Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device
Reexamination Certificate
1999-07-09
2001-09-11
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
C257S410000, C257S315000, C257S322000, C257S213000
Reexamination Certificate
active
06288419
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to floating gate stacks commonly found in floating gate memory cells, and in particularly to floating gate stacks having a metal control gate protected from oxidation during oxidation of the floating gate, methods of their fabrication and apparatus produced therefrom.
BACKGROUND
A flash memory device is a non-volatile memory, derived from erasable programmable read-only memory (EPROM) and electrically-erasable programmable read-only memory (EEPROM). Flash memory is being increasingly used to store execution codes and data in portable electronic products, such as computer systems.
A typical flash memory comprises a memory array having a large number of memory cells arranged in blocks. Each of the memory cells is fabricated as a field-effect transistor having a control gate and a floating gate. The floating gate is capable of holding a charge, and is separated, by a layer of thin oxide, from source and drain regions contained in a substrate. Each of the memory cells can be electrically programmed (charged) by injecting electrons from the drain region through the oxide layer onto the floating gate. The charge can be removed from the floating gate by tunneling the electrons to the source through the oxide layer during an erase operation. Thus the data in a memory cell is determined by the presence or absence of a charge on the floating gate.
Flash memories have a typical operating voltage in the range of about 1.8 to 5 volts. A high voltage, however, is usually required for programming and erase operations in a flash memory. This high voltage (Vpp) is in the range of the 10 to 13 volts, but can be higher. During a programming operation, electrons are injected onto the floating gate by applying the high voltage (Vpp) to the control gate and about one-half Vpp to the drain region while the source region is grounded. Electron tunneling from the floating gate during an erase operation is accomplished by applying Vpp to the source region, connecting the control gate to ground potential and leaving the drain region electrically unconnected or floating. Alternately, a large negative voltage could be applied to the gate and a higher voltage applied to the source to perform a negative gate erase operation.
The construction and operation of a basic stacked floating gate memory cell is described with reference to
FIGS. 1A
,
1
B and
1
C.
FIG. 1A
is a cross-sectional view of a typical floating gate memory cell used in flash memories. Memory cell
100
comprises a source region
102
and a drain region
104
. Source
102
and drain
104
are constructed from N+-type regions formed in a P-type semiconductor substrate
106
. Source
102
and drain
104
are separated by a channel region
108
. Memory cell
100
further includes a floating gate
110
formed by a first polysilicon layer, and a control gate
114
formed by a second polysilicon layer. Floating gate
110
is isolated from control gate
114
by an interpoly dielectric layer
112
and from channel region
108
by a thin gate oxide, or tunnel layer
116
. The source region
102
generally has a deeper junction than the drain region
104
for improving erase operations.
FIG. 1B
is the memory cell
100
of
FIG. 1A
during a programming operation. To program the memory cell
100
to store a charge, a positive programming voltage of about 12 volts is applied to control gate
114
. This positive programming voltage attracts electrons
120
from P-type substrate
106
and causes them to accumulate toward the surface of channel region
108
. The drain
104
voltage is increased to about 6 volts, and source
102
is connected to ground. As the drain-to-source voltage increases, electrons
120
begin to flow from source
102
to drain
104
via channel region
108
. Electrons
120
acquire substantially large kinetic energy and are referred to as hot electrons.
The voltage difference between control gate
114
and drain
104
creates an electric field through tunnel layer
116
, this electric field attracts the hot electrons and accelerates them towards floating gate
110
. Floating gate
110
starts to trap and accumulate the hot electrons, beginning the charging process. As the charge on the floating gate increases, the electric field through tunnel layer
116
decreases and eventually loses it capability of attracting any more of the hot electrons. At this point, floating gate
110
is fully charged. The charged floating gate
110
raises the memory cell
100
's threshold voltage (Vt) above logic 1 (High) voltage. Thus, when control gate
114
is brought to a logic 1 (High) during a read operation, the memory cell
100
will barely turn on. As known to those skilled in the art, sense amplifiers are typically used in a memory to detect and amplify the state of the memory cell.
FIG. 1C
is the memory cell
100
of
FIG. 1B
during an erase operation. The memory cell
100
is erased by discharging the floating gate. To erase the memory cell
100
, a positive voltage of about 12 volts is applied to source
102
while control gate
114
is connected to ground and drain
104
is left unconnected, electrically floating. Alternately, a negative voltage, such as −10 volts, can be applied to the control gate while the source is coupled to 5 volts. With a higher relative voltage at source
102
, negatively-charged hot electrons
120
are attracted and tunneled to source
102
through the tunnel layer
116
. The tunneling is stopped when the floating gate is discharged. The lack of negative charge on floating gate
110
returns the memory cell
100
's threshold voltage below logic 1 voltage. Thus, when a voltage on control gate
114
is brought to a logic 1 during a read operation, the memory cell
100
will turn on. While the construction and operation of memory cell
100
are typical of the general concept of floating gate memory cells, those skilled in the art will recognize that the foregoing description is not intended to cover all variations of floating gate memory cells known in the art.
As device sizes continually decrease, operating voltages must decrease. Reduced operating voltages generally require reduced gate resistance for acceptable operating characteristics. In addition, the realities of self-aligned contact (SAC) processing call for reduced gate height as devices are packed more tightly in the available die real estate.
Tungsten has been identified as one approach for reducing gate height and gate resistance for field-effect transistor (FET) gates in general. Tungsten, however, is incompatible with oxidation processes, causing peeling, stress and related problems with the tungsten layer. One solution to using tungsten in FET gates has been to selectively oxidize the gate sidewalls in an H
2
O/H
2
ambient. This ambient will oxidize the polysilicon sidewalls, but not the tungsten portion, thus eliminating the oxidation problems associated with tungsten. However, exposure of floating gate memory cells to water and hydrogen is known to cause certain reliability, erase uniformity and yield problems, so it is desirable to avoid this process in the case of floating gate memory cells. Yet the sidewalls of the floating gate must generally be oxidized in order to limit charge loss on the floating gate to acceptable levels, often on the order of one to two electrons per day.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved flash memory gate stacks and methods of their fabrication.
SUMMARY
One embodiment of the invention includes a floating gate stack. The floating gate stack includes a first polysilicon layer, a dielectric layer over the first polysilicon layer, a second polysilicon layer over the dielectric layer, and a barrier layer over the second polysilicon layer. The floating gate stack further includes a metal layer over the barrier layer and a cap layer over the metal layer. An oxidation bar
Pan Pai-Hung
Prall Kirk D.
Johnson Chauncey
Meier Stephen D.
Micro)n Technology, Inc.
Schwegman Lundberg Woessner & Kluth P.A.
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