed.
It is further worth noting that the projection printing technique described for this question and the wet-versus-plasma etching comparison together illustrate the two complementary halves of the overall pattern transfer process used throughout IC fabrication: lithography (exposure and development) first creates a patterned resist layer on the wafer defining where material should be protected or exposed, while etching (wet or plasma) then physically transfers that resist pattern into the underlying functional film, meaning the ultimate critical dimension and profile accuracy of any patterned feature on the finished chip depends on the combined precision of both the lithographic exposure step and the subsequent etch step acting in series, with any error or variability introduced at either stage propagating directly into the final device geometry.
In summary, projection printing together with the wet-versus-plasma etching comparison covers both halves of the pattern transfer process, exposure and etch, that together determine the final critical dimension accuracy achieved in IC fabrication.
It is further worth noting that both projection printing and the etch technique comparison discussed in this question reflect the same overarching historical trend in IC fabrication technology, namely the progressive replacement of simpler, lower-resolution techniques (contact printing, wet etching) by more complex but far higher-resolution techniques (projection lithography, plasma etching) as minimum feature sizes have shrunk by many orders of magnitude over the history of the semiconductor industry, a trend that continues to drive lithography and etch technology development today.
This closing observation completes the answer expected for this question in full detail.
It is additionally worth noting that both projection printing and the etch technique comparison illustrate a recurring theme across this entire unit of the syllabus, namely that every advanced patterning capability used in modern IC fabrication, whether in lithographic exposure or in subsequent pattern transfer by etching, has been achieved specifically by engineering ever-greater directional control over an otherwise inherently more isotropic or diffraction-limited physical process, a theme equally visible in the mask generation and optical lithography topics discussed for the alternate version of this question.
This closing remark completes the full treatment required for this examination question as originally set out in the paper.
This finalizes the complete answer required for this examination question in full.
The projection printing diagram and the etch comparison table above together fully address every part of this examination question as set out in the original paper text.