Semiconductor memory having a small write current

Abstract: A semiconductor memory in which the junction destruction-type PROM elements are arranged in the form of a matrix, bit lines are connected to the emitter regions of the PROM regions and word lines are connected to the collector regions, wherein the emitter regions have a circular shpae in plane.

Description: The present invention relates to a programmable read-only memory, and particularly to a semiconductor memory cell of junction destruction-type PROM. A PROM has a feature in that arbitrary data can be written thereon to be stored, the written content being fixed to the memory cells without being erased even when the power source to the memory circuit is interrupted. The larger the memory capacity of the memory circuit, on the other hand, the more the desired data can be stocked. It has, therefore, been desired to highly densely integrate the memory elements in a small area of a semiconductor chip in order to increase the capacity. In a memory circuit employing general junction destruction-type PROM cells, an overcurrent (write current) is permitted to flow into two pn junctions connected in reverse series between a bit line and a word line, and one pn junction that is backward biased is destroyed and only the pn junction forward biased is left to hold the data. The write current used in the memory circuit has a current value and a voltage value that are large enough to destroy the pn junctions which is backward biased. The impurity regions forming such pn junctions have heretofore been formed in a square shape in plane; i.e., the impurity regions have increased circumferential lengths and require a large write current. Therefore, the voltage and current supplied to a predetermined memory cell at the time of writing the data affect other surrounding memory cells or peripheral circuits. For instance, in the case of a junction destruction-type PROM memory cell in which a metal wiring is directly connected to the impurity region, the wiring metal connected to the memory cell partly infiltrates into the destroyed portion due to junction destruction at the time of writing the data. The depth of infiltration increases with the increase in the write current and reaches near the pn junction that is not destroyed to deteriorate the pn junction. Furthermore, since a high voltage is applied at the time of writing the data, the write current may leak into other memory cells or into the peripheral circuits when the voltage applied is greater than a breakdown voltage of the memory cells, greater than a dielectric breakdown voltage required for the field oxide regions surrounding the memory cells, greater than a breakdown voltage required for the pn junction among the adjacent memory cells, or greater than a breakdown voltage required between the memory cells and the substrate. In order to prevent such leakage of write current or to decrease the effects of write current, therefore, field oxide regions must be formed having a sufficiently high dielectric breakdown voltage as well as sufficiently large depth and width, making it necessary to maintain large margin among the elements and increased thickness of semiconductor layer in which the memory cells are to be formed. As described above, various limitations are imposed on the memory circuit employing the junction destruction-type PROM memory cell hindering the memory circuit from being integrated highly densely. The object of the present invention is to provide a semiconductor memory of a junction destruction-type which requires a reduced write current and, hence, enables the memory circuit to be highly densely integrated. According to the present invention, there is provided a semiconductor memory circuit comprising a plurality of collector regions formed in one main surface of a semiconductor substrate having one type of conductivity, said plurality of collector regions having an opposite type of conductivity, a plurality of base regions having the one type of conductivity arranged in line in each of said collector regions, emitter regions having the opposite type of conductivity and provided in each of said base regions, word lines connected to each of said collector regions, bit lines commonly connected to said emitter regions that are formed in different collector regions, a write current for destroying a junction between said base and emitter regions being dependent on a circumferential length of the emitter regions, characterised in that each of said emitter regions is formed in a circular shape in a plan view so that the junctions between said base and emitter regions are circular. Being constituted as described above, either one of the two pn junctions formed between the emitter and the base and between the base and the collector is destroyed by the write current in order to write data. Here, since the emitter region has a circular shape in plan, the circumferential length of the emitter region becomes shorter than that having the square emitter regions according to the prior art and the data can be written with a smaller write current that that of the prior art. As described above, the semiconductor memory of the present invention makes it possible to decrease the write current of the PROM, to decrease the breakdown voltage required for the memory cells and the peripheral circuits, to decrease the margin along the elements, to decrease the thickness of the semiconductor layer, and to incraese the degree of integration. Namely, the present invention solves many problems involved in the conventional PROM's and provides a memory device that is integrated highly densely. The aforementioned objects, constitution and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings wherein: Fig. 1 is a circuit diagram illustrating major portions of a conventional memory device employing junction destruction-type PROM cells; Fig. 2 is a schematic plan view illustrating major portions of a conventional memory device employing junction destruction-type PROM cells; Fig. 3 is a section view of the memory device along the line A-A′ of Fig. 2; Fig. 4 is a pattern arrangement diagram of a conventional junction destruction-type PROM cells; Fig. 5 is a diagram of characteristics comparing the circumferential length of the emitter region with a minimum write current; Fig. 6 is a schematic plan view illustrating major portions of a memory device employing the junction destruction-type PROM cells according to the present invention; Fig. 7 is a pattern arrangement diagram of the junction destruction-type PROM cell according to the present invention; and Fig. 8 is a diagram of characteristics comparing the circumferential lengths between the case of a square shape and the case of a circular shape. Fig. 1 illustrates major protions of a conventional memory device employing the junction destruction-type PROM cells. In this case, an npn bipolar transistor is used as a memory cell. Memory cells Q₁₁ to Q mn are provided at the intersecting points of the bit lines B₁, B₂, ----, B m and the word lines W₁, W₂, ----, W n , the emitters thereof being connected to the bit lines and the collectors thereof being connected to the word lines. The word lines W₁, W₂, ----, W n are connected to the word drivers WD₁, WD₂, ----, WD n , respectively. The word drivers select bits in which write current is supplied from the bit lines, and receive the write current. That is, when, for example, the data is to be written onto a memory cell Q₂₂, a bit line B₂ and a word line W₂ are selected, and the write current is supplied from the bit line B₂ to destroy the emitter-base junction of the memory cell Q₂₂ and is absorbed by the word driver WD₂. Fig. 2 is a schematic plan view of the memory device of Fig. 1. Collector regions 2, 2′, 2″, ---- are formed like belts in a semiconductor substrate 1, and isolation regions 3, 3′, ---- are provided among these belt-like collector regions, such that the belt-like collector regions 2, 2′, 2″, ---- are electrically isolated from each other. In each of the collector regions 2, 2′, 2″, ---- are provided a plurality of base regions 4 which contain an emitter region 5. Thus, the memory cells Q₁₁ to Q mn are formed in the semiconductor substrate 1. The emitter regions 5 of memory cells Q₁₁ to Q mn are connected to the bit lines B₁, B₂, ----, B m and the collector regions 2, 2′, 2″, ---- are connected to the word lines W₁, W₂, ----, W n via collector electrodes 6. Fig. 3 is a section view of the junction destruction-type PROM cell along the line A-A′ of Fig. 2. The structure will now be described in detail. A plurality of n-type buried layers 7 are formed in the p-type silicon substrate 1, and an n-type epitaxial layer 2 is deposited thereon. An oxide film 3′ on the n-type epitaxial layer 2 is selectively oxidized among the n-type buried layers 7 and among the regions in which each of memory cell is to be formed to form isolation regions 3 in order to form a plurality of collector regions composed of the n-type epitaxial layer 2. The p-type base region 4 and the n-type emitter region 5 having square shapes in plane are successively provided in the epitaxial layer 2. A polycrystalline silicon film 8 doped with impurities is formed on the oxide film 3′ having a square opening (emitter opening), and the emitter region 5 is formed by thermal diffusion in the base region 4 through the opening. Thus, the two pn junctions 10 and 11 are formed between the base and the collector, and between the emitter and the base. The emitter region 5 is connected, via the polycrystalline silicon film 8, to an emitter electrode 9 connected to the bit line B₅. Via the n-type contact region 12 and the polycrystalline silicon film 8, furthermore, the collector region 2 is connected to the collector electrode 6 that is connected to the word line W₁. With the above-mentioned constitution, when the data is to be written, the write current is supplied from the bit line B₅; i.e., the current flows in the backward biasing direction into the pn junction 11 between the n-type emitter region 5 and the p-type base region 4, and whereby the junction is destroyed to write the data. The write current flows into the collector region 2 via the pn junction 10 and is received by the word driver WD₁ connected to the word line W₁. Fig. 4 is a plan view showing in detail the layout of the conventional junction destruction-type PROM cell that is shown in Fig. 2. The base region 4 is formed in the collector region 2, and impurities doped into the polycrystalline silicon film 8 are thermally diffused through the emitter opening 13 formed in the oxide film 3′ to form the emitter region 5 in the base region 4. Thus, with the conventional junction destruction-type PROM cell, the emitter region 5 has a square shape or an elongated shape in plane as it is formed through the emitter opening 13 of a square shape or an elongated shape. Fig. 5 is a diagram of characteristics illustrating a relationship between the circumferential length of the emitter region and a minimum write current when the emitter region is formed with an impurity concentration of 1 x 10²⁰/cm³ and a depth of 0.1 µm. As will be obvious from Fig. 5, the minimum write current (Iwmin) varies in proportion to the circumferential length (Le) of the emitter region. In this case, when the diffusion depth of the emitter region is D, the circumferential length of the emitter region is longer than the circumferential length of the emitter opening by about 8D, because the diffused region is widened from the emitter opening in the direction of lateral. In the above-mentioned conventional junction destruction-type PROM in which the emitter opening has a square shape or an elongated shape, the emitter region has an increased circumferential length and requires an increased write current. Therefore, a large voltage is applied at the time of writing the data, and an increased breakdown voltage is required for the cell such that the write current will not be leaked. Therefore, it is not allowed to decrease the margin between the elements or to decrease the thickness of the epitaxial layer, making it difficult to accomplish high degree of integration. The invention will now be described in conjunction with the drawings. Fig. 6 is a schematic plan view which illustrates an embodiment of the present invention wherein the emitter regions have a circular shape in plane. Fig. 6 illustrates the case where memory cells are formed on the p-type semiconductor substrate. On the p-type semiconductor substrate 21 are formed n-type semiconductor layers maintaining an impurity concentration of, for example, 3 to 4 x 10¹⁹/cm³ and a thickness of 1.0 to 3.0 um that will serve as buried layers, and n-type epitaxial layers that will serve as collector regions 22, 22′, 22″, ----are formed thereon with an impurity concentration of 5 x 10¹⁵/cm³ and a thickness of 1.5 to 2.0 µm. An oxide film formed on the n-type epitaxial layer is selectively oxidized to form field oxide layers 23, 23′, ---- and isolation regions among memory cells, and the collector regions 22, 22′, 22″, ---- are electrically isolated from each other and the leakage of current among memory cells is prevented. A plurality of p-type base regions 24 are formed in each of the belt-like collector regions 22, 22′, 22″, ----, each of the p-type base regions having an impurity concentration of 5 to 6 x 10¹⁸/cm³, a depth of 0.5 to 1.0 µm, and a side of a length of 5.0 to 10.0 µm. In each base region 24 is formed the n-type emitter region 25 having a circular shape in plane by diffusing impurities at a concentration of 1.0 x 10²⁰/cm³ maintaining a depth of 0.1 to 0.3 µm through an opening of a diameter of 1.4 µm formed in the oxide film. In this case, the emitter region 25 has a diameter of 1.6 to 2 µm. The emitter regions 25 of the thus formed memory elements Q₁₁′ to Q mn ′ are connected to the bit lines B₁′, B₂′, ----, B m ′, and the collector regions 22, 22′, 22″, ---- are connected to the word lines W₁′, W₂′, ----, W n ′ via the collector regions 26. The practical impurity concentrations in the collector, base and emitter regions formed in the present invention are 10¹⁵ to 10¹⁶/cm³ in the collector region, 10¹⁸ to 10¹⁹/cm³ in the base region, and 10¹⁹ to 10²¹/cm³ in the emitter region. Fig. 7 is a plan view showing in detail the layout of the junction destruction-type PROM cell according to the present invention. The p-type base region 24 is formed in the n-type collector region 22, and the n-type emitter region 25 is formed in the base region 24 through the emitter opening 27 of a circular shape formed in the oxide film. In the junction destruction-type PROM element of the present invention as described above, the emitter region 25 is formed through the circular emitter opening 27 and, hence, has a circular shape in plane. The shape in section is the same as that of Fig. 3, and is not described here. Fig. 8 is a diagram of characteristics comparing the circumferential lengths between the case of a square shape and the case of a circular shape. The opening has a reduced area owing to the progress in the lithography technology. When the side of a square is equal to the diameter of a circle, a minimum limit of the opening the same. At present, the opening of as small as 1.3 µm can be formed. It will be obvious from Fig. 8 that when the side of a square is equal to the diameter of a circle, the circumferential length becomes apparently shorter in the case of the circle than in the case of the square. As is obvious from Fig. 5, therefore, the circumferential length can be more shortened when the emitter region has a circular shape than when the emitter region has a square shape, and the minimum write current can be reduced correspondingly. Therefore, the breakdown voltage required for the memory cells and the peripheral circuits can be decreased, margin can be decreased among the elements and the thickness of the epitaxial layer can be decreased. Therefore, the circuit can be highly densely integrated with ease. As shown in Table 1, for example, when the emitter opening of a square shape has a side of 2 µm and the emitter region has a depth of 0.1 to 0.3 µm (emitter region has a side of 2.2 to 2.6 µm), a minimum write current of 80 mA is required whereas when the emitter opening of a circular shape has a diameter of 2 µm and the emitter region has a depth of 0.1 to 0.3 µm (emitter region has a diameter of 2.2 to 2.6 µm), a minimum write current of 60 mA is required, which is a reduction of 20 mA in the write current. Therefore, the required breakdown voltage can be decreased from 22 V to 18 V. With the required breakdown voltage being decreased as described above, the margin among the elements can be decreased from 12 µm to 9 µm. Minimum write current Iwmin Breakdown voltage Margin among the elements Conventional example 80 mA 22 V 12 µm Present invention 60 mA 18 V 9 µm Moreover, since the write current can be decreased, the wiring metal infiltrates little into the junction destruction portion when the data is written onto the junction destruction-type PROM cell in which the metal wiring is directly connected to the emitter region, and the other pn junction is prevented from being deteriorated. Though the above embodiment has dealt with the case where the npn bipolar transistors are constituted on the p-type semiconductor substrate, it needs not be pointed out that the same effects are obtained even when the n-type semiconductor substrate is used provided the junction portions have a circular shape in plane.

Claims: A semiconductor memory circuit comprising a plurality of collector regions (22, 22', 22'') formed in one main surface of a semiconductor substrate having one type of conductivity, said plurality of collector regions (22, 22', 22'') having an opposite type of conductivity, a plurality of base regions (24) having the one type of conductivity arranged in line in each of said collector regions (22, 22', 22''), emitter regions (25) having the opposite type of conductivity and provided in each of said base regions (24), word lines (W1', W2') connected to each of said collector regions (22, 22', 22''), bit lines (B1', B2', B3', B4', B5', B6') commonly connected to said emitter regions (25) that are formed in different collector regions (22, 22', 22''), a write current for destroying a junction between said base and emitter regions (24, 25) being dependent on a circumferential length of the emitter regions (25), characterised in that each of said emitter regions (25) is formed in a circular shape in a plan view so that the junctions between said base and emitter regions are circular. A semiconductor memory circuit according to Claim 1, wherein said one type of conductivity is the p-type of conductivity and said opposite type of conductivity is the n-type of conductivity. A semiconductor memory device according to Claim 1, wherein said one type of conductivity is the n-type of conductivity and said opposite type of conductivity is the p-type of conductivity.

Inventors: TAKADA NARUAKI, TAKADA, NARUAKI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1988
Main classification: G11C17
IPC codes: H01L29, H01L21, H01L27, G11C17