Method of Fabricating Electrically Eraseable Read Only Memory Cell Having a Trench

(1) Field of the Invention

The present invention relates to an erasable electrical programmable memory cell of a type useful in VLSI technology, more specifically to an EPROM cell capable of providing a means to increase the density of cell array devices.

(2) Description of the Invention

More efficient utilization of device area in VLSI technology is a prominent objective in order to increase the density and number of devices and memory cells on a semiconductor chip in order to reduce cost and increase the speed of operation. A known technique is to place various elements, i.e. transistors, capacitors, etc. in trenches to achieve greater element density. Examples of such utilization of trench structures in EPROM devices are shown in U.S. Pat. Nos. 4,975,383 and 5,180,680.

Accordingly, an object of this invention is to describe an improved method and resulting device structure for making highly dense EPROM products.

There is provided an electrically erasable read only memory (EPROM) cell on a semiconductor substrate having a trench containing a source region at the bottom, floating gates on opposite sidewalls, separated from the substrate by a gate insulation layer. Drain regions are provided adjacent the sidewalls of the trench. A conductive word line stripe that is transverse to the trench provides a control gate. A suitable metallurgy system interconnects the source region, the drain regions, and the word line of each cell into an operative memory array.

The method of fabricating the EPROM cell of the invention involves forming a trench in a semiconductor substrate, forming a gate insulation layer on the surface of the trench, forming polycrystalline silicon floating gate layers on opposite sidewalls of the trench, implanting dopants in the surface of the substrate and bottom of the trench, and forming an insulating layer over the floating gates. A conductive word line stripe, that is transverse to the trench, is formed over the floating gates. A suitable metallurgy system is formed to interconnect the source and drain regions, and the word line stripe of each cell into an operative memory array.

FIGS. 1 through 5 is a sequence of cross sectional views, in broken section, which illustrate the process of the invention.

FIG. 6 is a top plan view of FIG. 5.

FIG. 7 is a cross sectional view which illustrates an alternate embodiment of the invention.

FIG. 8 is a top plan view of FIG. 7.

FIG. 9 is a detail view that illustrates a contact terminal for the source region.

FIG. 10 is a cross sectional view taken on line 10--10 of FIG. 9.

Referring now to the Figs. of the drawing, there is illustrated, methods for forming two preferred specific embodiments of the EPROM cell of the invention and methods for fabricating them. It should be understood that these embodiments are not intended to severely limit the scope of this invention, since it should be understood by those skilled in the art that modifications could be made to the structures and methods of fabricating without departing from the scope and spirit of the invention.

As shown in FIG. 1, a trench 12 is formed in a semiconductor substrate 10. In practice, when a device with an array of cells is produced, a plurality of parallel elongated trenches is formed in the substrate 10. The substrate can be any suitable semiconductor material, but is preferably a monocrystalline silicon substrate, with a crystal orientation of [100], and a background doping of a concentration of P type dopant in the range of 1 E 15 to 1 E 17 cm.sup.-3. The trench 12 will preferably have a depth in the range of 3000 to 10,000 Angstroms, a width in the range of 0.5 to 2.0 Micrometers, and spacing in the range of 0.1 to 2.0 micrometers. Most preferably the trench 12 has a depth of 0.5 to 0.7 micrometers, a width of 0.5 to 1.5 micrometers, and a spacing of 0.1 to 1.0 micrometers. The trench is formed by depositing a masking layer of a photoresist, and exposing and developing it to form openings that define the trench. The trenches are then formed by anisotropic reactive ion etching (RIE), most preferably by etching in a conventional anisotropic etching reactor using as the etching specie chlorine, hydrogen bromide, sulfur hexafluoride or carbon tetrachloride.

After the trench has been formed, the photoresist layer is removed and a first insulating layer 14 is formed on the surface of substrate 10, including the sidewalls and bottoms of the trench 12. The layer 14 will serve as a gate insulating layer in the trench and is preferably a layer of thermal SiO.sub.2, when the substrate is silicon, with a thickness on the order of 50 to 300 Angstroms. This layer 14 can be formed by heating the substrate to a temperature of 900.degree. to 1000.degree. C., in the atmosphere of dry oxygen for a time of 1/2 to 3 hours. A first layer 16 of polycrystalline silicon is deposited on the surface of trench 12. The layer 16 preferably has a dopant concentration on the order of 1 E 19 to 1 E 21 cm.sup.-3 either by deposition of undoped polysilicon and then doping with phosphorus oxychloride or P31 ion implantation, or doped in situ, and a thickness in the range of 500 to 3000 Angstroms.

The major portion of layer 16 is then removed leaving only areas on opposing sidewalls of the trench 12. In forming an array of cells, a plurality of opposed areas of layer 16, which will constitute floating gates 17, are formed in the trench 12 with a longitudinal spacing of approximately 0.5 to 3.0 micrometers. This configuration can be seen more clearly in plan views FIGS. 6 and 8. Layer 16 is preferably shaped by a two step process. The portions of layer 16 on the surface are removed by masking and subtractive etching, as indicted in FIG. 2. The masking layer (not shown) is removed and the substrate exposed to anisotropic dry etch. This removes the portion of layer 16 on the bottom of trench 12 and takes a portion off the top edges, resulting in floating gates 17, as shown in FIG. 3.

As indicated in FIG. 4, dopant ions are introduced into the substrate 10, adjacent to the surface between the trenches, forming drain regions 18 and in the bottom of the trench between the gate electrodes 17, to form source regions 20. Preferably arsenic ions are implanted with an acceleration voltage of 40 to 120 KEV, with a dosage in the range of 5 E 14 to 1 E 16 ions/cm.sup.2 followed by an annealing step at conditions 800.degree. to 900.degree. C. The resultant regions 18 and 20 will preferably have an average dopant concentration on the order of 1 E 19 to 1 E 21 cm.sup.-3.

As shown in FIG. 5, a second insulating layer 22 is formed over floating gate electrodes 17. This layer can be a thermal SiO.sub.2 layer, a chemical vapor deposition SiO.sub.2, or a composite layer of SiO.sub.2 and Si.sub.3 N.sub.4. The techniques for forming such layers are well known. The layer 22 preferably has a physical thickness in the range of 100 to 400 Angstroms and an equivalent dielectric layer (silicon dioxide) thickness of between about 50 to 300 Angstroms.

A conductive word line stripe 24, that is transverse to the trenches, is formed over floating gates 17. Stripe 24, separated from floating gates 17 by a thin insulating layer 22 serves as a control gate electrode for the memory cell. The stripe 24 can be formed of any suitable conductive material. Preferably stripe 24 is formed of doped polycrystalline silicon. Stripe 24 can be formed by depositing a layer of polycrystalline silicon and shaping the stripe using conventional masking and etching techniques. The polycrystalline silicon is preferably deposited by low pressure chemical vapor deposition (LPCVD) using a silane or Si.sub.2 H.sup.6 ambient gas as the source of silicon within a chamber having a pressure of between about 0.001 mTorr to 10 Torr and a temperature of between about 500.degree. to 700.degree. C. The polycrystalline silicon substantially fills the trenches and provides a relatively planar surface. The relationship of the word lines 24 with the trenches and floating gates is illustrated more clearly in FIG. 6.

An alternative word line stripe structure is shown in FIGS. 7 and 8. In this embodiment stripe 26 is a polysilicon stripe deposited by LPCVD using a silane or Si.sub.2 H.sub.6 ambient gas as the source of silicon within a chamber having a pressure of between about 0.001 mTorr to 10 Torr and a temperature of between about 500.degree. to 700.degree. C. Note that stripe 26 is a conformal layer, i.e. having a relatively uniform thickness. The surface of stripe 26 is non-planar, since it is deposited on a non-planar surface. In order to restore a measure of planarity to the surface, a layer of SiO.sub.2 or borophospho silicate glass is deposited by spin-on techniques, and an etch back performed. This fills the depressions with insulation material 28. A passivating layer can then be deposited, and a metallurgy layer formed thereon.

An electrical contact is made to source region 20, as shown in FIGS. 9 and 10. The contact is preferably in the end of the trench 12. An opening 34 is made to region 20, as most clearly shown in FIG. 10, and the opening filled with a conductive material 36, preferably tungsten, titanium-tungsten, or polycrystalline silicon using well known etching and deposition techniques. A suitable metallurgy stripe 38 is deposited to contact plug 36 to supply the voltage to region 20 which is necessary for operation. The metallurgy stripe 38 is typically aluminum, aluminum/silicon/copper, aluminum/copper, or the like.

The operation of the cell of the invention may be understood by those skilled in the art with reference to the following table:

It is of course understood that erasure can be also accomplished by ultraviolet (UV) light.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it is will understood by those skilled in the art that various changes and modification can be make in the invention without departing from the spirit and scope of the invention.