The NVM Insider, Issue 6 - Page 2- Tech Tidbits

Tech Tidbits: Converting Sidense OTP to Mask Programmable ROM

Todd Humes, VP of Product Engineering and Jim Lipman, Director of Marketing, Sidense

For large amounts of on-chip code and data, mask ROM provides an inexpensive and easily programmed storage mechanism. However, the inability to configure ROM after wafer processing means that information stored in the ROM cannot be changed in the field, either to correct errors or to provide updated storage, which can lead to expensive silicon re-spins and time-to-product delays.

This is why you should consider using Sidense OTP for your silicon development and prototyping. There are several reasons why field-programmable OTP makes sense:

• You can commit to silicon before your code is finalized, which will accelerate your time-to-product
• You have the flexibility of trying different versions of your chip’s code
• Your chip can support additional features that may not be available on initial silicon
• Sidense OTP is very dense and has fast access times
• The conversion to mask ROM for cost reduction is easy to achieve

Using Sidense OTP in place of ROM does require slightly more area. In addition, the programming time Sidense OTP requires increases approximately linearly with bit count. Since ROM is programmed during chip fabrication, there is no programming time or cost at test. While these considerations are not very important for small bit count applications, they are at high bit counts, for example, when the OTP is storing hundreds of thousands of bits of boot code or firmware. Verifying Sidense OTP also results in additional test time.

These OTP factors are acceptable at a prototype phase of chip development, but may become too expensive during production, especially for high bit-count applications. However, the ability to complete code development during silicon fabrication and make and test code changes during prototyping coupled with the ability to accelerate market entry and reduce final product cost is a compelling argument for using field-programmable OTP during product development.

You can reduce programming time for SiPROM macros by programming one bit per bank simultaneously instead of a single bit at a time. This reduces programming time by a factor of two or four, depending on the number of banks in the macro configuration you are using. However, converting all or part of your OTP macro to mask ROM can significantly reduce overall programming time along with ATE test cost.

Conversion to mask ROM requires a single GDS layer change – diffusion (thin gate oxide) – within a standard process flow. Since many customers do their prototype development using a non-critical (and less expensive) mask set, when they go into production they will be purchasing a production quality mask set anyway, so they do not incur any incremental costs of this layer change. If a customer uses a dedicated prototype mask set to accelerate product time to market, the cost of changing this layer is easily justified.

There are many benefits of converting either all or part of a Sidense OTP macro to mask ROM once all or part of the bit storage is frozen.

• Significant cost savings due to the reduced time for ATE programming of the macro. For differential or redundant read-mode macro operation, the programming time savings is even greater than for single-cell read mode
• Since the memory footprint does not change when converting field-programmable OTP to mask ROM, NRE costs for layout are minimal
• You can “mix and match” OTP and ROM in the same macro for applications such as factory or field trimming of analog circuitry and sensor conditioning; product customization (one silicon chip with several variants) where functionality is enabled by OTP programming; chip identification, lot date coding, and other ID functions; future code patching; and end customer personalization, such as that required for digital hearing aids.

The following example shows a 128 Kbit (8K x 16) Sidense OTP macro used for storing code. 112 Kbits of code storage have been finalized and the macro section with this stored code converted to ROM. After the conversion, the programming time for this macro is reduced by 87.5%.

Contact This e-mail address is being protected from spambots. You need JavaScript enabled to view it for more details on implementing ROM in your design.