Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2002-07-25
2003-03-11
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S688000
Reexamination Certificate
active
06531388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device. More particularly, the present invention relates to a method of forming an aluminum film on a semiconductor substrate and atop which film an anti-reflective layer is to be formed.
2. Description of the Related Art
As semiconductor devices become more highly integrated, the wiring patterns of the devices are becoming finer. In addition, an RC delay caused by the wiring is known to be the main characteristic of a semiconductor device that determines the operating speed of the device. For this reason, a multi-layer wiring structure is widely used. An aluminum film, to which a small amount of silicon (Si) or copper (Cu) is added, is used to form the multi-layer wiring structure. Specifically, the aluminum is sputtered onto a semiconductor wafer to fill a contact hole or a via hole and thereby form a metal wiring layer.
Subsequently, the metal wiring layer must be patterned. However, the desired pattern can not be formed by a projection exposure method if a photosensitive resin, such as photoresist, were deposited directly on the aluminum film, because aluminum is highly reflective. The exposure light would reflect from the aluminum film causing a metal bridge to form between wirings particularly in a stepped area. This problem is referred to as a “notching phenomenon”. In order to solve this problem, dye has been added to the photoresist to reduce the influence of the reflected light. However, this technique compromises the microscopic processing quality, and so it is not adopted in cases where a fine wiring structure is desired.
On the other hand, another technique for obviating the notching phenomenon resides in forming an anti-reflective layer on the aluminum film and using a typical photoresist film having a superior microscopic processing quality. The anti-reflective layer has a low degree of reflectivity with respect to the exposure light. In this respect, a layer of titanium nitride (TiN) has been mainly used as the anti-reflective layer.
A multi-chamber physical vapor deposition apparatus is the most widely used apparatus for forming an aluminum film on a semiconductor wafer. Hereinafter, a method of depositing aluminum using a multi-chamber physical vapor deposition apparatus will be described with reference to FIG.
1
.
Referring to
FIG. 1
, a lot comprising 25 or 26 sheets of silicon wafers is loaded in a cassette of a loadlock chamber. Then, the pressure in the loadlock chamber is reduced until a vacuum is maintained in the loadlock chamber. Thereafter, a wafer
10
coated with an insulating layer
12
is moved from the loadlock chamber into an RF-etching chamber wherein an etching process is performed to remove impurities from the wafer.
Next, the wafer
10
is conveyed into a reaction chamber, that is a PVD chamber. In the PVD chamber, an aluminum film
14
is formed on the wafer by a sputtering method. The aluminum fills a contact hole or a via hole (not shown).
Then, the pressure in the PVD chamber is reduced and the wafer is moved into a cooling chamber. Thereafter, cooling water and cooling gas, such as argon (Ar), are injected into the cooling chamber so that the wafer is simultaneously water-cooled and air-cooled. Then, the wafer
10
is returned to the cassette of the loadlock chamber.
Once all wafers have been returned to the cassette of the loadlock chamber, a venting process is carried out whereupon the wafers may be unloaded. That is, vent gas, such as nitrogen (N
2
) or argon (Ar), is supplied into the loadlock chamber through a vent line connected to the loadlock chamber until the pressure of the loadlock chamber reaches 760 Torr, whereupon the wafers are removed from the PVD chamber.
In the above-described aluminum deposition process, the stress on the aluminum film varies depending on the material of the cassette in which the wafers are stored. The cassette has 25 or 26 slots in which the wafers are received, respectively, and is mainly made of polypropylene (hereinafter, referred to as “PP”) or polybutylene terephtalate (hereinafter, referred to as “PBT”).
FIG. 2
is a graph showing the difference in thermal conductivity according to the cassette material. The temperature of a hot plate on which the cassettes are placed is set to 100° C. The surface temperature of the cassettes on the hot plate was measured over time. The thermal conductivity of the cassette is thus the gradient of the curve of the change in temperature over time. As shown in
FIG. 2
, a cassette made of PP has a thermal conductivity of 0.3° C./min. On the contrary, a cassette made of PBT has a thermal conductivity of 1.2° C./min, or one that is four times that of a PP cassette.
In the above-described aluminum deposition process, when the PP cassette is used, the venting process is carried out just after the wafer in the last slot has been conveyed into the loadlock chamber. Consequently, the wafer from the last slot is cooled relatively fast compared to the other wafers. Therefore, the stress on this last wafer is relatively high. More specifically, a thin film has an intrinsic stress according to deposition conditions, such as the temperature and pressure. When the cooling process is carried out after the deposition process, the thin film and an underlying layer experience different degrees of thermal expansion, whereby the thin film is subject to thermal stress. Therefore, a rapidly cooled aluminum film is subject to a high level of stress.
Still further, the aluminum film
14
as shown in
FIG. 1
is subject to a large amount of thermal stress caused due to differences in thermal expansion between the aluminum film
14
and the underlying insulating interlayer
12
. At the same time, the aluminum film
14
is subject to tensile stress created by the anti-reflective layer
16
formed thereon. When these stresses are applied to the aluminum film
14
, the aluminum film
14
undergoes plastic deformation. The stresses are relieved to some extent because aluminum is a relatively soft material. On the contrary, the titanium nitride layer is a relatively hard material. Accordingly, when the aluminum film
14
undergoes a thermal cycle, from the cooling process to the succeeding process in which an anti-reflective layer
16
comprising titanium nitride (TiN) is formed thereon, shear stress occurs at the interface between the aluminum film
14
and the anti-reflective layer
16
. If the value of the shear stress exceeds a critical value, the TiN anti-reflective layer
16
is locally delaminated from the aluminum film
14
, which defect is referred to as a “ball defect” (as seen at “A” in FIG.
1
).
When the PBT cassette is used, all of the wafers returned to the cassette are rapidly cooled, and not just the last wafer, because the cassette has a high degree of thermal conductivity, i.e., a thermal conductivity that is four times that of the PP cassette. Therefore, all of the wafers are subject to high levels of stress. Accordingly, ball defects are produced in all of the wafers after a TiN anti-reflective layer is formed thereon.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to obviate the above-described problems of the prior art. More specifically, an object of the present invention to provide a method of manufacturing a semiconductor device, which prevents a ball defect from occurring at the interface between an aluminum film and an anti-reflective layer formed thereon.
To achieve this object, aluminum is deposited on a substrate and then slowly cooled. The slow cooling is carried out to relieve the thermal stress in the aluminum film. Typically, the thermal stress is produced when the aluminum film is subjected to a reflow process. At least part of the slow cooling process is a forced cooling of the aluminum film, and preferably also includes holding the substrate for a predetermined period of time in the cooling chamber before the forced cooling begins. The substrate is then left as is for more than 3 minutes (passive c
Bae Jong-Yong
Lee Seung-Hwan
Gurley Lynne
Samsung Electronics Co,. Ltd.
Volentine & Francos, PLLC
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