High Density Memory With Field Shield

This invention relates to integrated semiconductor memory circuits and more particularly to a memory with a high density of cells, each of which employs a capacitor for storing a binary digit of information.

Integrated semiconductor memory circuits, particularly those employing cells which include essentially a storage capacitor and a switch have achieved high memory cell densities. One of the simplest circuits for providing a small dynamic memory cell is described in commonly assigned U.S. Pat. No. 3,387,286, filed July 14, 1967, by R. H. Dennard. Each cell employs a storage capacitor and a field effect transistor acting as a switch to selectively connect the capacitor to a bit/sense line.

In also commonly assigned U.S. Pat. Nos. 3,811,076 by W. M. Smith, and 3,841,926, by R. A. Garnache and W. M. Smith, both filed on Jan. 2, 1973, there is disclosed a one device field effect transistor memory cell of the type described in the hereinabove identified Dennard patent which utilizes a layer of doped polysilicon and an N+ diffusion region in a P type conductivity semiconductor substrate separated by a dielectric medium disposed on the surface of the semiconductor substrate for forming the storage capacitor of the cell. The polysilicon layer extends beyond the storage capacitor to act as a field shield between adjacent cells by applying a negative bias or fixed negative potential to the polysilicon layer. The N+ diffusion region of the storage capacitor is formed by using a doped segment of an insulating layer disposed on the surface of the semiconductor substrate and outdiffusing the dopant into the substrate.

In IBM Technical Disclosure Bulletin, Vol. 21, No. 9, February 1979, pp. 3823-3825, there is disclosed a one device memory cell which advantageously uses two layers of polysilicon.

Although the cells described hereinabove do provide memories having a high density of cells in a planar or two dimensional arrangement, yet each cell does require a significant given area of semiconductor surface. To reduce the size of the given area for each cell, structures have been made wherein the cell is formed in a three dimensional arrangement, such as described in commonly assigned U.S. Pat. No. 4,335,450, filed on Jan. 30, 1980, by D. R. Thomas, wherein there is disclosed a cell having a transistor disposed on a sidewall of a groove or trench with the storage node disposed below the transistor.

Also, commonly assigned U.S. Pat. No. 4,397,075, filed on July 3, 1980, by J. J. Fatula, Jr. and P. L. Garbarino, and commonly assigned U.S. patent application having Ser. No. 182,724, filed on Aug. 29, 1980, by S. A. Abbas and R. C. Dockerty, describe a dense, vertical cell having the storage capacitor in a well or trench. Somewhat similar cells are disclosed in U.S. Pat. No. 4,327,476, filed on Dec. 7, 1979, and in IBM Technical Disclosure Bulletins, February 1975, by G. V. Clarke and J. E. Tomko, pp. 2579-2580, July 1982, by C. G. Jambotkar, pp. 593-596, and February 1984, by B. El-Kareh, R. R. Garnache and F. R. White, pp. 4699-4701.

Furthermore, commonly assigned U.S. Pat. No. 4,462,040, filed on May 7, 1979, by I. T. Ho and J. Riseman discloses a one device dynamic random access memory utilizing a trench having vertical sidewalls with the storage capacitor and the transfer device located within the trench, and U.S. Pat. Nos. 4,271,418, filed Oct. 29, 1979, and 4,225,945, filed Oct. 29, 1979, teach a one device memory cell formed in a groove or trench with the storage node located at the bottom of the trench, the bit/sense line at the top of this substrate and the transfer device on the sidewall of the trench.

It is an object of this invention to provide a memory cell, in a three dimensional arrangement including a groove or trench, requiring a very small area of the surface of a semiconductor substrate, which has a high storage capacity and means disposed within the groove or trench for isolating adjacent cells.

In accordance with the teachings of this invention, a memory cell, formed in a groove or trench, in a semiconductor substrate is provided which includes a storage capacitor located at the bottom of the trench, a bit/sense line disposed at the surface of the substrate, a transfer device located on the sidewall of the trench between the capacitor and the bit/sense line and a field shield for electrically isolating the storage capacitor.

The foregoing and other objects, features and advantages of the invention will be apparent form the following and more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.

FIG. 1 is a plan view of an array of memory cells of the present invention,

FIG. 2 is a sectional view of the array of cells illustrated in FIG. 1 of the drawings taken along line 2--2,

FIG. 3 is a sectional view of the array of cells illustrated in FIG. 1 of the drawings taken along line 3--3, and

FIG. 4 is a sectional view similar to that of FIG. 2 but taken at an early step during the making of the array.

Referring to the drawings in more detail, there is illustrated in FIG. 1 a plan view of an array of memory cells of the present invention. The array includes four memory cells 10, 12, 14 and 16 formed in trenches 18, 20, 22 and 24, respectively, located in a P type conductivity semiconductor substrate 26, as indicated also in the sectional views in FIGS. 2 and 3 of the drawings. Each of the cells 10, 12, 14 and 16 includes a first N+ diffusion region 28 disposed in the semiconductor substrate 26 at the bottom and along the lower portions of the sidewalls of the trenches 18, 20, 22 and 24. The first N+ diffusion region 28 acts as a storage node and serves as a first plate of a storage capacitor 30. A second N+ diffusion region 32 is disposed at the surface of the semiconductor substrate 26 adjacent to each of the trenches 18, 20, 22 and 24. The second N+ diffusion regions 32 are arranged in parallel, illustrated vertically in FIG. 1, to serve as bit/sense lines 32 for the array with each of the lines 32 associated with the cells of one column of the array. A first layer of silicon dioxide 34 is grown within the trenches 18, 20, 22 and 24 and over the second N+ diffusion regions 32, with the layer 34 being thicker over regions 32 due to the higher oxidation rate in the presence of the high N+ doping.

A first layer of doped polysilicon 36 is deposited over the first layer of silicon dioxide 34 within the trenches 18, 20, 22 and 24 and over the surface of the substrate 26. The first layer of doped polysilicon 36 serves as a second plate of the storage capacitor 30 and as a field shield between the storage capacitors 30 and the bit/sense lines 32 of each of the memory cells. An opening 38, as indicated in FIG. 2 of the drawings, is formed in the first layer of polysilicon 36 on one sidewall of each of the trenches 18, 20, 22 and 24 between the bit/sense line 32 and the adjacent edge of the first N+ diffusion region 28. A second layer of silicon dioxide 40 is grown over the remaining portions of the polysilicon layer 36. A second layer of doped polysilicon 42 is deposited over the second layer of silicon dioxide 40 and over the first layer of silicon dioxide 34 within the opening 38. The second polysilicon layer 42 serves as the control or gate electrode of a switch, transfer device or field effect transistor 44 which includes first and second N+ diffusion regions 28 and 32 acting as source/drain electrodes. The trenches 18, 20, 22 and 24 may now be completely filled with any suitable material such as polyimide 46. Conductive lines 48, which may be made of copper doped aluminum, are arranged in parallel, illustrated horizontally in FIG. 1, so that each of the lines 48 contacts the gate electrodes 42 of one row of cells of the array to act as a word line.

It can be seen from the teachings of this invention that a very dense array of cells is formed with each of the cells having a large storage capacitor within a trench along with a switching device and a field shield which prevents current or charge leakage between adjacent cells when an appropriate voltage is applied to the first polysilicon layer 36.

Writing data into and reading from the memory array illustrated in FIGS. 1-3 of the drawings is preformed in a manner similar to that described in the prior art, e.g., in the hereinabove cited Dennard patent.

The cells 10, 12, 14 and 16 of the array of the present invention may be fabricated by any known process. For example, the N+ bit/sense line diffusion region 32 can be formed by defining a chemical vapor deposition (CVD) arsenic doped oxide layer into strips with a subsequent thermal drive in of the arsenic doping into the silicon substrate 26. A second CVD undoped oxide layer can be defined in strips running orthogonal to the doped oxide strips. The intersection of these orthogonal strips can then be used as a mask for etching the trenches 18, 20, 22 and 24 that are self aligned to the bit/sense line diffusion regions 32. The CVD oxide layers may subsequently be removed, or left in place to reduce bit/sense line capacitance. The trenches 18, 20, 22 and 24 may be formed by using reactive ion etching techniques and may be 5 microns deep with an opening of 1.5.times.1.5 microns or 2.25 square microns. The N+ diffusion regions 28 may be made by chemically vapor depositing a thin layer of doped silicon dioxide within the trenches, filling the trenches with any suitable material such as a photoresist to a desired level, e.g., as taught in commonly assigned U.S. Pat. No. 4,295,924, filed Dec. 17, 1979, by R. R. Garnache and D. M. Kenney, and issued on Oct. 20, 1981, and then etching the unwanted upper portions of the doped silicon dioxide. After removing the unwanted material from the trench, which defines the desired level of doped silicon dioxide within the trench, the dopant, e.g., arsenic, within the doped silicon dioxide is outdiffused into the bottom and sidewalls of the trenches to form the N+ diffusion regions 28. Furthermore, to form the opening 38 within the first polysilicon layer 34, after depositing the first polysilicon layer 34, the trenches 18, 20, 22 and 24 are filled with any suitable photoresist 50, as indicated in FIG. 4 of the drawings, and, with the use of an appropriate mask 52 having openings 54, wells 56 are formed in the photoresist 50 by using known reactive ion etching techniques. The mask 52 may be any known multilevel or multilayer photoresist mask having a non-erodable layer, e.g., polysilicon. After the wells 56 are formed in the photoresist 50, an appropriate wet etchant, such as HNO.sub.3 /HF, or a freon dry plasma etchant, may be used to form the openings 38 within the first polysilicon layer 36.

Although the first layer of polysilicon 36 may be continuous throughout the array, except for the openings 38, it should be understood that the second layer of polysilicon 42 must be appropriately etched into strips so that adjacent word lines 48 do not contact each other. To prevent storage node to storage node punch-through the P type doping between and below these nodes 28 should be about 5.times.16.sup.16 to 1.times.10.sup.17 atoms/cm.sup.3 of boron. Also, if desired, the substrate 26 may have an N type conductivity with the diffusion regions 28 and 32 being of a P+ type conductivity without departing from the spirit of this invention. Likewise, dual oxide/nitride layers may be used where the silicon dioxide layers are indicated.

As can be understood from the above description and drawings of this invention, a significant density advance has been made in fast dynamic random access memories wherein, with presently known technology, memory chips each of which may have millions of cells formed therein, although for purposes of clarity only four are illustrated in the drawings, can be manufactured. Furthermore, each of these cells, which has an area of only four square or four features, is capable of storing a binary digit of information in a relatively large storage capacitor, thus, extremely sensitive sense amplifiers for detecting the stored signals in this very dense memory are not required. The vertical structure and arrangement of the cell elements within the trench provides a storage node capacitance which is relatively independent of cell area at the surface of the semiconductor substrate while electrically isolated from other cell elements resulting in a memory with improved density, signal strength and performance.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.