Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2000-04-17
2004-07-20
Trinh, Michael (Department: 2822)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S241000, C438S268000, C438S270000, C438S587000
Reexamination Certificate
active
06764901
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to the field of memory devices and, in particular, to a circuit and method for a folded bit line memory cell with a vertical transistor and a trench capacitor.
BACKGROUND OF THE INVENTION
Electronic systems typically store data during operation in a memory device. In recent years, the dynamic random access memory (DRAM) has become a popular data storage device for such systems. Basically, a DRAM is an integrated circuit that stores data in binary form (e.g., “1” or “0”) in a large number of cells. The data is stored in a cell as a charge on a capacitor located within the cell. Typically, a high logic level is approximately equal to the power supply voltage and a low logic level is approximately equal to ground.
The cells of a conventional DRAM are arranged in an array so that individual cells can be addressed and accessed. The array can be thought of as rows and columns of cells. Each row includes a word line that interconnects cells on the row with a common control signal. Similarly, each column includes a bit line that is coupled to at most one cell in each row. Thus, the word and bit lines can be controlled so as to individually access each cell of the array.
A memory array is typically implemented as an integrated circuit on a semiconductor substrate in one of a number of conventional layouts. One such layout is referred to as an “folded digit line” architecture. In this architecture, sense amplifier circuits are provided at the edge of the array. The bit lines are paired in complementary pairs. Each complementary pair in the array feeds into a sense amplifier circuit. The sense amplifier circuit detects and amplifies differences in voltage on the complementary pair of bit lines as described in more detail below.
To read data out of a cell, the capacitor of a cell is accessed by selecting the word line associated with the cell. A complementary bit line that is paired with the bit line for the selected cell is equilibrated with the voltage on the bit line for the selected cell. The equilibration voltage is typically midway between the high and low logic levels. Thus, conventionally, the bit lines are equilibrated to one-half of the power supply voltage, V
CC
/2. When the word line is activated for the selected cell, the capacitor of the selected cell discharges the stored voltage onto the bit line, thus changing the voltage on the bit line.
The sense amplifier detects and amplifies the difference in voltage on the pair of bit lines. The sense amplifier typically includes two main components: an n-sense amplifier and a p− sense amplifier. The n-sense amplifier includes a cross-coupled pair of n-channel transistors that drive the low bit line to ground. The p−sense amplifier includes a cross-coupled pair of p−channel transistors and is used to drive the high bit line to the power supply voltage.
An input/output device for the array, typically an n-channel transistor, passes the voltage on the bit line for the selected cell to an input/output line for communication to, for example, a processor of a computer or other electronic system associated with the DRAM. In a write operation, data is passed from the input/output lines to the bit lines by the input/output device of the array for storage on the capacitor in the selected cell.
Each of the components of a memory device are conventionally formed as part of an integrated circuit on a “chip” or wafer of semiconductor material. One of the limiting factors in increasing the capacity of a memory device is the amount of surface area of chip used to form each memory cell. In the industry terminology, the surface area required for a memory cell is characterized in terms of the minimum feature size, “F,” that is obtainable by the lithography technology used to form the memory cell. Conventionally, the memory cell is laid out with a transistor that includes first and second source/drain regions separated by a body or gate region that are disposed horizontally along a surface of the chip. When isolation between adjacent transistors is considered, the surface area required for such a transistor is generally 8F
2
or 6F
2
.
Some researchers have proposed using a vertical transistor in the memory cell in order to reduce the surface area of the chip required for the cell. Each of these proposed memory cells, although smaller in size from conventional cells, fails to provide adequate operational characteristics when compared to more conventional structures. For example, U.S. Pat. No. 4,673,962 (the '962 Patent) issued to Texas Instruments on Jun. 16, 1997. The '962 Patent discloses the use of a thin poly-silicon field effect transistor (FET) in a memory cell. The poly-silicon FET is formed along a sidewall of a trench which runs vertically into a substrate. At a minimum, the poly-silicon FET includes a junction between poly-silicon channel
58
and the bit line
20
as shown in
FIG. 3
of the '962 Patent. Unfortunately, this junction is prone to charge leakage and thus the poly-silicon FET may have inadequate operational qualities to control the charge on the storage capacitor. Other known disadvantages of such thin film poly-silicon devices may also hamper the operation of the proposed cell.
Other researchers have proposed use of a “surrounding gate transistor” in which a gate or word line completely surrounds a vertical transistor. See, e.g.,
Impact of a Vertical &PHgr;
-
shape transistor
(
V&PHgr;T
)
Cell for
1
Gbit DRAM and Beyond
, IEEE Trans. On Elec. Devices, Vol 42, No.12, December, 1995, pp. 2117-2123. Unfortunately, these devices suffer from problems with access speed due to high gate capacitance caused by the increased surface area of the gate which slows down the rise time of the word lines. Other vertical transistor cells include a contact between the pass transistor and a poly-silicon plate in the trench Such vertical transistor cells are difficult to implement due to the contact and should produce a low yield.
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 realizable memory cell that uses less surface area than conventional memory cells.
SUMMARY OF THE INVENTION
The above mentioned problems with memory cells and other problems are addressed by the present invention and which will be understood by reading and studying the following specification. A memory cell is described which includes a vertical transistor and trench capacitor.
In particular, an illustrative embodiment of the present invention includes a memory cell for a memory array with a folded bit line configuration. The memory cell includes an access transistor that is formed in a pillar of single crystal semiconductor material. The access transistor has first and second sources/drain regions and a body region that are vertically aligned. The access transistor also includes a gate that is coupled to a wordline disposed adjacent to the body region of the access transistor. A passing wordline is separated from the gate by an insulator for coupling to other memory cells adjacent to the memory cell. A trench capacitor is also included. The trench capacitor includes a first plate that is formed integral with the first source/drain region of the access transistor and a second plate that is disposed adjacent to the first plate and separated from the first plate by a gate oxide. In another embodiment, the second plate of the trench capacitor surrounds the second source/drain region. In another embodiment, an ohmic contact is included to couple the second plate to a layer of semiconductor material.
In another embodiment, a memory device is provided. The memory device includes an array of memory cells. Each memory cell includes a vertical access transistor that is formed of a single crystalline semiconductor pillar that extends outwardly from a substrate. The semiconductor pillar includes a body and first and
Forbes Leonard
Noble Wendell P.
Micro)n Technology, Inc.
Schwegman Lundberg Woessner & Kluth P.A.
Trinh Michael
LandOfFree
Circuit and method for a folded bit line memory cell with... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Circuit and method for a folded bit line memory cell with..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Circuit and method for a folded bit line memory cell with... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3206460