Understanding Maskset Type – MPW, MLM, MLR and Single-Maskset

Tapeout is a major milestone in every ASIC project lifecycle. It means the design phase is completed and you are ready to send out the GDSII to the fab for production.

The term “tapeout” was coined in 70’s. Historically, engineered used a magnetic tape to store all the ASIC design files. The event of carrying out the tape to the foundry was thus called “tapeout”.

Before wafer production can start, a Maskset needs to be ready. Foundries have mask making partners and use them to produce masksets optimized for project requirements.

Creation of a maskset is often the most expensive NRE-related cost in ASIC production.

Typically, foundries provide 3 types of maskset services (see the drawing below for more details).

MPW – Multi-project wafer

If you have any doubts about your design functionality or performance, MPW service will help you lower the risk involved with a costly dedicated maskset. MPW process’s goal is to help customers get samples of their ASIC (or IP) before taping out using MLM or Full Maskset. The outcome of MPW process is bare dies, with the number of dies varying between the different fabs and process nodes. The MPW maskset consists of various projects from different customers. Distributing the maskset price among projects drive the cost down, the typical price of MPW shuttle is 90% of full maskset price.

• MPW runs involve the fabrication of multiple IC designs from different customers or projects on a single silicon wafer.
• Each project usually shares the same maskset for a certain technology node (for example: 65nm or 180nm).
• MPW is the best way to share the high costs of mask production, making it more cost-effective for low-volume or prototyping purposes for example universities.
• It’s commonly used by small to medium-sized companies, startups, or research institutions to reduce manufacturing costs.

MPW shuttle starts on specific dates. Ask your fab about MPW schedules and how often it occurs.

MLM – Multi Layer Mask

MLM (Multi Layer Mask) or MLR (Multi Layer Reticle) services help reduce the tapeout NRE cost (full maskset cost). This method allows combining up to 4 masks into one, and hence reducing the total number of masks that need to be created. As the number of masks is reduces — the NRE reduced as well.

What is the catch with MLM? The drawback is the wafer price. While the NRE price decreases — the wafer price increases. Therefore MLM is recommended for projects that require low or medium production volume.

You can think of MLM as a financial service. You pay lower NRE (vs. full maskset), but on the other hand, you pay a higher wafer price. If you know your production projection it’s very easy to calculate whether to use MLR or Full Maskset. The decision here is pure economic.

Foundries do not like MLM. It slows down their production throughput. Therefore – ask you fab if they support MLM.

Full Maskset or Single Maskset

Full maskset uses the most optimized masks for production volume. This is the real thing.

• In a single maskset approach, an IC design is produced in the foundry using a set of photomasks designed specifically for that particular project.
• The masks contain the pattern of the integrated circuit (IC) to be manufactured – based o the submitted GDSII. Each layer of the IC requires its own mask.
• Single maskset fabrication is typically used for high-volume production runs of a single design. It’s expensive but offers the advantage of optimized masks tailored for the specific design and therefore a lower wafer price.

I hope this article explains the difference between Full maskset, MLR and MPW shuttle and you are able to select the right tapeout for your project. Tapeout is normally a very stressed and exciting period; don’t forget that tapeout is also a good cause for celebration.