Patent Application
20070246354
1. Field of the Invention
The present invention relates to the field of glow-discharge deposition and etching systems.
2. Prior Art
Glow-discharge plasma deposition systems are used in the general IC industry for physical vapor deposition of metal and other films. A glow-discharge is a self-sustaining type of plasma, i.e., a partially ionized gas containing an equal number of negative and positive charges as well as some other number of non-ionized gas particles. In plasma systems, atoms are dislodged, or sputtered, from the surface of target material by collision with high energy particles. Some of the sputtered material arrives at the surface of a silicon wafer that faces the target and adheres to it there, thereby coating the surface with a film of sputtered material. Film thickness is in general proportional to deposition time and power. These metal films are used for a variety of purposes, including device interconnection, diffusion barrier, resistors, electrodes, etc.
The most commonly used systems in the industry today are magnetron sputtering systems. This type of sputtering increases the percentage of electrons that cause ionizing collisions by utilizing magnetic fields to help confine the electrons near the target surface. The plasma, sputtering target and wafer are typically contained in a deposition chamber. The stationary or rotating magnet is located immediately above the target. The magnet generates a plasma above the target and very close to the target face. The density of this plasma is relatively uniform. In turn, this translates to a deposited film on the wafer that is mostly uniform in thickness. Typical percentage standard-deviation (% STD) for a 0.5 μm thick aluminum layer is ˜0.5% (thickness range=150 Å). This non-uniformity is acceptable for most VLSI device applications. For the purposes of this work we shall refer to films with thickness greater than 0.1 μm as “thick films.”
Films needed for diffusion barriers, Schottky diodes, etc., may range in thickness from approximately 100 Å to 1000 Å. In this work, we refer to these films as “medium-thick films.” These films, such as TaN, TiN, CoSi2, PdSi2, etc., typically exhibit increased non-uniformity. This increased % STD is due to various effects, such as ab initio deposition and non-uniformities due to chamber issues (shields, dep rings, gas flow, wafer edge effects, etc.).
For 300 Å TiN, % STD may be as high as 2.5%, which translates to a range of ˜15%. One should note that although the actual thickness may not be increasing per se (15% of 300 Å is 45 Å), however, as a percentage of film thickness, and hence film properties, the % STD increases. Medium-thick films therefore exhibit an even greater non-uniformity in properties such as sheet resistance, conductivity, etc.
“Thin films,” that is, films whose thickness is less than 50 Å, in particular those films sputtered from multi-component targets, can exhibit % STD as high as 6% or more. The deposition of these very thin films is difficult to control without utilization of “averaging techniques,” such as wafer movement across a plasma region and very careful chamber design. Unfortunately, such systems are very expensive for general use in VLSI. Some of these systems in the industry are known generally as MRAM or Optical Systems.
An exemplary prior art glow-discharge plasma deposition system may be seen in
Glow-discharge plasma etching is the reverse of glow-discharge plasma deposition, the material being removed from a substrate rather than deposited, typically through a mask. While the effects of non-uniformity in etching rates across a wafer are usually not as significant as non-uniformities in deposition rates, still uniformity in etching rates is desirable to minimize etching time and minimize the time of exposure to the plasma etch of the layer below the layer being removed.
a illustrates the approximate non-uniformity of the film thickness contour based on the original magnetic field distribution with no magnetic filter in accordance with the present invention.
b illustrates the approximate thickness of the magnetic material to alter the film thickness proportionately.
a illustrates the deposition obtained without use of a magnetic filter in with the present invention.
b illustrates the effect of the magnetic filter on the resulting deposition thickness in accordance with the present invention.
In preferred embodiments, the present invention comprises the addition of a new device to a glow-plasma system to improve deposition uniformity in a commercial system designed for deposition of thick films. This new device is referred to herein as a “Magnetic Filter.” Such a Magnetic Filter can improve the as-deposited % STD of very thin, multi-component films by a factor of 5× or more, and add negligible system cost to the overall system and no additional cost in the operation of the system.
A prior art glow-plasma deposition system of the general type shown in
In accordance with the present invention, a magnetic filter 38 is placed between the metal backing plate 30 and the magnet 35, as may be seen in
a illustrates the approximate non-uniformity of the film thickness contour based on the original magnetic field distribution.
Thus in this particular application of the invention, one major benefit is the very low cost of adding a Magnetic Filter, which makes it an insignificant portion of the overall system cost. A second major and critical benefit is that the Magnetic Filter is positioned between the target and the magnet, and is external to the deposition chamber. As such, it is not deposition chamber intrusive, and therefore does not interfere directly with critical chamber process parameters such as pressure, temperature, electric potential, etc. Due to the latter, a third major benefit is that this technique can be applied equally well to all plasma systems for improved uniformity irrespective of their use as deposition or etch systems.
Other applications could require positioning of the Magnetic Filter within the magnetic field so as to cause a desired alteration of the magnetic field at a particular location in the space of interest. This may be inside or outside the plasma chamber.
The degree and type of magnetic filtering has been described in terms of global and local modifications of a generic plasma deposition production tool with a multi-component target for deposition of a sputtered thin film. Changes to plasma conditions due to the magnetic filter material have been shown in terms of normalized magnetic field strength changes. Deposited film improvements are shown in terms of contour maps and % STD. Although the initial, as-deposited film properties are highly non-linear and dependent on various properties, a magnetic filter can be deduced for any set of conditions with undue experimentation. It is thus possible to tailor film material parameters to suit particular application needs. In one particular case, this filtering technique is used for integrated circuit products. The degree and type of modification is not immediately apparent and cannot be deduced from present available knowledge published in the literature, but is determined empirically.
Although the data presented herein is for a composite film deposited in a generic, plasma production tool, specifically an Applied Materials metal deposition system, this technique is not limited to this composite film and this particular tool. Instead, the method is equally and similarly applicable to all plasma deposition, plasma doping and plasma etch tools that utilize either a plasma or ionized gasses to assist process conditions. Since the magnetic filter may not be intrusive to the process chamber, it is envisaged that other materials and tools can be altered and modified in a similar manner.
The present invention improves thin film as-deposited uniformity so that much more uniform thin films are manufacturable. It thus reduces the thin film cost per wafer, since more expensive deposition tools are not necessary. By improving product device uniformity across a wafer, device yield per wafer is also improved. When material trimming is necessary, the present invention reduces trim energy variability across a wafer, and hence reduces trim time. It also eliminates device yield loss particularly near the wafer edge. The present invention also reduces the film deposition rate during processing, and thus enables better wafer-to-wafer timing control for very thin processes (and generally very short deposition times), without reducing power supply set-points to levels where the power supply output is not well controlled.
In general, manufacturers of plasma systems try to optimize the magnet sets to improve within-wafer film uniformity. However there is a limit on the uniformity that can be achieved that way. The present invention takes the deposition (or etching) uniformity a step further to allow thin film deposition without the expensive and time-consuming task of attempting to re-engineer the entire magnet set and deposition chamber.
Other advantageous applications and effects that may be achieved by the present invention include:
a. Sputter rate changes for single or multi-component target. Globally changing the field changes the plasma voltage, which in turn changes the sputter rate of each element. This results in different sputtered-film compositions and equivalent parametric changes.
b. Plasma damage reduction. By creating a more uniform field globally across a wafer, one can optimize the process so that the plasma voltage is reduced and hence reduce plasma damage.
c. Radial field uniformity changes. An example is where one would want thicker deposition at the edge of a wafer to compensate for CMP (chemical mechanical polishing) increased erosion at the wafer edge. Similarly one can compensate for increased etch rates at the wafer edge.
d. Plasma annealing. Creating or modifying a non-uniform plasma for uniform equivalent annealing across a wafer.
e. Plasma doping. Creating or modifying a non-uniform plasma for uniform equivalent doping across a wafer.
f. Increased sputter yield from sputter targets, thereby reducing process cost-of-ownership.
In the claims to follow, systems in which the present invention is applicable are referred to as plasma systems, though are to be understood to include systems using an ionized gas, and are independent of the use of the system, such as, by way of example, for deposition, doping and annealing.
Thus while certain preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.
