Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2001-05-29
2003-09-30
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S680000, C438S799000, C438S660000, C438S663000
Reexamination Certificate
active
06627547
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to metallization processes for use in making devices such as semiconductor devices, and to devices formed using those metallization processes.
2. Related Art
Formation of a metal layer is a common step in the formation of some devices, such as, for example, semiconductor devices. In particular, a metal layer can be formed so as to fill in vias or cover steps formed during fabrication of a semiconductor device. The formation of a metal layer over vias having a high aspect ratio (i.e., ratio of the depth of the via to the width or diameter of the via) or steps having a relatively large height has been subject to several problems, such as cusping and voiding.
In one previous method of forming a metal layer on a semiconductor wafer, the metal layer is formed using a two step process. In the first step, a relatively thick portion of the metal layer is deposited while the semiconductor wafer is held at a relatively cold temperature (i.e., preferably less than or equal to 200° C. ). The thickness of this portion must be adequate, in view of relevant process parameters (e.g., the geometry being metallized and the metal being used), to ensure that adequate metal is present to avoid the formation of voids during the metal reflow that occurs during the second step. For example, when the metal is an aluminum alloy, this thick portion preferably has a thickness equal to about 50% to 75% of the total thickness of the metal layer to be formed. Further, this portion is preferably deposited at a rate greater than about 150 Å/sec. In the second step, the remainder of the metal is deposited while the semiconductor wafer is held at a relatively high temperature (e.g., when the metal is an aluminum alloy, about 400° C. to about 500° C. ) that allows the deposited metal to reflow through grain growth, recrystallization and bulk diffusion. The rate of deposition of the aluminum in the second step is preferably slower than that during the first step, but is preferably greater than about 50 Å/sec., and more preferably between about 100 Å/sec. and about 200 Å/sec. Further, the deposition rate can be increased during the second step to increase the process throughput. However, this method does not minimize the number of defects formed in the metal layer (such as result from cusping and/or voiding, for example) as much as desired.
In another previous method of forming a metal layer on a semiconductor wafer, the metal layer is also formed using a two step process including a first, cold deposition step followed by a second, hot deposition step. However, in this method, a relatively thin portion of the metal layer (e.g., 25% of the overall thickness) is deposited while the semiconductor wafer is held at the cold temperature, while a relatively large portion of the metal layer (e.g., 75% of the overall thickness) is deposited while the semiconductor wafer is held at the hot temperature. When the metal is an aluminum alloy, the wafer can be held at a temperature of about 200° C. for a period of about 10 seconds during the cold deposition step. During the hot deposition step, a heated gas (typically argon) is flowed against the backside of the wafer to heat the wafer and the deposited metal. The wafer can be heated to a temperature of about 375° C. to about 500° C. For the illustrative temperatures given, the wafer is typically held at that temperature for about 3-5 minutes. However, the heated gas flow is kept relatively low (e.g., less than about 15 sccm and typically in the range between about 10 sccm and about 15 sccm) so that the pressure within the process chamber can be kept low (e.g., less than about 2 mtorr). Since the heated gas flow is kept relatively low, the wafer is not heated as fast as is desirable to minimize the number of defects formed (e.g., by cusping and/or voiding) in the metal layer. Increasing the temperature of the heated gas has been tried as a means to improve this method; however, the increased gas temperature causes the steady state temperature of the wafer during the hot deposition step to increase, thus increasing the likelihood of damaging the wafer (in particular, metallization that has been previously formed on the wafer). Causing the heated gas to impinge on the wafer at multiple locations has also been tried; however, while this can cause the distribution of defects to be more evenly spread throughout the metal layer, it does not adequately reduce the overall number of defects.
SUMMARY OF THE INVENTION
The invention enables a layer of metal to be formed on a substrate with few or no voids formed in the layer. According to the invention, a layer of metal can be formed on a substrate using a cold deposition step followed by a hot deposition step. The cold deposition step need only be performed for a time sufficient to deposit metal over the entire surface on which the metal layer is to be formed. In the hot deposition step, further metal may be deposited while the substrate is rapidly heated to a target temperature. In particular, the invention enables the substrate to be heated more rapidly than has been the case in previous methods for depositing a metal layer using a cold deposition step followed by a hot deposition step. The rapid heating of the substrate results in the rapid heating of the metal deposited on the substrate. Heating this metal quickly causes the metal atoms to become mobile very quickly; in particular, the mobility of the most recently deposited metal atoms (which are typically furthest from the site of heat application) is enhanced. As a result, the deposited metal is far less susceptible to cusping and voiding than has been the case with previous methods for depositing a metal layer on a substrate. The rapid heating of the substrate can be accomplished by, for example, flowing a heated gas against the substrate at a flow rate that is higher than heretofore thought feasible.
The invention provides several advantages over previous methods of forming a metal layer. First, the invention enables a hot deposition step to be completed in a shorter period of time than has been the case in previous similar methods, thus providing increased throughput. Additionally, the invention may produce metal layers having few or no voids and, in particular, fewer voids than produced by previous methods. In particular, the invention can be used to reliably (i.e., so that 100% step coverage is achieved) fill tapered vias having an aspect ratio greater than 1:1, particularly when the via depth is about 0.5 micrometers or less. Further, the invention enables these advantages to be accomplished without increasing the temperature to which the substrate is heated, thus avoiding the increased potential for damage to the substrate and/or previously deposited or formed layers, lines or other structures associated with the use of higher temperatures.
In one embodiment of the invention, a method of forming a layer of metal on a surface of a substrate includes the steps of depositing a first amount of the metal on the substrate surface, then depositing a second amount of metal on the first amount of metal while heating the substrate from a cold temperature to about 95% of a target hot temperature at an average rate that is greater than or equal to about 10° C. /sec., more preferably greater than or equal to about 15° C. /sec., and most preferably greater than or equal to about 25° C. /sec. The deposition of the first amount of metal need only be performed long enough to ensure that the metal is deposited to cover the substrate surface. The deposition of the second amount of metal can occur for long enough to complete the formation of the metal layer. Alternatively, the heating can be discontinued before the metal layer is complete and the remaining amount of metal deposited without application of heat (e.g., as the substrate cools). Heating the substrate quickly causes the atoms of the deposited metal to become mobile very quickly (for example, increases the mobility of the atoms enough
Berry Renee R.
Cypress Semiconductor Corporation
Graham David R.
Nelms David
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