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What is Mask ROM (MROM)?
Mask ROM or MROM refers to a special kind of Read Only Memory which is manufactured along with the contents written or programmed in it by the manufacturer of the Integrated Circuit and not by the user.
Technically, the term ‘mask’ here signifies that the chip was masked off during the photolithography process. This term is used to differentiate this ROM ‘chip’ from the ‘category’ of ROM that includes PROMs and others.
- A mask ROM refers to that particular chip that is manufactured with the relevant contents already instilled in it.
- The data stored in a mask ROM is permanent since it is primarily a non-volatile memory.
- There is a grid of word lines and bit lines in a mask ROM joined selectively to form an arbitrary look up table.
Understanding Mask ROM
Mask Read Only Memory or mask ROM or simply MROM is a memory chip that comes with the contents in it already.
The term is typically used to differentiate this chip from the ROM category which includes PROMs or Programmable Read Only Memory and EPROMs or Electrically Erasable Programmable Read Only Memory.
Moreover, this term is also used to differentiate these particular memory chips from the memories that are programmable by the users such as FPGAs or Field Programmable Gate Array and others.
The mask ROMs are quite inexpensive memory chips and are considered to be the original ROMs that were the first hard wired devices that came with a set of pre-programmed data and instructions.
In addition to that, these specific types of memory chips also include a software mask which is burned on the chip while designing the semiconductor as a part of the manufacturing process.
Ideally, the manufacturer takes the specification of the ROM from the customer in a specific format and in a tabular form and then creates the consequent mask for the paths to generate the desired output.
The preferred data is changed into a custom mask layer during the final bonding of the interconnections on the memory chip and this gives it the name.
In the older technology of the solid state ROM, there were combinational logic gates that are manually joined for mapping the n-bit address input on the arbitrary values of the m-bit data output to create a look-up table.
On the other hand, in the mask ROM, the grid of bit lines and word lines are joined selectively together with the transistor switches to signify a random look-up table.
This consisted of an expected propagation delay and a standard physical layout.
There are several advantages of using a mask ROM but the most significant one of all is that the cost of production of it is very low.
In fact, it all depends on the cost of the IC used for making it and different major factors of it such as its size per bit.
Even when these specific types of memory chips are manufactured in large quantities, the overall cost is significantly lower in comparison to any other type of secondary memory.
And, a mask ROM is far more compact and smaller in size in comparison to the other types of semiconductor memory. This also makes the mask ROM quite cheaper.
In addition to the above benefits, a mask ROM is less vulnerable to damages since the data stored in it are permanent.
This means that when you use a mask ROM you will be able to protect your data from damaging it due to any likely mistakes.
Since the mask storage is a type of binary storage, it is very secure and cannot be modified.
This makes them the most useful storage for storing and protecting sensitive information.
Most importantly, it is easy to create and needs very little maintenance and therefore it is so popular.
The data in the mask ROM is usually encoded physically onto the circuit and therefore it can be programmed only during the fabrication stage.
You may consider this to be a limitation of sorts for the mask ROM because it brings in a large number of severe disadvantages.
Some of the disadvantages are:
- The users need to contact the manufacturer to create the custom design which means that it is economical only when the mask ROMs are manufactured and purchased in large quantities
- It is for the same reason the turnaround time of completing the design for a custom mask ROM and the users getting the finished product is pretty long
- Mask ROMs are not a very practical and useful alternative memory especially for the Research and Development team because they need to amend the contents of the memory repeatedly in order to further refine their design and
- There is no other way to fix a faulty mask ROM shipped other than recalling it and physically replacing every unit of the product.
Therefore, design errors in the mask ROMs can be very costly.
Also, the overall lifespan of the mask ROM is pretty short which means that you will need to replace them quite frequently which will add to the cost factor.
And finally, a mask ROM is pretty slow in operation in comparison to other traditional memories.
Design and Production
Using rewritable non-volatile memory such as EEPROM or Electrically Erasable Programmable Read Only Memory and UV-EPROM or Ultraviolet Erasable Programmable Read Only Memory for the development stage of a project is a common practice.
And, when the code is finalized, a switch to mask ROM is made as well.
A mask ROM, just as the name suggests, is masked off during the production stage itself. Here, the term ‘Mask’ signifies those parts of the Integrated Circuit which are slender electronic circuits that help in processing the data.
These masks are usually small blocks of codes stored in a covered region during the manufacturing process.
This circuit is typically sheltered with opaque plates. These plates are called photo-masks in which there are holes or transparencies that let the light pass on to a few specific areas.
Where there are no such holes, the light is blocked. As you can very well imagine, this creates a distinct pattern with lights.
The mask ROM is also specially designed to endure high temperature and not lose data simply due to this reason.
This is done with special focus on the flash memories that tend to lose the data stored in them beyond a particular temperature threshold.
Ideally, due to the rise in temperature to about 275oC, the charge stored in a memory may leak away rendering the data stored in a CMOS based flash memory with an average life expectancy of about 10 years unreadable in just a matter of a couple of hours.
And, in most of the cases these are not depleted completely.
A mask ROM comes with a predetermined set of root key IDs that does not allow the users to change the data stored in it. However, the data in the device can be changed by the user by using the version of the mask ROM.
When produced in high volumes, a mask ROM can be a very cost-effective and useful alternative to these:
- Programmable Read Only Memory or PROM
- Erasable Read Only Memory or EROM
- Electrically Erasable Read Only Memory or EEPROM
- Non Volatile Read Only Memory or NVRAM and
- Flash Memory.
A mask ROM is not produced as the other memory chips used in a computer. This is because the transistors in it are typically arranged even before the actual manufacturing process of the semiconductor commences.
While designing the mask ROM, there are a few specific considerations to make by the chip designers.
One of the most significant considerations is the width as well as the placement of the transistor.
As for all the other parameters, these are usually laid out by the process engineers before that.
In general, there are not too many manufacturers of mask ROM chips in comparison to the number of producers of EPROM chips, EEPROM chips, PROM chips, NVRAM chips, and Flash memory chips.
One of the most significant reasons for this momentous difference in the number of manufactures of mask ROM chips and other memory chips is that these chips are cost-effective only when these are produced in large numbers.
The semiconductor used to produce the mask ROM chips is described as that specific material that is half insular and half conductive.
In its simplest form however this can be changed by using electrical impulses.
Typically, there are two types of semiconductors used namely:
- N-type semiconductors that have a much higher concentration of electrons and
- P-type semiconductors that have a lower concentration of electrons.
It is due to this particular difference in the two types of semiconductors that determines the overall differences in the design in the production of the mask ROM chips.
Typically, if the semiconductor has a lower concentration of electrons it will not allow hardwiring a large number of transistors in it.
If done, it will surely affect the storage density capabilities as well as the capacitance of the Integrated Circuit.
There are different quality standards followed by the manufacturers of mask ROMs. These are:
- SMD 5962-98586, which is followed for 512 x 8-bit custom mask ROM, microcircuit, BiCMOS or Bipolar CMOS or Complementary Metal Oxide Semiconductor, memory, digital, and monolithic silicon
- SMD 5962-98644, which is followed for 32k x 8-bit mask programmable ROM, microcircuit, Complementary Metal Oxide Semiconductor or CMOS, memory, digital, SOI or Silicon On Insulator, radiation hardened, monolithic silicon
- SMD 5962-97517, which is followed for 32k x 8-bit mask programmable ROM, microcircuit, Complementary Metal Oxide Semiconductor, memory, digital, radiation hardened, monolithic silicon and
- SMD 5962-97544, which is followed for 8k x 8-bit mask programmable ROM, microcircuit, Complementary Metal Oxide Semiconductor, memory, digital, radiation hardened, monolithic silicon.
There are a few specific steps followed during the fabrication process of the mask Read Only Memory. These are:
- Shaping the gate oxide film along with the sources and drain regions as well as the gate electrode on one part of the semiconductor substrate to create the Metal Oxide Semiconductor or MOS transistor
- Applying a BPSG or Borophosphosilicate Glass film over the whole exposed surface of the consequential structure obtained after forming of the MOS transistor in order to smooth the surface
- Creating the metal electrode on those particular parts of the BPSG film that are disposed on the drain and source regions
- Creating a pad on those particular parts of the BPSG film that are disposed on another part of the semiconductor substrate which is adapted to establish a connection with an external circuit
- Applying a coat of a nitride film on the whole exposed surface of the consequent structure after creating the pad and removing a part of the nitride film disposed on the pad to expose it
- Applying a coat of a photo resist film over the whole exposed surface of the consequent structure after removing a portion of the nitride film from over the pad
- Removing a part of the photoresist film disposed on top of the gate electrode as well as the parts over the drain and source regions of the MOS transistor that is to be entrenched with ROM code ions
- Exposing the nitride film partially along with the drain and source region portions that are disposed next to the gate electrode
- Removing the exposed part of the nitride film in order to expose the BPSG film partially
- Removing the exposed part of the BPSG film selectively in order to expose the gate electrode along with the drain and source region portions disposed next to the gate electrode
- Implanting the Read Only Memory code ions into the exposed gate electrode as well as on the drain and source region portions
- Applying the Poly Imide Iso Indro Quizolin Dione or PIQ film over the whole exposed surface of the resulting structure after the implantation of the code ions as well as the removal of the part of the PIQ film that is disposed over the left over photo resist film
- Removing the residual photoresist film and
- Curing the left over PIQ film by using a heat treatment and creating an impurity diffusion region at the same time in that particular part of the semiconductor substrate where it is disposed under the gate electrode.
In addition to that, a special fabrication method is also required to be followed to designate the multiple MOS transistors of the mask ROM.
This particular method for fabricating the whole structure of the memory chip substantially is necessary to make sure that the selected transistors operate as designated in the enhancement mode.
The steps involved in this particular stage of fabrication of the mask Read Only Memory are almost the same as above.
Working Process of Mask ROM
The mask of the ROM works in a specific process which is called photolithography.
The mask typically acts as a kind of Intellectual Property protection and disallows reproduction of the product by anyone else other than the Integrated Circuit manufacturer.
Even the user cannot change the data stored in a mask ROM. However, users rely on the mask ROM in particular to store the final code of their projects.
There are MOS transistors in a mask ROM. A few of these specific transistors are meant to operate in a depletion mode so that they can specify operation in one logic state.
On the other hand, some of the other designated transistors are meant to operate in the enhancement mode.
Different Applications of Mask ROM
There are several useful applications of mask ROM but some of the most major ones are:
- In the network operating systems
- In the server operating systems
- For storing fonts of the laser printers especially and
- For storing sound data for some electronic musical instruments especially.
There are some ICs that may contain only mask ROM but a few others may contain a wide variety of other devices along with it as well.
Since the size of the mask ROM is very small, these chips are ideal for using in a lot of smaller devices as well such as:
- Smartphones and
- Gaming consoles.
Another common use of a mask ROM is as a ‘fallback’ or an external backup in case the main ROM stops working.
More often than not, mask ROM chips are used in computer systems that need more sustainability in the long term.
In the computers, a mask ROM can also be used to store the essential firmware for an Integrated Circuit.
Apart from that, it can also be used as a secondary memory for a lot of different purposes such as for storing data.
And, there are a few specific types of mask ROMs that may even be used to safeguard the passwords.
Apart from that, you will also find quite a few specific types of microcontrollers that contain mask ROM in them for storing all of their firmware and the bootloader. The firmware helps in protecting the data and information from the hackers.
A few microcontrollers or tiny computers on the Integrated Circuits, as said earlier, come with a non-volatile memory and a mask ROM for development purposes and for finalized code respectively.
Therefore, with all the advantages and design features, functionalities and applications, the mask ROM is still a popular choice in spite of the fact that these are quite slow in operation in comparison to the conventional Read Only Memory chips.
So, as you can see from this article, the mask ROM is a useful memory chip to store microcode of projects, though it has some significant downsides.
There are several applications of this particular memory chip in computers, especially those particular ones that need long term sustainability.