Etching a substrate: processes – Gas phase etching of substrate – Application of energy to the gaseous etchant or to the...
Reexamination Certificate
1999-03-05
2003-07-08
McDonald, Rodney G. (Department: 1753)
Etching a substrate: processes
Gas phase etching of substrate
Application of energy to the gaseous etchant or to the...
C216S067000, C216S071000
Reexamination Certificate
active
06589437
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to plasma reactors having an RF applicator and more particularly to a method and an apparatus for actively controlling the density of the species generated within such a plasma reactor using time-modulation.
2. Background Art
Plasma (or “dry”) etch processing is vitally important to several of the largest manufacturing industries in the world. In particular, plasma etch processing is indispensable in the manufacture of semiconductors, integrated circuits and microelectronic devices. These products are essential to numerous industries including the computer, electronics, biomedical and aerospace industries.
Plasma etch processing is used to modify the surface properties of a workpiece. For example, in semiconductor fabrication the workpiece is a semiconductor wafer and the plasma etch process removes patterned material from the surface of the wafer. Etch requirements such as etch depth may vary between wafers depending on, for example, the hole depth. Thus, a wafer having one pattern usually has different etch requirements than another wafer having another pattern.
These differing etch requirements dictate that such processing parameters as process gas chemistry, temperature, pressure, flow rates and power also differ between etch processes. Moreover, these processing parameters for each etch process are usually precise. Accordingly, each etch process typically has a narrow process window. In fact, several specialized “recipes” of processing parameters exist for etch process, but determining these recipes can consume a great deal of time and cost. Therefore, there is a need to broaden these narrow process windows.
One reason these process windows are so narrow is because of competing etch and deposition mechanisms within the plasma. A process window is bounded by inadequate etch selectivity at one extreme and inadequate etch stop depth at the other extreme. At one extreme, the etch selectivity means that the etch process removes one type of material while leaving other materials unaffected. This selectivity may be to any material, such as selectivity to the photoresist or the underlying substrate. The etch selectivity is inadequate when surfaces that are not meant to be etched are etched. Moreover, inadequate etch selectivity may cause overetching of a hole depth or pattern and reduce yield.
Etch selectivity is often enhanced by introducing polymer precursors in the process gas. In general, a polymer precursor is contained within the process gas and the polymer is deposited on any surface where there is no oxygen coming off the surface. Conversely, when oxygen is coming off the surface, the oxygen tends to prevent a net deposition of polymer. Thus, the polymer deposits on the surfaces of non-oxygen containing materials and not on the surfaces of oxygen containing materials.
At the other extreme the process window is bounded by excessive deposition, or inadequate etch stop depth. If too much polymer precursor is permitted to deposit on the surface of non-oxygen materials, polymer deposition can occur on top of an oxygen-containing material and then etch stop occurs. Etch stop is the cessation of etching prior to the full etching, and usually occurs during the etching of holes in the workpiece due to an excess deposit of polymer in the hole. The depth at which the etch stop occurs is called the etch stop depth. Inadequate etch stop depth means that the etch stop depth is always less than the desired etch depth.
Between these two extremes the etch process has adequate etch selectivity and adequate etch stop depth. In other words, the workpiece has an adequate layer of polymer on the non-oxygen containing surface so that the surface is sufficiently protected from etching and there is a sufficient etch rate to etch the workpiece to a desired depth. However, because a process window is so narrow it is easy to diverge from the window. Too little polymer and inadequate etch selectivity occurs. Too much polymer deposition and inadequate etch stop depth occurs.
One example of an etch process with a narrow process window is a self-aligned contact (SAC) etch process. Typically, a photoresist mask is placed over an oxide layer to be etched. The oxide may be BPSG, undoped silicate glass (USG) or some other oxide and may have various layers of varying oxide materials. Furthermore, there is a silicon substrate with a bottom nitride layer overlying the substrate in order to isolate a poly line conductor from the substrate. The poly line is placed over the insulated substrate and capped with an insulating nitride layer. In addition, there may be various other layers over the poly line. The thickness of the nitride layer overlying the poly line can be as thin as 400 angstroms, while the overall thickness of the poly line insulating layer can be from less than 1 micron to 2 microns.
The SAC etch process requires that a hole be etched into the oxide layer down to the substrate. However, as the hole nears the substrate the poly line may occupy a part of the hole. The poly line insulating layer, including the nitride layer and possibly other materials and layers, are an etch stop layer. The purpose of this etch stop layer is to keep the etchant from “blowing through” and exposing the poly line conductor. Later, a conductive material will be deposited within the hole and contact must be made with the substrate but not the poly line.
The nitride layer over the poly line must be protected with polymer to prevent etching, but the oxide must not be protected because it needs to be etched. in order to accomplish this, the processing parameters call for a relatively low source power and process gas flow. For example, for a process gas of C
4
F
8
, the flow rate is between 12 and 14 standard cubic centimeters per minute (SCCM). The size of the process window is a mere 1 SCCM. If the flow is increased by 2 SCCM etch stop will occur, and if the flow is decreased by 2 SCCM excessive etching will occur. Thus, the process window is so narrow that a change in the gas flow rate of less than about 10% and only 1 SCCM will put the gas flow rate out of the process window.
Another example of an etch process with a narrow process window is a high-aspect ratio etch process. In a high-aspect ratio etch process the aspect ratio, or ratio of the hole depth to the hole diameter, is large. in general, the greatest aspect ratio achievable with current technology is between 5:1 and 6:1 because etch stop tends to occur at higher aspect ratios.
For example, if the process gas contains polymer precursors (e.g., fluorocarbons and fluorohydrocarbons) and the chemistry is “leaned up” so that there is a lower carbon (C) to fluorine (F) ratio, this tends to form less polymer. This means that a deeper hole and higher aspect ratio hole should be able to be etched into the workpiece. The problem, however, is that at higher aspect ratios the photo-resist mask will be erode and lead to “blowing out” of the hole.
If, to avoid this problem, the process gas chemistry is made more “rich” by increasing the flow and putting in more carbon rich chemistry and higher pressures, then a thin polymer layer is formed on the photo-resist. This permits better passivation of the photo-resist. However, more polymer is also deposited in the hole and at some point this excessive deposition of polymer will cause the hole to taper off and etch stop. Typically, if the source power is 10-20% too low etch stop will occur, and if the power is 10-20% too high the etch selectivity will be severely degraded. This narrow process window also applies to other process parameters such as gas flow rate and pressure.
In order to broaden these process windows, an inductively coupled plasma (ICP) reactor or a capacitively coupled plasma (CCP) reactor is often used. In general, the common elements of the two types of plasma reactors include a reactor chamber and a workpiece support within the chamber. The workpiece is placed on the support and a process gas is introduced into the cha
Applied Materials Inc.
Bach Joseph
McDonald Rodney G.
Wallace Robert
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