JPH0298898A - Diode read-only memory - Google Patents

Abstract

PURPOSE:To increase storable data capacity by controlling and setting the level of break-down voltage every Zener diode means of each memory cell, and providing a means to detect the level. CONSTITUTION:A row decoder 2b, Zener diodes 3 to 5 respectively connected between a bit line 24 and a word line 25 so as to be inversely biased, and sense amplifiers 8 connected to the Zener diodes 3 to 5 respectively constituting single memory cell through the bit line 24 are provided. In response to the voltages successively applied from a row decoder 2b, the Zener diodes 3 to 5 where the break-down voltages VZ different in level are provided individually are successively energized to the selected word line 25, and to which voltage level the Zener diode is responded and energized is detected by a sensing amplifier 8. Thus, the storable data capacity can be increased.

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates generally to diode read-only memory devices, and more particularly to diode read-only memory devices using Zener diodes with controlled breakdown voltages.

[Conventional technology]

FIG. 4 is a circuit diagram showing a conventional diode read-only memory device (hereinafter referred to as ROM). This diode R
OM is described, for example, in the book entitled ``rVLsI System Design'' (John S., Morogal, 1982).
Wiley & 5ons). Referring to FIG. 4, this diode ROM includes a row decoder 2a connected to receive row address signals RAO to RAn, and each bit line 24 and word line 2.
5, and a sense amplifier 8 connected to each diode 23 via a bit vA24. Each diode 23 constitutes a memory cell. Each word 1iI25 is connected to and selected by the row decoder 2a. Each sense amplifier 8 is connected to receive a chip selection signal CE applied from the outside via a terminal 9. This diode RO
M is for reading the signal written in the memory cell in word single Q, and the data t read by the sense amplifier 8
Output No. A DO or Dm. [Invention solves the problem 1. In the conventional diode ROM shown in FIG. In a read operation, row decoder 2a applies high level ffl pressure to one of word lines 25 in response to row address signals RAO to RAn. The sense amplifier 8 reads and outputs the written data No. 713 by detecting the current flowing through the three diodes 23. In this way, in the conventional diode ROM, one memory cell has one memory cell depending on the connection state of the storage diode 23.
Binary data t, ! of ゛ or 'O'' were written, and the amount of data that could be stored was small. This invention was made to solve the above-mentioned problems. An object of the present invention is to obtain a diode read-only memory device with an increased data capacity. a plurality of memory cells configured by Zener diode means connected to be reverse biased; and voltage setting means for controlling a breakdown voltage for each Zener diode means and setting it to one of three or more levels; and voltage detection means for detecting the level of breakdown voltage set by applying reverse bias voltages of different levels to the Zener diode means.
Zener diode means are provided as memory cells. Writing of the data signal to be stored is performed by controlling and setting the breakdown voltage level for each Zener diode means. Since the breakdown voltage can be set to one of three or more levels for each memory cell and the level of the set breakdown voltage can be detected, the storable data capacity can be increased. [Practice of invention? 41 FIG. 1 is a circuit diagram of a read-only memory (hereinafter referred to as 'FROM') using a Zener diode, showing one embodiment of the present invention. Based on Figure 1, this diode RON
i is connected to a row decoder 2b connected to receive row address signals RAO to RAn, and a bit line 2, respectively.
Zener diodes 3.4 and 5 are connected to be reverse-biased between the word line 25 and the word line 25, and a sense amplifier 8 is connected to the Zener diodes via the bit line 24. Each Zener diode constitutes one memory cell. zener diode 3
．． 4 and 5 have a breakdown voltage Vz of 5.
6, and 7V (,: set. This diode ROM responds to the chip selection C3 CE applied from the outside via the knee 7'9, and outputs the data 1.i stored in the memory cell. The number is read out in word rat order.Next, the read operation will be explained.First, the high level chip a us i;
The number CE is given to the sense amplifier 8, and the sense amplifier 8
Power becomes four-sexed. Row decoder 2b selects one of word lines 25 in response to row address signals RAD to RA rr. At this time, a voltage of 4.5 to 7.5 V is sequentially applied to the selected word line 25 from the row decoder 2b. In response to this voltage, the Zener diodes 3, 4 and 5 to which the breakdown voltage Vz of the violet level is set are sequentially turned on, and the sense amplifier 8
It is detected which Zener diodes connected to the selected word line 25 are rendered conductive in response to the pressure level. Therefore, the breakdown 711 pressure Vz set in each Zener diode is detected,
As a result, the data signal stored in each memory cell is read out. FIG. 2 is a cross-sectional view showing the structure of a memory cell, ie, a Zener diode, used in the diode ROM shown in FIG. Referring to FIG. 2, this Zener diode is formed using the emitter and base of a 1q20pnl transistor in a bipolar IC. An rl type epitaxial layer 11 is formed on a p type silicon substrate 10 by epitaxial growth. By selectively thermally diffusing boron B or the like into this layer 11, a p vibration isolation layer 12 is formed. The isolation layer 12 isolates the semiconductor elements formed in the n-type epitaxial layer 11. Also, minute i! N-type epitaxial layer 11 between IT layers 12
Boron ions such as B+ are selectively implanted and diffused widely into the upper layer of p! A u-base layer 13 is formed. Further, by selectively thermally diffusing highly concentrated boron B or the like into a portion of the p-type base layer 13, a p+-type scattered layer 14 is formed. It is formed in order to reduce the contact resistance between the p+ type scattering layer 14, the p type base layer 13, and the metal wiring 17 described later. Arsenic ions, etc., are selectively implanted into the upper layer of the p-type base layer 13 and diffused to form the nu1 emitter layer 15. This n-type emitter layer 15 forms a pn junction with the p-type base layer 13 at its bottom and side surfaces. An oxide film 16 is formed on the upper surface of the n-type epitaxial layer 11, and a metal wiring 17 is electrically connected to the n-type emitter layer 15 and the p+-type scattering layer 14 through a contact hole provided in the oxide film 16. be done. Plasma nitride film 18
is by plasma CVD method at 250 to 400°C,
It is formed on the metal interconnect #jA 17 and the oxide film 16. The plasma nitride film 18 has high moisture resistance. Generally, the width W of the depletion layer between the pn junctions and the electric field ε applied to the pn junctions are expressed as follows when the pn junctions are step junctions. ε匡V/W...
・ (2) Here, q is the charge amount, Na is the acceptor concentration, Nd is the donor concentration, ε is the dielectric constant of silicon, vbl
is the diffusion voltage (built-in voltage) of the p1 junction, and v is the reverse voltage. Zener breakdown occurs when the electric field ε exceeds a predetermined level (approximately 10'V/cm). Equations (1) and (2) ) As is clearer, the higher the impurity concentrations Na and Nd, the narrower the width W of the depletion layer, and the reverse voltage V' at the same level.
In contrast, the electric field ε applied to the pn junction becomes higher. That is, Zener breakdown is more likely to occur in pn junctions with high impurity concentrations of Na and Nd. In the Zener diode shown in FIG. 2, a relatively shallow p-type base layer 13 and an n-type emitter layer 15 are formed, so the region where the impurity concentrations Na and Nd reach their maximum values is located deep just below the oxidized rtA 16. This is an area of zero. Therefore, from equation (1), the p-type base layer 13 near the depth 0
The width of the depletion layer at the pri junction between the side surfaces of the emitter layer 15 and the mouth-shaped emitter layer 15 is the narrowest. Therefore, the depletion layer of the pn junction near depth 0 has approximately 10
When a voltage is applied between the p-type base layer 13 and the nN emitter layer 15 so that a reverse electric field of V/cm or more is applied, Zener breakdown occurs at the p-n junction. On the other hand, when a reverse high voltage of about IOV is applied to a Zener diode having the structure shown in the second factor, the breakdown voltage Vz increases. The reason for this is thought to be as follows. That is, when a voltage of about 10 V in the opposite direction is applied between the p-type base layer 13 and the n-type emitter layer 15, the voltage between the side surfaces of the p-type base layer 13 and the n-type emitter layer 15 under the oxide film 16111 increases. p
The highest electric field occurs at the n-junction. This high electric field causes
Electrons and holes (hereinafter referred to as hot carriers)
moves, and hot carriers with high energy are injected into the oxide film 16. The plasma nitride film 18 formed on the oxide film 16 has an excellent bavscivation effect and is an indispensable insulating film as the final protective film of the IC. contains hydrogen. This hydrogen is diffused into the oxide film 16 by heat treatment in another step after the plasma nitride film 18 is formed. As a result, the hydrogen diffused in the oxide film 16 and the injected hot carriers cause the following reaction. e−1h”+H2−”2B −(3) That is, as shown in equation (3), the electron h−e− and the hole h
The bonding energy of + hot carriers is hydrogen molecule H,
(bond energy is about 4.5 eV). The FAMed hydrogen atoms 11 cause the following reaction in the region immediately below the oxide film 16. SiH+H-+Sl +H2... (4) As a result, a Si fraction (trivalent silicon) is generated which becomes an interface state. Silicon S1 in this reaction is contained in the substrate (p-type base layer 13 and Ill-type Eftha layer 15). When an acceptor-type W-plane center is generated by injection of hot carriers, the width W of the depletion layer between the base layer 13 and the 0-type emitter layer 15 immediately below the oxide film 9 tends to increase. As a result, according to equation (2), the strength of the electric field applied between the depletion layers is relaxed, and the breakdown voltage Vz increases to y?
do. Breakdown voltage Vz, L5? The degree to which
It changes depending on conditions such as the impurity concentration of the p-type base layer 13 directly under the oxide film 16, the 112 concentration in the plasma nitride film 18, and the current density. In the diode ROM shown in FIG.
The breakdown voltage Vz is set by controlling and setting the 21 degrees in stages. That is, in the step 11) of this diode ROM, the Zener diode 3.4 is connected using the resist as a mask.
, and every 5. As a result, as described above, the breakdown voltage Vz is set for each 4.-. 3 is a circuit diagram of a Zener diode ROM showing another embodiment of the present invention. Referring to FIG. 3, in addition to the circuit shown in FIG. A sense amplifier 2 includes a column decoder 2C connected to receive CArn.
1 is selectively read out in response to a signal output from the column decoder 2C (i4' is given to the output buffer 722. Therefore, the data 1.'i stored in the memory cell bit by bit is The output buffer 22 outputs the read data signal in response to the chip selection signal CE. [Effects of the Invention] As described above, according to the present invention, each Zener diode means of three memory cells Since a means for controlling and setting the level of the breakdown voltage is provided and a means for detecting the level is provided, the amount of data stored in the memory cell is increased, and the data capacity that can be stored is increased. Nineteen read-only memory devices were added.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a Zener diode read-only memory showing one embodiment of the present invention. FIG. 2 is a cross-sectional view showing the structure of a Zener diode used in the read-only memory shown in FIG. FIG. 3 is a circuit diagram of a Zener diode read-only memory showing another embodiment of the invention. FIG. 4 is a circuit diagram showing a conventional diode read-only memory. In the figure, 2a and 2b are row decoders, 2C is a column decoder, 3+4.5 is a Zener diode with different breakdown voltage, 8.21 is a sense amplifier, 2
4 is a bit line, and 25 is a word line. In addition, in the drawings, the same number 70 indicates the same or equivalent parts.

Claims (1)

[Scope of Claims] A plurality of memory cells each formed by Zener diode means connected to be reverse biased between a word line and a bit line, and a breakdown voltage for each Zener diode means of the memory cell. voltage setting means for controlling and setting to one of three or more levels; and voltage setting means connected to the first two plurality of memory cells via the word line and bit line, and applying reverse bias voltages of different levels to the Zener diode means. voltage detection means for detecting a level of a set breakdown voltage by providing a diode read-only memory device.