What Is X-Ray Lithography?
X-ray lithography is a high-resolution patterning technique to fabricate micro and nanoelectronic devices. It employs X-ray radiation with wavelengths between 0.4 and 4 nanometres to transfer geometric patterns from a mask onto a light-sensitive resist layer deposited on a substrate, typically silicon.
How Does X-Ray Lithography Work?
X-ray lithography works similarly to photolithography but with X-rays. The following is a step-by-step breakdown:
- Substrate Preparation: A thin layer of resist is deposited on the substrate, typically a silicon wafer.
- Mask Making: An X-ray mask containing the desired circuit pattern is created. This mask is similar to a photographic negative, with the desired circuit features being transparent to X-rays, and blocked areas made of an X-ray absorbing material like gold.
- Alignment: The X-ray mask is precisely positioned very close to the resist-coated substrate (unlike photolithography which can use lenses for projection). This proximity is because X-rays cannot be focussed using lenses.
- X-ray Exposure: Powerful X-rays, typically generated by a synchrotron radiation source, are directed through the mask. The X-rays pass through the transparent areas of the mask, exposing and hardening the resist in those corresponding regions on the substrate. The areas under the opaque parts of the mask that block the X-rays remain unexposed.
- Development: After exposure, the wafer is treated with a chemical developer that selectively removes the unexposed resist. This leaves behind the desired circuit pattern defined by the exposed and hardened resist.
- Etching (Optional): In some cases, the remaining resist layer becomes a mask for further etching processes that transfer the circuit pattern onto the underlying substrate material.
What Are The Advantages & Disadvantages Of X-Ray Lithography?
Advantages of X-Ray Lithography
- High Resolution: Due to the much shorter wavelength of X-rays than visible light, X-ray lithography achieves higher resolution than traditional lithography techniques. This allows for the creation of extremely tiny features on chips, enabling more complex and powerful devices.
- Reduced Diffraction: X-rays are less prone to diffraction (bending) than light, resulting in sharper and more precise circuit patterns.
Limitations of X-Ray Lithography
- Complexity And Cost: X-ray sources and masks are expensive and require specialised facilities, making the overall process costly.
- Proximity Printing: The need for proximity between the mask and resist adds complexity to the manufacturing process and can introduce challenges in maintaining consistent quality.
Due to these limitations, X-ray lithography is primarily used for high-end chip production where miniaturisation and extreme precision are crucial.
Is X-Ray Lithography Better Than Photolithography?
The similarities between X-ray lithography and photolithography end at their use of electromagnetic radiation to create nanometre-scale circuits on silicon wafers.
The former boasts impressive resolution and sharp feature definition, making it ideal for cutting-edge chip development where cramming in more transistors is crucial. However, its high cost and complex process limit its use to specialised applications.
In contrast, photolithography is the workhorse for everyday chip production due to its affordability, established infrastructure, and faster turnaround times. While it struggles to achieve the same level of miniaturisation as X-ray lithography, its practicality makes it the preferred choice for most chip manufacturing needs.
