Mask ROM
The mask programming refers to creating the desired user data content of a read only memory ( ROM) using one or more masks by the semiconductor manufacturer. Depending on the structure of the memory cell is a ROM, the programming is carried out either on the final wiring mask ( metallization ) on masks for adjusting the thickness of the gate oxide or by ion implantation.
Setting the data content by the user
Classic components were in the past ROM memory devices in which, for example, smaller operating systems or application programs have been saved. Furthermore, microprocessors, or application specific integrated circuit ( ASIC ) having a read only memory, also be realized in this technology. In principle, programmable logic devices such as field programmable gate arrays ( FPGA ) or programmable logic device ( PLD ) can be realized in this technology.
The user develops the data content for the device with the semiconductor manufacturer and provides the semiconductor manufacturers the data content (usually in electronic form ) are available.
The semiconductor manufacturer produces in the semiconductor factory uniform basic components to the programming step. Only in the following programming step, the individual data content of the user is programmed into the device. Then the now programmed devices through the other single production run to completion.
Quantities and cost
The production of mask-programmed devices is economical only for a very large number of items. Furthermore, it is considered that many thousands or tens of thousands of individual devices included on a wafer. Usually only the production of a variety of individual wafers is economically viable for the semiconductor manufacturer.
This is only cost-effectively when there is a high demand for ROMs with the same data content. The semiconductor manufacturer needs to produce the individual components masks. These masks are manufactured according to the data content created by the user. These individual masks are usually very expensive and must be paid by the user. The payment of the masks, either directly as a lump sum or the cost will be assessed to the components produced.
In the production of modules, the user can dispense with the programming of the device, thus saving the cost of the programming process. Taking into account the production costs ( one-off costs for masks and unit costs per component ) by the semiconductor manufacturer and the savings in production assemblies arise in larger numbers cost advantages.
Implementation period and changes
When a change of the data content is required, new masks have to be produced by the semiconductor manufacturer. In addition, only new components must be produced with the modified data content. The running time by the semiconductor factory is in the range from a few days to a few weeks. Only then are the elements with changed data content for the customer are available.
Disadvantages
The data content of mask-programmed devices can not be subsequently changed. If these components include a data error, they are usually worthless and can not be used by the user. Furthermore, when a malfunction of the module to update the content data is no longer possible. In this case, only the exchange of the device helps (either unsolder or replacement of socketed components ). Here, there is a risk that already delivered modules must be called back to clients for rework.
Alternatives
To mask-programmed devices can be an alternative Flash - EPROM, E2PROMs or battery-buffered RAM memory devices. In point change in the data contents all three component types are always an alternative, because these components of the data content can be changed later.
A related with the mask programming technology are one-time programmable devices One Time Programmable. The devices can be programmed only once. However, unlike the mask programming this device can be programmed by the user.