Methods of forming gated semiconductor assemblies

Details Methods of forming gated semiconductor assemblies Methods of forming gated semiconductor assemblies

1998-04-07

2003-10-21

Pham, Long (Department: 2811)

Semiconductor device manufacturing: process

Making field effect device having pair of active regions...

Having insulated gate

C438S266000, C438S268000, C438S791000

Reexamination Certificate

active

06635530

ABSTRACT:

TECHNICAL FIELD
The invention pertains to gated semiconductor assemblies, such as, for example, erasable, programmable read-only memories (EPROMS), electrically erasable proms (EEPROMS), and flash EEPROMS.
BACKGROUND OF THE INVENTION
Read-only-memories (ROMs) are memories into which information is permanently stored during fabrication. Such memories are considered “non-volatile” as only read operations can be performed.
Each bit of information in a ROM is stored by the presence or absence of a data path from the word (access) line to a bit (sense) line. The data path is eliminated simply by insuring no circuit element joins a word and bit line. Thus, when the word line of a ROM is activated, the presence of a signal on the bit line will mean that a 1 is stored, whereas the absence of a signal indicates that a 0 is stored.
If only a small number of ROM circuits are needed for a specific application, custom mask fabrication might be too expensive or time consuming. In such cases, it would be faster and cheaper for users to program each ROM chip individually. ROMs with such capabilities are referred to as programmable read-only-memories (PROMs). In the first PROMs which were developed, information could only be programmed once into the construction and then could not be erased. In such PROMs, a data path exists between every word and bit line at the completion of the chip manufacture. This corresponds to a stored 1 in every data position. Storage cells during fabrication were selectively altered to store a 0 following manufacture by electrically severing the word-to-bit connection paths. Since the write operation was destructive, once the 0 had been programmed into a bit location it could not be erased back to a 1. PROMs were initially implemented in bipolar technology, although MOS PROMs became available.
Later work with PROMs led to development of erasable PROMs. Erasable PROMs depend on the long-term retention of electric charge as the means for information storage. Such charge is stored on a MOS device referred to as a floating polysilicon gate. Such a construction differs slightly from a conventional MOS transistor gate. The conventional MOS transistor gate of a memory cell employs a continuous polysilicon word line connected among several MOS transistors which functions as the respective transistor gates. The floating polysilicon gate of an erasable PROM interposes a localized secondary polysilicon gate in between the continuous word line and silicon substrate into which the active areas of the MOS transistors are formed. The floating gate is localized in that the floating gates for respective MOS transistors are electrically isolated from the floating gates of other MOS transistors.
Various mechanisms have been implemented to transfer and remove charge from a floating gate. One type of erasable programmable memory is the so-called electrically programmable ROM (EPROM). The charge-transfer mechanism occurs by the injection of electrons into the floating polysilicon gate of selected transistors. If a sufficiently high reverse-bias voltage is applied to the transistor drain being programmed, the drain-substrate “pn” junction will experience “avalanche” breakdown, causing hot electrons to be generated. Some of these will have enough energy to pass over the insulating oxide material surrounding each floating gate and thereby charge the floating gate. These EPROM devices are thus called floating-gate, avalanche-injection MOS transistors (FAMOS). Once these electrons are transferred to the floating gate, they are trapped there. The potential-barrier at the oxide-silicon interface of the gate is greater than 3 eV, making the rate of spontaneous emission of the electrons from the oxide over the barrier negligibly small. Accordingly, the electronic charge stored on the floating gate can be retained for many years.
When the floating gate is charged with a sufficient number of electrons, channel function is inhibited. The presence of a 1 or 0 in each bit location is therefore determined by the presence or absence of a conducting floating channel gate in each program device.
Such a construction also enables means for removing the stored electrons from the floating gate, thereby making the PROM erasable. This is accomplished by flood exposure of the EPROM with strong ultraviolet light for approximately 20 minutes. The ultraviolet light creates electron-hole pairs in the silicon dioxide, providing a discharge path for the charge (electrons) from the floating gates.
In some applications, it is desirable to erase the contents of a ROM electrically, rather than to use an ultraviolet light source. In other circumstances, it would be desirable to be able to change one bit at a time, without having to erase the entire integrated circuit. Such led to the development of electrically erasable PROMs (EEPROMs). Such technologies include MNOS transistors, floating-gate tunnel oxide MOS transistors (FLOTOX), textured high-polysilicon floating-gate MOS transistors, and flash EEPROMs. Such technologies can include a combination of floating gate transistor memory cells within an array of such cells, and a peripheral area to the array which comprises CMOS transistors.
A prior art EPROM device is described with reference to semiconductor wafer fragment
10
of
FIGS. 1-3
.
FIG. 1
is a top view of wafer fragment
10
, and
FIGS. 2 and 3
are cross-sectional side views along the lines labelled X—X and Y—Y, respectively, in FIG.
1
. Wafer fragment
10
comprises a substrate
12
, having field oxide regions
14
formed thereover. Substrate
12
can comprise, for example, lightly doped monocrystalline silicon. To aid in interpretation of the claims that follow, the term “semiconductive substrate” is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.
Field oxide regions
14
can comprise, for example, silicon dioxide. An active region
15
extends over and within substrate
12
between field oxide regions
14
. A floating gate
16
and a control gate
18
are formed over the active region. Gates
16
and
18
can comprise, for example, conductively doped polysilicon.
Floating gate
16
is separated from substrate
12
by a tunnel oxide layer
20
. Gates
16
and
18
are separated from one another by an insulative layer
22
which can comprise, for example, a combination of silicon dioxide and silicon nitride, such as the shown ONO construction wherein a silicon nitride layer
17
is sandwiched between a pair of silicon dioxide layers
19
. The silicon nitride comprises Si
3
N
4
, although other forms of silicon nitride are known. Such other forms include silicon enriched silicon nitride layers (i.e., silicon nitride layers having a greater concentration of silicon than Si
3
N
4
, such as, for example, Si
4
N
4
). An advantage of silicon-enriched silicon nitride layers relative to Si
3
N
4
is that the silicon-enriched silicon nitride layers frequently do not require separate, discrete antireflective coatings formed between them and a photoresist. However, silicon enriched silicon nitride is difficult to pattern due to a resistance of the material to etching. Silicon enriched silicon nitride layers are formed to have a substantially homogenous composition throughout their thicknesses, although occasionally a small portion of a layer (1% or less of a thickness of the layer) is less enriched with silicon than the remainder of the layer due to inherent deposition problems.
Wafer fragment
10
further comprises silicon dioxide layers
24
and
26
extending along sidewalls of gates
16
and
18
, and comprises a silicon dioxide layer
28
over control gate
18
. Layers
24
,
26
and
28
can electrically insul

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