Method of Making a Semiconductor Read Only Memory

This invention relates to a semiconductor read only memory (ROM) which can be more efficiently manufactured than conventional memories with higher productivity.

A conventional mask ROM (read only memory) is manufactured by the use of photo masks on which patterns are written corresponding to the desired data to be stored in the memory cells. First, the user presents a desired specification to the maker. The maker must then produce the logic design and pattern design, manufacture the mask and make the wafer later at the user's request. The maker could not start to manufacture the ROM until the specification of the user was finished.

An object of this invention is to provide an improved semiconductor read only memory which can be more efficiently produced.

The semiconductor read only memory according to this invention includes a plurality of MOS transistors formed on a substrate and polycrystalline or amorphous silicon resistances connected to the source regions and the drain regions of the MOS transistors. The partially completed ROM can then be stored as stock. Whenever desired the resistances can be selectively activated by laser annealing or otherwise to a low resistance corresponding to the desired data.

This and other objects and advantages of the invention will become more apparent and more readily appreciated from the following detailed description of the presently referenced exemplary embodiment of the invention taken in conjunction with the accompanying drawings, of which:

FIGS. 1(A) to (E) are sectional views illustrating steps of an embodiment of this invention;

FIG. 2 is a plan view of a device of FIG. 1;

FIG. 3 is an equivalent circuit diagram of a device of FIG. 1;

FIGS. 4(A) to (D) are sectional views, similar to FIG. 1, of an alternate embodiment of this invention;

FIG. 5 is a plan view, similar to FIG. 2, of a device of FIG. 4; and

FIG. 6 is an equivalent circuit diagram, similar to FIG. 3, of a device of FIG. 4.

Polycrystalline or amorphous silicon film used widely as the electrodes and the interconnections of semiconductor devices maintains its high resistance or nonconductivity when implanted with impurity ions. Thermal activation is required to put the film to practical use. The resistance of polycrystalline or amorphous silicon film with or without impurities is conventionally decreased by enlarging the grain size thereof with laser annealing, and the film is then used as electrodes and interconnections.

This invention is based on the above-mentioned characteristic of polycrystalline or amorphous silicon. Referring now to FIGS. 1(A) to (D), there are shown schematic views of the invention. P-type semiconductor substrate 11 is thermally oxidized in a steam atmosphere at 1000.degree. C. and a field oxide film 12 is formed on the surface of substrate 11. Thereafter an active area 13 is produced by photoengraving (FIG. 1(A)). Substrate 11 is thermally oxidized again in an oxygen atmosphere at 1000.degree. C. including 3% hydrogen chloride, and an oxide film 15 of 700 to 900 Angstroms thickness is formed on the exposed surface. Polycrystalline or amorphous silicon film 14 of 3500 Angstroms thickness is formed on the surface of oxide film 15 and 12 using the chemical vapor deposition (CVD) method, and silicon film 14 is activated in an oxyphosphorus chloride atmosphere at 1000.degree. C. serving as diffusion sources (FIG. 1(B)).

Thereafter, silicon film 14 is patterned by photoengraving and gate electrodes and interconnections are formed. The exposed oxide films are etched using the gate electrodes and interconnections as masks, and gate electrodes gate oxides 15 and windows 10 are formed. Source regions 16, drain regions 17 and connecting regions 18 are formed by ion implantation of arsenic through windows 10. A plurality of MOS transistors are serially connected with connecting regions 18 used commonly as source and drain regions of neighboring transistors (FIG. 1(C)). CVD-oxide film 19 of 7000 Angstroms thickness is formed as a protecting film on the surface of substrate 11, and source, drain and connecting regions are activated in a nitrogen atmosphere at 1000.degree. C. Contact holes 9 are opened by selective etching and polycrystalline or amorphous silicon film 20 of 3500 Angstroms thickness is formed by the CVD method. Then arsenic is diffused into silicon film 20 by ion implantation, and silicon film 20 is changed to a silicon resistance film. In this condition, silicon resistance film 20 is in nonconductive or high resistance (FIG. 10(D)).

Thereafter, a protecting insulator film 21 of phosphosilicate glass of 7000 Angstroms thickness is covered on the surface of substrate 11, and contacting holes 22 and 23 are opened through protecting insulator film 21 and CVD-oxide film 19. As a result of it, source region 16 and drain region 17 being exposed.

Aluminum is evaporated on the surface of substrate 11, and source electrode 24 and drain electrode 25 are formed by selective photoengraving. Thereafter a passivating film 26 of phosphosilicate glass is covered on the surface of substrate 11. In the above-mentioned steps, each source and drain region of the MOS transistors have been connected by polycrystalline or amorphous silicon resistances. Thus, the equivalent circuit shown in FIG. 3 (aftermentioned in detail) has been provided as half finished goods which can be stored. However, the silicon resistances are nonconductive or high resistant yet. Laser beams L are selectively irradiated (shown by arrows A) on the surface upon the desired silicon resistances to store the data responding to the specifications of the user after sintering at 450.degree. C. in a nitrogen atmosphere including 10% hydrogen (FIG. 1(E)). FIG. 2 illustrates a plan view of the NAND type ROM formed in a matrix, similar elements being assigned the same reference numbers as in FIG. 1. Laser beam irradiation is satisfactory to partially irradiate the predetermined silicon resistances by computer control and also using the hard mask to expose desired portions.

In this embodiment of the invention, referring to FIG. 3, resistances R2 and R4 (shown by arrows A) are activated by laser beams so that the values of resistances R2 and R4 are decreased. Thus, MOS transistors TR2 and TR4 do not operate normally as enhance mode transistors because of the short between source and drain electrodes of these transistors by activated resistances R2 and R4. However, other transistors TR1 and TR3 operate normally as original enhance mode transistors with high nonactivated resistances R1 and R3.

After this activation, if transistor TR2 is selected by the input of transistor TR2 being low level and the inputs of another transistor being high level, transistors TR1 and TR3 operate and transistor TR4 is equivalent to a resistance. The output level of transistor TR2 is then low level, which is its selective level. Thus, a NAND type mask ROM integrated circuit is provided.

The ROM of the present invention is manufactured by laser annealing at a final step in the overall process of wafer fabrication so that half finished goods having nonactivated resistances can be stocked when the specifications of users are not yet determined. A complete data stored ROM is manufactured by partial activation of predetermined regions responding to the specifications of users. Thus, the length of time (lead time) between order and delivery of goods can be shortened, and productivity thus remarkably improved.

This invention applies to other ROMS as will be explained by FIGS. 4 to 6. Since the following steps are almost the same as the above-mentioned explanation, details are omitted here. FIGS. 4(A) to (D) illustrate a flow of the fabrication of a NOR type mask ROM in accordance with this invention.

After a field oxide film 42 is formed on the surface of semiconductor substrate 41, active area 43 is formed by photoengraving (FIG. 4(A)). Thereafter MOS transistors having gate electrodes 44, gate oxide films 45, source regions 46 and drain regions 47 and simultaneously connecting regions 48 are formed (FIG. 4(B)).

Polycrystalline or amorphous silicon resistances 50 are connected between source regions 46 and connecting regions 48 through CVD oxide films 49, but at this time these resistances 50 are nonconductive or high resistant (FIG. 4(C)). Drain electrode 52 is connected to drain region 47 through glass film 51 and passivation film 53 is formed over all of the surface of the substrate. Desired portions A are activated by irradiation of laser beam L.

FIG. 5 illustrates a plan view of FIG. 4, and similar elements are assigned the same reference number as in FIG. 4. FIG. 6 illustrates an equivalent circuit of the device shown in FIG. 4, resistances R2 and R4 shown by arrows A being activated by laser beams. Transistors TR3 and TR5 can be operated in the enhance mode, but transistors TR2 and TR4 cannot operate normally. In this result, if transistor TR2 is selected, the output level is a high level, but if transistor TR3 is selected, the output level is a low level. Thus, a NOR type mask ROM is provided.

In this embodiment, the silicon resistances have been connected only to source regions, but it is also possible that other silicon resistances can be connected to drain regions.

In these embodiments of the invention, the desired ROM can be completed merely by activating predetermined portions corresponding to the specification of the user, so the productivity of the ROM is remarkably improved.

Many changes and modifications in the above-described embodiments of the invention can be carried out without departing from the scope thereof. Accordingly that scope is limited only by the scope of the appended claims.