Photomasks

Masks in photolithographic production

A photomask displays the pattern of an integrated circuit and works as a template in the photolithographic production of the resulting device. Masks are used in the fabrication of high-end electronic components, semiconductor devices, or displays. Even though transistors are becoming smaller and smaller, photomask-based UV lithography is still the industry-standard workhorse for micro-fabrication.

A photomask usually consists of a metal covered soda-lime or quartz plate with transparent openings. The metal absorbs the light at different wavelengths: Standard masks use chrome as absorption material for i-, g- and h-line. The transparent image on the photomask represents the master template, the pattern which is transferred by a mask aligner or stepper into a photosensitive layer by photolithography. For semiconductor or display applications a whole set of photomasks is required to produce the complete device. For semiconductor manufacturing the photomask is protected by a foil (pellicle) to avoid any contamination.

The image on the photomask is patterned by laser lithography or e-beam depending on the requirements.  A a master template for photolithographic manufacturing, the mask has to fulfill stringent requirements. Those include key specifications like linewidth uniformity, pattern position accuracy, edge roughness and minimum feature size. In addition, to enable a large process window for the final process, the photomask specifications have to be considerably better than the target application.

Requirements

 

  • High line width uniformity and low edge roughness, considerably better than required for the final application
  • Precise pattern positioning and plate-to-plate accuracy to enable accurate alignment of multilayer structures
  • Good Mura condition to avoid disturbances in regular periodic patterns, especially for display applications
  • High repeatability to ensure stable photomask quality

Application images

 

Photomask production

Ever since 2007, our VPG systems and their current successors, the VPG+ systems, have proven themselves to be the ideal solution for the high-volume production of demanding photomasks – particularly in the fields of electronic packaging, color filters, light emitting diodes, and touch panels.

Photomasks for Advanced Packaging Applications

3D and 2.5D packaging allows significantly smaller chips with higher performance compared to other technologies. For all advanced packaging applications lithography is a key fabrication process. The VPG+ series presents the solution to fabricate high quality photomasks required for advanced packaging applications. VPG+ systems offer high speed, automatic distortion compensation and excellent resolution in order to master these applications.

Photomask structures

An example written by a VPG+. The squares are 2 μm in size.

Photomasks produced by Emulsion Lithography

Heidelberg Instruments also provide solutions for emulsion lithography processes. This traditional lithography technique is used with highly sensitive material and permits high throughput rates with low intensity laser sources. The typical structure size here is 20 µm and applications can be found in the photomasks fabrication for the production of circuit boards, chip packaging, lead frames, ball grid arrays (BGA)-chip-packaging etc. – Please contact us for a consultation if you are interested in a dedicated emulsion lithography Tool.

Key benefits

 

  • High resolution

    enables the production of photomasks for optical, UV, and DUV mask-based lithography.

  • CD uniformity

    to create a production master with well-defined structure sizes.

  • Edge roughness

    considerably better than required for the target application.

  • Placement accuracy

    for pattern consistency throughout the complete mask set and to reduce Mura effects.

Products

Our VPG+ series, both the VPG+ systems for small and those for large exposure area, are particularly suited for the production of photomasks. The ULTRA represents our laser mask writer specifically for the semiconductor industry. Of course our other direct-write systems may be used in the fabrication of photomasks as well, for example the DWL series.

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