EEPROM having coplanar on-insulator FET and control gate

An electrically erasable programmable read-only memory CEEPROM) includes a field effect transistor and a control gate spaced apart on a first insulating layer, a second insulating layer formed over the field effect transistor and the control gate and a common floating gate on the second insulating layer over the channel of the field effect transistor and the control gate, the floating gate thus also forms the gate electrode of the field-effect transistor. The EEPROM devices may be interconnected in a memory array and a plurality of memory arrays may be stacked on upon another. The invention overcomes the problem of using a non-standard silicon-on-insulator (SOI) CMOS process to make EEPROM arrays with high areal density.

FIELD OF THE INVENTION 
This invention relates to semiconductor memories and, more particularly, to 
electrically erasable programmable read-only memories (EEPROMs). 
BACKGROUND OF THE INVENTION 
Silicon-on-insulator (SOI) technology has made great strides in recent 
years, and may possibly replace bulk silicon as the technology of choice 
for future VLSI circuits. SOI technology has dielectric isolation, rather 
than the twin tubs of conventional CMOS, and makes it practical to use the 
isolated silicon island as a circuit element. 
The most commonly used EEPROMS use a floating gate and control gate (word 
line) elements in combination with programming either by hot-electron 
injection or by Fowler-Nordheim tunneling through a thin dielectric, and 
erasing by Fowler-Nordheim tunneling. 
EEPROM cells may be fabricated using a standard CMOS process on bulk 
silicon without any additional processes. Such is described in a 
publication by K. Ohsaki et al., IEEE Journal of Solid State Circuits, 
Vol. 29, No. 3, p. 311, March 1994 entitled "A Single Poly EEPROM Cell 
Structure for Use in Standard CMOS Processes". The EEPROM cell consists of 
adjacently placed NMOS and PMOS transistors. The EEPROM cell uses only a 
single polysilicon layer which is patterned to provide a common 
polysilicon gate with respect to the NMOS and PMOS transistors. This 
polysilicon gate serves as the floating gate of the EEPROM cell. With bulk 
CMOS, this EEPROM implementation is very space consuming, requiring about 
48 lithography squares, making the cell impractical for most applications. 
The present state of the art in EEPROM design is represented, for example, 
in the publication by H. Kume et al. entitled "A 1.28 .mu.m.sup.2 
Contactless Memory Cell Technology for a 3V-Only 64 Mbit EEPROM", in 1992 
International Electron Devices Meeting, Technical Digest, p. 991. An 
EEPROM device consists of an n-channel field effect transistor having a 
floating gate of polysilicon and a control gate (word line) above the 
floating gate in a stack. The small cell area of 1.28 .mu.m.sup.2 is based 
on 0.4 .mu.m CMOS process (4 squares). The program/erase mechanism uses 
Fowler-Nordheim tunneling. 
EEPROMs are useful for low power portable electronics and as microcodes for 
application specific integrated circuits (ASIC's) and for microprocessors. 
SUMMARY OF THE INVENTION 
The present invention relates to semiconductor devices, arrays of such 
devices and stacked arrays of such devices, suitable for electrically 
erasable programmable read-only memories (EEPROMs). The EEPROM device 
consists of a floating gate, a control gate, and an insulated-gate 
field-effect transistor (IGFET). Both the control gate and the FET are 
made from the same layer of semiconductor on an insulator layer. Being 
made from the same layer, the control gate and the FET are thus coplanar. 
The floating gate lies on top of both the control gate and the FET. The 
region of the floating gate on top of the FET thus also forms the gate 
electrode of the FET. The control gate is capacitively coupled to the 
floating gate. 
The present invention, with both the FET and the control gate being 
co-planar and lying on an insulator surface, can be readily fabricated 
using the standard silicon-on-insulator (SOI) technology, where the 
silicon layer of the SOI wafer can be used to form both the control gate 
and the FET. Since the floating gate also forms the gate electrode of the 
FET, the fabrication process of this EEPROM device can also be used to 
fabricate other FETs which are not part of the EEPROM device. Thus, the 
present invention readily enables the integration of EEPROM devices of the 
present invention and standard SOI CMOS devices on the same chip. 
The EEPROM device of the present invention can also be fabricated using 
polysilicon-on-insulator or amorphous silicon-on-insulator, with both the 
control gate and the FET made from a polysilicon layer or an amorphous 
silicon layer. Since polysilicon-on-insulator or amorphous 
silicon-on-insulator can be formed readily on top of standard CMOS 
integrated circuits, or on top of one another, multiple layers of arrays 
of EEPROM devices of the present invention may be stacked on top of one 
another. By stacking layers of arrays, the areal density of the EEPROM 
cells can be increased.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a top view of an electrically erasable programmable 
read only memory (EEPROM) cell 10 is shown. EEPROM cell 10 includes field 
effect transistor (FET) 12 having a floating gate 14. The source 13 of FET 
12 is coupled over lead 11 to a voltage potential such as ground and the 
drain 15 of FET 12 is coupled to bit line 16. Floating gate 14 extends 
over a word line 18 with an overlap area 20 sufficient to provide 
capacitive coupling of a predetermined value. 
A plurality of EEPROM cells 10 may be interconnected an array as shown in 
FIG. 8 to form an EEPROM array. The EEPROM cells may be arranged in 
columns and rows. EEPROM cells in a row are coupled to a respective word 
line. EEPROM cells in a column are coupled to a respective bit line. 
An array of EEPROM cells may be fabricated on a silicon-on- insulator (SOI) 
wafer 24 such as shown in FIG. 2. A substrate 25 which may be for example 
silicon has an insulating layer 26 thereover which may be for example 
silicon dioxide. A semiconductor layer 27 which may be single crystal, 
polycrystalline or amorphous is positioned on insulating layer 26. 
Semiconductor layer 27 may be for example Si, Ge, SiC, SiGe, GaAs, GaN, 
InGaAs or InP. Wafer 24 may be fabricated by bond and etch back 
techniques. Bond and etch back consists of bonding a silicon wafer to 
another silicon wafer with one or both wafers having a layer of silicon 
dioxide thereon. One of the silicon wafers is then etched down to a thin 
layer. Or, wafer 24 may be fabricated by implantation of oxygen into a 
silicon wafer 25 and subsequently annealed to form a buried oxide layer. 
The process is known as separation by implantation of oxygen (SIMOX). 
FIG. 3 shows a cross section view of FIG. 1 along the lines 5--5 at a first 
stage in fabrication before the gate oxide and subsequent layers are 
formed. Semiconductor layer 27 may be patterned to form first and second 
semiconductor regions that are spaced apart. The first region is for the 
source 13, drain 15 and channel region 39 of FET 12. The second region is 
for the control gate (word line 18). The openings 28, 29 and 30 in 
semiconductor layer 27 are filled with a dielectric for example silicon 
dioxide 32 such as by chemical vapor deposition (CVD). The upper surface 
of patterned semiconductor layer 27 and silicon dioxide 32 is polished 
such as by chemical mechanical polishing (CMP) to form a planar surface 
33. 
Source 13, drain 15 and the body of FET 12 and control gate (word line) 18 
are doped p type such as for example to about 4.times.10.sup.17. Control 
gate (word line) 18 may be doped heavily p++ such as for example to about 
2.times.10.sup.20 to reduce its resistance. 
Next as shown in FIG. 4, a thin silicon dioxide layer 36 is formed for 
example by CVD to provide a gate insulator for FET 12. 
Next, a layer 38 of polycrystalline semiconductor material which may be for 
example polysilicon is formed over a thin film insulator layer 36, which 
may be for example silicon dioxide, formed over patterned semiconductor 
layer 27. Layer 38 is subsequently patterned by lithographic techniques to 
form floating gate 14 as shown in FIGS. 1 and 4. 
Using floating gate 14 as a mask, source 13 and drain 15 are doped n type 
by ion implantation such as for example to about 1.times.10.sup.20 leaving 
the channel region 39 p type. Floating gate 14, acting as a mask, is also 
doped n type in the process. 
A layer 40 of dielectric which may be for example silicon nitride is formed 
over floating gate 14 and insulator layer 36. Layer 40 is subsequently 
etched such as by reactive ion etching (RIE) to form sidewalls 42 shown in 
FIG. 5. 
Next, insulator layer 36 is etched through where not protected by floating 
gate 14 or sidewall 42 to expose the semiconductor material of control 
gate (word line) 18 and source 13 and drain 15 shown in FIG. 1. Next, a 
layer of refractory metal such as titanium is deposited over the exposed 
semiconductor material for example silicon of control gate (word line) 18, 
source 13, drain 15 and floating gate 14. The layer of refractory metal is 
annealed to form, for example, titanium silicide 44 on control gate (word 
line) 18, source 13, drain 15, and floating gate 14 as shown in FIGS. 1, 6 
and 7. 
FIG. 6 is a cross section view along the line 6--6 of FIG. 1. In FIG. 6, 
FET 12 may be fabricated on 200 nm thick semiconductor material. Silicon 
dioxide layer 36 may be 5 nm thick. Floating gate 14 may be about 200 nm 
thick. 
FIG. 7 is a cross section view along the line 7--7 of FIG. 1. In FIG. 7, 
control gate (word line) 18 is made from the same semiconductor layer as 
FET 12. 
FIG. 8 is a top view of an memory array 50 of EEPROM cells interconnected 
on one layer to form a random access memory. EEPROM cells 51-61 are 
arranged in rows and columns. The control gates (word lines) of EEPROM 
cells 51-54 are coupled in series to word line 64. The control gates (word 
lines) of EEPROM cells 55-57 are coupled in series to word line 65. The 
control gate (word lines) of EEPROM cells 58-61 are coupled in series to 
word line 66. Word lines 64-66 correspond to rows 0-2 in memory array 50 
and carry control signals W0-W2 respectively. Word lines 64-66 are coupled 
to word line drivers 68, 69, and 70 which may be for example CMOS 
circuits. The source 13 of FETs 12 of each EEPROM cell is coupled to a 
predetermined voltage such as ground potential by way of a first metal 
wiring level (not shown). The drain 15 of FETs 12 of EEPROM cells 51, 55 
and 58 are coupled to bit line 72. The source 13 of FETs 12 of EEPROM 
cells 52, 56 and 59 are coupled to bit line 73. The drain 15 of FETs 12 of 
EEPROM cells 53, 57 and 60 are coupled to bit line 74. The drain 15 of 
FETs cells 54 and 61 are coupled to bit line 75. Bit lines 72-75 
correspond to columns 0-3 and carry data signals D0-D3 respectively. Bit 
lines 72-75 may be metal lines on a second wiring level and may contact 
the drain terminals of FETs in the column in the array by way of shared 
vias from the second wiring level to two FETs from adjacent rows. 
A typical operation of the memory array 50 is as follows. To erase a bit, 
the word line is raised from 0 to 10 volts and the bit line is held at 0 
volts. To program a "1", the word line is lowered from 0 to -7.5 volts and 
the bit line is raised from 0 to 2.5 volts. To program a "0", the word 
line is lowered from 0 volts to -7.5 volts and the bit line is held at 0 
volts. To read data out, the word line of the selected cell is raised from 
0 to 2.5 volts and the respective bit line is biased at a positive voltage 
for example 1 volt and the current through the selected bit line is 
measured using a suitable sense amplifier. 
FIG. 9 is a cross section view showing a plurality of stacked layers 81-83 
where each layer may be a memory array similar to memory array 50 shown in 
FIG. 8. In FIG. 9, the word lines of each layer are accessed from the side 
of the array layer, as in FIG. 8 for one layer. The vias or studs 91 and 
92 may contact all of the FETs 12 in a vertical column and then the vias 
or studs are connected in respective columns to form respective bit lines. 
Thus, a metal bitline for one of the memory cells on the upper-most memory 
array layer also serves as the bit line for all the memory cells belonging 
to the same vertical column, one cell from each of the lower memory array 
layers. An insulation layer not shown is formed above a stacked layer 
prior to forming the next memory layer thereover. Memory array 50 may be 
fabricated above a bulk silicon wafer or an SOI wafer. 
While there has been described and illustrated an EEPROM array and stacked 
array containing EEPROM devices having a coplanar on-insulator FET and 
control gate or word line, it will be apparent to those skilled in the art 
that modifications and variations are possible without deviating from the 
broad scope of the invention which shall be limited solely by the scope of 
the claims appended hereto.