Method of forming tunnel oxide layer

Details Method of forming tunnel oxide layer Method of forming tunnel oxide layer

2002-01-10

2003-09-30

Whitehead, Jr., Carl (Department: 2813)

Semiconductor device manufacturing: process

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

Having insulated gate

C438S264000

Reexamination Certificate

active

06627501

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of forming a tunnel oxide layer, and more particularly, to a method of forming a tunnel oxide layer by utilizing the rapid thermal oxidation (RTO) method and annealing the tunnel oxide layer in-situ by utilizing the rapid thermal annealing (RTA) method.
BACKGROUND OF THE INVENTION
Generally, according to the difference in accessing functions, the memory devices can be divided into random access memory (RAM) and read only memory (ROM), wherein RAM must keep the supplying power uninterruptedly for reserving the stored data and is thus called as a volatile memory, and ROM does not lose the stored data even with the break of supplying power and is thus called as a nonvolatile memory. In addition, according to the varieties of the ways for storing data, ROM can be further divided into mask read only memory (MROM), programmable ROM (PROM), erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), and flash memory, etc. Because the data stored in a non-volatile memory can be kept after the power is shut off, non-volatile memory device is applied widely in the industries of computer and electronics. Particularly, as the increasing popularization of portable electric devices, such as notebook computer and telecommunication equipment, etc., and the increasing device integration, the technical demands for electrically erasable programmable ROM and flash memory, which can be accessed like disk drives, is increasing day by day.
Referring to
FIG. 1
, it shows a cross-sectional view of a flash memory cell. In the flash memory cell, a tunnel oxide layer
16
, floating gate
18
, dielectric layer
20
, and control gate
22
are stacked in order on a P-type substrate
10
, and the N-type drain
12
and source
14
are formed under the upper surface of substrate
10
, wherein the composition of tunnel oxide layer
16
can be, for example, silicon dioxide (SiO
2
), and the composition of the floating gate
18
and control gate
22
can be, for example, poly-silicon, and the composition of dielectric layer
20
can be, for example, silicon dioxide or silicon nitride (Si
3
N
4
).
If the source
14
and the substrate
10
are connected to ground, and the control gate
22
and the drain
12
are applied with high-voltage electricity, the carrier multiplication phenomenon will occur between the N-type drain
12
and the P-type substrate
10
. A portion of the hot electrons resulted from the carrier multiplication phenomenon are absorbed by the drain
12
, and the other potion of them pass through the tunnel oxide layer
16
and into the floating gate
18
, so as to make the floating gate
18
charged. Because there is a potential barrier in the composition of tunnel oxide layer
16
and dielectric layer
20
that connect to the floating gate
18
, the electrons within the floating gate
18
cannot escape but stay in the floating gate
18
. While a voltage is applied to the control gate
22
for accessing the data in the aforementioned flash memory cell, with the charged floating gate
18
, data “1” is stored. On the other hand, with no electron staying within the floating gate
18
, data “0” stored in the flash memory cell. As to deleting the data stored in memory cell, by applying an appropriate negative voltage to the control gate
22
, the electrons within floating gate
18
are induced to leave floating gate
18
through the tunnel oxide layer
16
, and thereby the data stored in the flash memory cell is deleted.
In the memory cells of EEPROM and flash memory, the written/erased action of data are performed by prompting hot electrons passing through the tunnel oxide layer to get enter/exit from the floating gate. Thus, the quality of electricity of the tunnel oxide layer has great influence on the stability of memory device, and the reliability of device is lowered by many impurities or charges, and the unnecessary variation of electricity is induced.
The stability of electricity of thermal oxide layer, such as tunnel oxide layer, etc., is influenced principally by the increase of charge concentration of oxide layer resulted from the trapped charges induced during the operation of device by impurity defects or unsaturated bonding of the interface between silicon substrate and oxide layer, so that the charges are kept within the oxide layer. In order to lower the charge concentration in the oxide layer, after the oxide layer is formed, an annealing process is used to lower the concentration of impurity defects or unsaturated bonding, so that the amount of trapped charge is decreased, and the charge concentration of oxide layer is lowered.
Generally, most of the silicon dioxide for the tunnel oxide layer is formed and annealed with the use of a thermal oxidation furnace by the furnace process. An annealing process is a kind of metal smelting technique in wide application, and the annealing principle is to use thermal energy to increase the energy of lattice atoms and material defects, so that vibration and diffusion of lattice atoms and defects are increased, and the arrangement of atoms of material is rearranged, and the material defect of are lowered. In virtue of the disappearance of defects, recrystallization is performed, and further grain growth is performed. The purpose of the annealing process is to eliminate the material defects and rearrange the structure of material, so after the oxide layer is formed, the annealing process can be applied to eliminate the defects of oxide layer and enhance the quality of electricity of oxide layer.
Referring to
FIG. 2
,
FIG. 2
shows a cross-sectional view of a conventional thermal oxidation furnace for forming and annealing the tunnel oxide layer. Thermal oxidation furnace
100
is mainly composed of a quartz tube
102
that has been annealed by high-temperature heating, a heater
104
, and thermocouples (not shown) used for measuring the furnace tube temperatures.
For forming a silicon dioxide film served as the tunnel oxide layer with the use of a thermal oxidation furnace
100
, about from 100 to 150 pieces of substrate
106
are first put on boats
112
made of, for example, quartz, and then into a thermal oxidation furnace
100
, wherein an appropriate amount of nitrogen (N
2
) is injected into the thermal oxidation furnace
100
through a gas inlet
108
. Then, after the temperature of thermal oxidation furnace
100
is raised, oxygen (O
2
) and hydrogen (H
2
) followed are injected into the thermal oxidation furnace
100
to form a silicon dioxide film by thermal oxidation, wherein for the prevention of hydrogen explosion caused by the hydrogen accumulated in the thermal oxidation furnace
100
, the amount of oxygen must be at least greater than a half of the amount of hydrogen, and the gas after reaction has to be exhausted through the gas outlet
110
. Then, hydrogen and oxygen are stopped being injected into the thermal oxidation furnace
100
, and the system temperature is lowered with the use of nitrogen.
Subsequently, the thermal oxidation furnace
100
is used to anneal the tunnel oxide layer, and a reactive gas, such as nitrogen, is injected into the thermal oxidation furnace
100
through the gas inlet
108
. The temperature of the thermal oxidation furnace
100
is raised to an appropriate high temperature by the use of heater
104
, and substrate
106
is placed at the high-temperature environment for a period of time. At this time, the lattices of the atoms of the tunnel oxide layer on substrate
106
are rearranged with the thermal energy resulting from the high temperature. Subsequently, after the temperature of the thermal oxidation furnace
100
is lowered, substrate
106
is taken out of the thermal oxidation furnace
100
, and the process of forming the tunnel oxide layer is completed.
Although more than one hundred of substrates can be treated simultaneously by forming and annealing silicon dioxide film with the use of furnace process for performing thermal process, yet it takes quite a long time for the process to be completed, which

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