Optical Lithography and the MicroWriter ML3

Optical Lithography

Lithography is the process by which a geometric pattern is transferred onto a substrate coated with a radiation-sensitive polymer film called photoresist. The resist changes its solubility in a specific developer solution depending on whether it has been exposed to radiation. In positive resists, exposure breaks polymeric chains, making the irradiated regions more soluble and therefore removable during development. In negative resists, exposure causes cross-linking, rendering the irradiated regions insoluble and leaving them behind after development.

The achievable resolution is fundamentally limited by diffraction. For projection-based systems, the minimum resolvable feature size \(l_m\) is given by:

\[l_m = k_1 \frac{\lambda}{\mathrm{NA}}\]

where \(\lambda\) is the exposure wavelength, \(\mathrm{NA} = \bar{n}\sin\theta\) is the numerical aperture of the objective (with \(\bar{n}\) the refractive index of the medium and \(\theta\) the half-angle of the cone of light collected by the lens), and \(k_1\) is a process-dependent factor that encodes resist chemistry, source coherence, and sensitivity. A closely related quantity is the depth of focus:

\[\mathrm{DOF} = k_2 \frac{\lambda}{\mathrm{NA}^2}\]

This expresses the axial range over which the image remains acceptably sharp. Importantly, increasing NA improves resolution linearly but degrades DOF quadratically; it is therefore generally preferable to reduce \(\lambda\) rather than push NA when seeking higher resolution.

Two strategies are commonly used to reduce \(\lambda\): excimer laser sources (e.g. KrF at 248 nm, ArF at 193 nm) and immersion lithography, where the medium between objective and wafer is replaced by ultra-pure water (\(\bar{n} \approx 1.33\)), effectively reducing the in-medium wavelength and increasing resolution by the same factor.

For shadow printing (contact and proximity modes without a projection lens), the relevant resolution limit is instead set by diffraction at the gap \(g\) between mask and substrate:

\[\mathrm{CD} \simeq \sqrt{\lambda \, g}\]

Contact printing (\(g \to 0\)) offers the best shadow-mode resolution (~1 µm) at the cost of mask wear; proximity printing introduces a gap of 10–50 µm to protect the mask but degrades resolution by a factor of several.

A further critical parameter is overlay registration: the ability to align successive lithographic layers to one another. Industrial processes may require tens of sequential exposures, each of which must be aligned to sub-nanometre precision relative to the previous layer. Alignment markers patterned in an early step serve as the fixed reference frame for all subsequent layers.

Laser Writing (Maskless direct-write lithography)

Laser writing eliminates the physical mask entirely. A focused laser beam is scanned across the resist-coated substrate under computer control, exposing the pattern defined in a CAD file. Because only one point is illuminated at a time, there are no inter-feature diffraction effects from a mask aperture array. The pattern can be changed between exposures simply by modifying the CAD file, making laser writing ideal for prototyping.

The trade-off is throughput: sequential point-by-point exposure is inherently slower than a single-shot full-field exposure through a mask. Overlay accuracy is limited by the positioning precision of the motorised stage and the accuracy of the marker-based alignment procedure.

The MicroWriter ML3 (Durham Magneto Optics) is a direct-write laser lithography system operating at \(\lambda = 385\ \mathrm{nm}\) with a semiconductor (LED/laser diode) light source. The pattern to be exposed is designed in a standard CAD environment and uploaded to the machine's DMO software, which projects the pattern directly onto the photoresist via computer-controlled optics. Different objective lenses are available to trade resolution against field of view. Because the resist layer is transparent at 385 nm, the autofocus acts on the substrate surface underneath the resist, and a focus correction must be applied to account for the optical path length introduced by the resist film (which depends on resist thickness and refractive index).

MicroWriter ML3 — Step-by-Step Protocol

Preparation

  • Design the pattern in a CAD program; organise different lithographic layers as separate CAD layers or files;
  • spin the substrate with the chosen resist stack following the spin-coating protocol;
  • note the total resist stack thickness required for the focus correction:

Software and sample loading

  • Open the Durham Magneto Optics (DMO) software;
  • load the sample onto the stage;
  • in "Align wafer", using the real-time internal camera, locate the sample on the stage;
  • input the substrate thickness in the software so the system can calculate the correct focus position;
  • press autofocus button.

Alignment

  • In "Alignment" select "Center wafer",
  • identify at least three alignment markers distributed across the sample, ideally one near each corner or at three extremes. Register each marker, the software uses these to define the sample coordinate system,
  • allow "Rotation" and then press the button with a red dot in a cartesian plane, to set the origin and orientation of the coordinate axis at the centre of the sample. The transformation corrects for sample translation, rotation, and any residual stage non-orthogonality.

Pattern upload and job setup

  • In "Prepare Pattern" press the sum button in "Job list",
  • chose "File name" to upload the CAD file, the software automatically aligns the CAD coordinate system to the axis defined by the markers,
  • chose the positions of the center of the CAD that is wanted to expose, the layer, the quality of exposition and the resolution. In case is wanted to use, to this particular CAD a different dose or focuse with respect to the global one, change dose correction and focus correction.

Focus and focus correction

  • Then proceed to "Exposure" and insert the wanted global dose in "Resisty sensitivity" and a global focus correction in case is wanted,
  • change the objective to ×10 (or the maximum magnification allowed),
  • press "Autofocus" to allow the system to find focus on the substrate surface beneath the (transparent) resist,
  • in "Tools" select "Not generate position automatically" to manually choose the focuse positions within the sample boundaries where the focus check is performed.

Exposure

  • Press "Start Rendering" to check the duration of the exposure,
  • press the button with a green stoplight to begin the exposure,
  • after exposure, proceed with development following the appropriate resist protocol.