JPS6367341B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a read-only memory (hereinafter referred to as "ROM") whose stored contents are determined by ion implantation. In particular, the present invention relates to an integrated circuit having a ROM and a RAM (Random Access Memory) on the same chip, in which a code corresponding to the stored contents of the ROM can be easily identified.

[Conventional technology]

The most commonly used ROM that uses semiconductor technology is a memory that determines the stored content by changing a photomask. Among these, the method using a mask for the ion implantation process is used in many integrated circuits because the chip area is small compared to the memory capacity. However, in the case of the ion implantation method, it is impossible to easily visually identify the memory contents during the integrated circuit manufacturing process, and manufacturing management is extremely difficult when manufacturing many types of ROM at the same time.

In order to improve this point, a mask other than the ion implantation mask that determines the memory contents of the ROM (for example, an aluminum process mask) is used to write the code corresponding to the memory contents, such as numbers, etc., separately from the functions of the integrated circuit. There is a known method for providing identification on an integrated circuit in the following format. But with this method, the mask ROM
Since a mask that is not originally required is also required for the production of the product, there is a drawback that the production cost increases in terms of production management.

Therefore, the applicant uses selective ion implantation to
We proposed a technology for writing ROM codes onto ROM chips (Japanese Patent Application Laid-open No. 34786/1986).

In addition, a technique has been proposed in which ions are implanted to the extent that the threshold value of the gate region of the MOS transistor that constitutes the memory cell of the ROM is reduced to zero, and fixed memory is stored in the RAM (Japanese Patent Laid-Open No. 52-87332). Publication No.).

[Problem that the invention seeks to solve]

However, writing a code indicating the contents stored in ROM can be applied to integrated circuits made up only of ROM, but in integrated circuits in which ROM and RAM are formed on the same chip, a special identification code is written in the ROM. Providing a memory area is wasteful in terms of area, so using RAM and ROM only when the power is turned on.
There was a need for an integrated circuit that could semi-fixably store a ROM identification code in a RAM cell.

Furthermore, if the memory cells of the RAM are fixedly stored, data cannot be written to the memory cells of the RAM as a RAM, resulting in a portion that cannot be used as a RAM.

The present invention solves the above-mentioned problem by allowing ROM identification codes to be read from RAM cells only when power is turned on, and thereafter allowing ROM and RAM to be written and read as normal RAM on the same chip. The purpose is to provide integrated circuits for the future.

[Means for solving problems]

The present invention includes a read-only memory in which memory contents are written by ion implantation and a static random access memory on the same chip, and ions corresponding to the contents of the read-only memory are implanted into some of the cells of the random access memory. In the integrated circuit in which the implantation is applied, the flip-flop of the memory cell at a particular address of the random address memory corresponds to the content stored in the read-only memory for one transistor of the pair of transistors forming the flip-flop. Ion implantation is performed to change the threshold value according to the code to change the threshold value, and the energy and impurity concentration of the ion implantation are in a predetermined state when the flip-flop is powered on, and when inputting data, it operates as a random access memory. It is characterized by

[Effect]

Selective ion implantation is performed on only one transistor of a pair of transistors in a flip-flop that constitutes a memory cell at a specific address in static RAM to increase the threshold value of that transistor.
Modify it to the extent that it operates as RAM, put the flip-flop in an unbalanced state, and store the ROM identification code.

When the power is turned on, the flip-flop is in a predetermined state due to the unbalance of the threshold voltages of the transistors, so reading the RAM cell causes the flip-flop to read the ROM.
The identification code can be read and recognized. After that,
It can be freely written to and read from as normal RAM.

〔Example〕

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 1 is a schematic diagram illustrating the principle of an embodiment of the present invention. 1 is a chip. On this chip 1, a RAM 2 and a ROM 3 are formed.
This figure 1 shows RAM2 for easy explanation.
and ROM 3 are shown as a simple matrix composed of memory cells 4 and 5. this
Address signal lines 6 to 9 and output lines 11 to 14 are connected to the RAM 2, respectively. Also,
Address signal lines 16-19 and output lines 21-24 are connected to the ROM 3. Also,
In the memory cell 5 of FIG. 1, the shaded area indicates the ion-implanted area. Note that actual integrated circuits are manufactured on a large scale, have more than one ROM, and have a large storage capacity.

FIG. 2 is a circuit diagram of the memory cell 4 of the RAM 2. A flip-flop circuit is constructed using transistors 26 and 27 as inverters and transistors 28 and 29 as load resistance elements. A power supply voltage V _DD is applied to this cell.

This memory cell 4 includes, for example, a transistor 28
By performing ion implantation in the region surrounded by the dotted line, the threshold voltage becomes somewhat unbalanced with respect to the transistor 29, and immediately after applying the power supply voltage _VDD , this flip-flop becomes "0" or "1".
It can be set so that one of the two conditions is always constant. The degree of this ion implantation is
By not setting the threshold value to zero, but by setting the energy and impurity concentration to a level that allows data to be input and written to this flip-flop, the power supply voltage can be reduced.
Immediately after applying V _DD , the flip-flop can be set to the predetermined state described above. Even under this somewhat unbalanced condition, under normal operating conditions
It can be configured so that it has no effect on operating as RAM. The present invention
Ion implantation into a part of the RAM is performed using the same mask at the same time as the ion implantation when writing ROM contents onto the same chip. By appropriately selecting the area surrounded by the dotted line, the degree of unbalance can be set so that it always remains constant immediately after power is applied and has no effect during normal operation.

Now, it is assumed that the memory contents of ROM3 are ion-implanted in the shaded area, and correspondingly, the memory cells 4 of RAM2 at a specific address (in this example, the cell group specified by address line 6) are implanted. Ions are implanted into the load resistance element transistor 28 at the same time as the ROM according to the ROM code. Due to this ion implantation, the output of RAM2 becomes output line 1.
1 is obtained, "0" is obtained on the output line 12, "1" is obtained on the output line 13, and "0" is obtained on the output line 14. If read with a hex code, this will be "5", and it can be identified that ROM3 is the fifth code manufactured. This code number and the content to be written in the ROM 3 are determined in advance in correspondence.

Here, in general, integrated circuits with built-in ROM3 and RAM2 are tested in order to perform logic tests.
It has a structure that allows testing the contents of RAM2. Therefore, if the output of this RAM 2 is used to light a lamp or the like, it can be easily identified that the code is 5. Note that the diagonal line on the left side of the memory cell of the output line 11 designated by the address line 6 in FIG. 1 indicates that ions are implanted into the transistor 28 in FIG. 2.

FIG. 3 shows a different pattern of an embodiment of the invention. Compared to the example pattern above, ROM3
An output line 22 for the address line 19 of the memory contents of
The only difference is in the contents of and 23. this
In order to identify only the contents stored in ROM3, it is necessary to judge all the contents of address lines 16 to 19.
The ROMs 3 in FIG. 1 and FIG. 3 cannot be distinguished. However, in the present invention, ions are implanted into the memory cells of RAM2 so as to output codes corresponding to ROM3. That is, the address line 6 of the ion-implanted RAM 2 causes the output line 11 to
"1" is given to the output line 12, "1" is given to the output line 13, and "0" is given to the output line 14, which is "7" in hex code. Therefore,
It can be easily identified that the stored contents of ROM3 correspond to the seventh code.

In the above example, RAM2 is 4 bits, so only 16 types of ROM3 can be identified.
By increasing the number of bits of 2, the types of codes in the ROM 3 that can be identified can be easily increased.

〔Effect of the invention〕

As explained above, the present invention is a static type
By changing the threshold value of one of the paired transistors in the flip-flop that makes up the memory cell of the RAM by ion implantation, the flip-flop is made unbalanced so that the flip-flop will be in a predetermined state when the power supply voltage is applied.

Therefore, when the power supply voltage is applied, the code of the ROM can be read by reading the initial state of the memory cell of this RAM. Therefore, the ROM code can be easily known from the output of the RAM when the power supply voltage is applied, and after that, it can be freely written and read as a RAM.

Further, the manufacturing cost of an integrated circuit in which a ROM and a RAM are formed on the same chip can be reduced, and there is an advantage that a special area for storing the identification code of the ROM is not required.

[Brief explanation of drawings]

FIG. 1 is a diagram explaining the principle of the present invention. FIG. 2 is a memory cell circuit diagram of the RAM of the above example. FIG. 3 is another pattern diagram of the ROM in FIG. 1. 1...chip, 2...RAM, 3...ROM, 4,5
...Memory cell, 6-9, 16-19...Address signal line, 11-14, 21-24...Output line, 26-
29...Transistor.

Claims (1)

[Claims] 1. A read-only memory in which memory contents are written by ion implantation and a static random access memory are provided on the same chip, and some of the cells of the random access memory are provided with the contents of the read-only memory. In an integrated circuit with corresponding ion implantation, a flip-flop of a memory cell at a specific address of the random address memory is stored in the read-only memory for one transistor of a pair of transistors forming the flip-flop. Ion implantation is performed to change the threshold value according to the code corresponding to the content, and the energy and impurity concentration of the ion implantation are in a predetermined state when the flip-flop is powered on, and for data input, it is used as a random access memory. An integrated circuit characterized in that it is functional.