Coating processes – Measuring – testing – or indicating – Thickness or uniformity of thickness determined
Reexamination Certificate
1999-10-20
2001-05-08
Beck, Shrive (Department: 1762)
Coating processes
Measuring, testing, or indicating
Thickness or uniformity of thickness determined
C427S248100, C427S255280, C427S255360
Reexamination Certificate
active
06228420
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a chemical vapor deposition process. More particularly, the present invention relates to a method that improves the inconsistency of the wafer thin film thickness by chemical vapor deposition.
2. Description of the Related Art
Chemical vapor deposition is defined as the formation of a nonvolatile solid film on a substrate by the reaction of the vapor phase reactants that contain the desired components. If the reactant is a liquid at room temperature and atmospheric pressure, the reactant is vaporized before it is introduced into the reaction chamber.
There are two types of design for the reactant supply unit of a chemical vapor deposition system or reactor for the reactant, which is a liquid at room temperature. The first design heats the liquid reactant to increase the vapor pressure of the liquid reactant. The second design delivers a fixed amount of the carrier gas into the container of the liquid reactant. Using the carrier gas, the partial vapor pressure of the liquid reactant increases the flow rate of the liquid reactant is thus increased. The latter design has been more widely used because it is more convenient to control.
FIG. 1
is a schematic diagram showing parts of a chemical vapor deposition system using a carrier gas to deliver the reactant in a Very Large Scale Integrated circuit (VLSI) processing. As shown in
FIG. 1
, the liquid reactant, for example, tetrakisdimethyl-amidotitanium (TDMAT) is contained in an ampule
10
(or a bubbler chamber). The carrier gas, for example, helium (He), is delivered in a fixed flow rate into the ampule
10
by a mass flow controller
12
(MFC) to carry the TDMA vapor into a reaction chamber
14
for a chemical vapor deposition reaction after passing through the piping and combining with gases required for other processing conditions.
Using the mass flow controller
12
(MFC) to control the flow rate of the carrier gas delivered to the TDMAT, the flow rate of TDMAT is also controlled. By presetting the processing recipe of a particular process, the deposition system or reactor is automated to perform the chemical vapor deposition process.
After the reactor has been functioning for a certain time period, the thickness of the thin film deposited on the wafer, however, is different from the preset value without altering the pre-set condition. In another words, the flow rate of the carrier gas is supposed to remain unchanged. The extent of the difference between the preset value and the actual thickness of the deposited thin film increases as the number of times of the reactor is used increases.
These results indicate that the amount of the reactant being delivered into the reaction chamber
14
alters as the number of times and the duration of the operation accumulate. The resulting film thickness thus deviates from the preset value, which would lead to an inconsistent thin film thickness deposited on the different batches of the wafer. The quality of the product is therefore out of control.
Furthermore, there are at least two reaction chambers for each chemical vapor deposition system and each reaction chamber has its individual ampule
10
. Since the number of operations accumulated for each reaction chamber is different, the extent of the deviation of the thickness of the deposited thin film from the preset value is different for each reaction chamber. In another words, the consumption rate of the reactant is different for different reaction chambers. The consumption rate of the reactant in each ampule, therefore, can not be exactly determined to provide a dependable schedule for replacing the ampule.
SUMMARY OF THE INVENTION
Based on the foregoing, the current invention provides a method to maintain a consistent thickness of the thin film deposited by chemical vapor deposition, in which a compensative coefficient K is provided and the actual deposition time T is determined by multiplying the compensative coefficient K by the preset deposition time T
0
for example, T=K×T
0
. The compensative coefficient K is determined from equation (1).
K=m
0
/m(t) (1)
where m
0
and m(t) are the initial mass flow rate of the reactant and the mass flow rate of the reactant after an accumulated time period t, respectively. The unit for K is weight per unit time, for example, kg/sec. m(t) is determined from equation (2)
m(t)=m
0
×(0.414+0.217×e
(V0/V(t))
) (2)
where V
0
is the volume occupied by the gas phase in the ampule, when the ampule is first filled the reactant. V(t) is the volume occupied by the gas phase in the ampule after the reactant is delivered into the ampule for an accumulated time period t and V
0
≦V(t). Based on equation (2), m(t) is affected by the volume occupied by gas in the ampule. After the chemical in the ampule is consumed, the volume occupied by the gas phase in the ampule increases and the concentration of the gas phase of the reactant thus becomes diluted.
The volume occupied by the gas phase V(t) after an accumulated time period is determined by equation (3), where
V(t)=V
0
+4.57×10
−4
t (3).
When 90% of the reactant in the ampule is consumed, the ampule can be replaced. The time when the ampule should be replaced is determined by the following inequality equation (4).
W
0
−∫m(t)*t≦(1−0.9)*W
0
(4)
where W
0
is the initial total weight of the reactant in the ampule.
According to the preferred embodiment of the present invention, a method to maintain a consistent thin film thickness deposited from chemical vapor deposition is provided. The method can be applied to a chemical vapor deposition system or reactor which includes at least a reaction chamber, an ampule, a mass flow controller and an information management device. The method further includes setting the values for m
0
, V
0
and W
0
in the data management device, followed by obtaining the accumulated time period t from the mass flow controller. After this, the following calculations are conducted by the data management device:
(a) the value of t is substituted into equation (3) to determine the volume occupied by the gas phase in the ampule V(t);
(b) the value of V(t) is substituted into equation (2) to obtain the mass flow rate of the reactant m(t);
(c) the value of m(t) is substituted into equation (1) to obtain the value of the compensative coefficient K.
The actual deposition time is determined by multiplying the value of K by the initial preset deposition time T
0
. The actual deposition time T is then used to control the mass flow controller.
According to the preferred embodiment of the present invention, a method to maintain a consistent thickness of the thin film deposited by chemical vapor deposition is provided. The method includes a determining step to determine whether the ampule is to be replaced based on the equation (4). When 90% of the chemical solution is consumed, and only 10% remains, the ampule is replaced.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.
REFERENCES:
patent: 4388342 (1983-06-01), Suzuki et al.
patent: 5741547 (1998-04-01), Akram et al.
Chiu Hao-Kuang
Lai Chien-Hsin
Lin Juen-Kuen
Yu Fu-Yang
Barr Michael
Beck Shrive
Thomas Kayden Horstemeyer & Risley
United Microelectronics Corp.
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