Etching a substrate: processes – Gas phase etching of substrate – Irradiating – ion implanting – alloying – diffusing – or...
Reexamination Certificate
1999-07-12
2004-10-26
Goudreau, George A. (Department: 1763)
Etching a substrate: processes
Gas phase etching of substrate
Irradiating, ion implanting, alloying, diffusing, or...
C216S063000, C438S714000, C134S001100, C134S001200, C134S021000, C134S022110
Reexamination Certificate
active
06808647
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for plasma processing. More particularly, the invention relates to methods and apparatus for minimizing process sensitivity to chamber conditions during etching processes.
2. Background of the Related Art
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited and removed from a substrate during the fabrication process. Substrate etching methods and apparatus used in device manufacture for the purpose of removing material from the substrate are well known. Typical etching techniques include wet and dry etching. However, wet etching is typically limited to fabrication of components with lateral dimensions of a micron or greater. Solid state devices and integrated circuits are now routinely fabricated with sub-micron or even nanometer scale components. Therefore, dry etching is now the preferred etching method.
One dry etching technique is commonly known as plasma enhanced etching (“plasma etching”). Plasma etching is very well suited for the manufacture of nanometer sized devices. A conventional plasma etching reactor includes a reactor chamber and an apparatus for producing a plasma within the reactor chamber. The plasma may be produced inductively, e.g., using an inductive RF coil, and/or capacitively, e.g., using a parallel plate glow discharge reactor. Typically, the plasma is struck and maintained both capacitively and inductively.
In general, plasma etching involves positioning a mask on an upper substrate surface to define an exposed portion of the substrate to be etched. The substrate, or batch of substrates, is then placed in the reactor chamber. Etching gases are introduced into the reactor chamber and a plasma is struck. During processing, the reactive species in the plasma etch the exposed portion of the metal, dielectric, or semiconductive material by contacting the exposed portion of the substrate.
At the molecular level, the etch process is a reaction between the reactive species in the plasma and the exposed surface layers of the substrate. The species include free radicals, ions and other particles. Although the reaction between the substrate and the free radicals is essentially chemical in nature, it is greatly enhanced with the ion bombardment which contributes to the etching and provides activation energy to the surface reaction. The reaction between the plasma and a substrate yields etch byproducts, i.e., small volatile molecules that desorb from the surface and subsequently are diffused into the reactor chamber. Most of the volatile byproducts are then pumped out of the reactor chamber.
Etching of a single layer of material generally comprises two primary steps: a main etch process and an overetch process. The main etch process removes the bulk of the material from the exposed substrate surface to form the desired feature. The overetch process is needed in order to remove residual material from the substrate while avoiding undercutting (i.e., isotropic etching) and excessive loss of selectivity between interfacing layers, such as a polysilicon/oxide interface. The chemistry and process parameters for each step are selected to achieve anisotropy, constant, and preferably high, etching rates, uniformity, high selectivity, and reproducibility.
Successful etching requires a controlled process to ensure uniform etching at a controlled and constant rate with respect to each individual substrate as well as from one substrate to the next. Uncontrolled changes in etching rates can lead to changes in device geometries and dimensions. Preferably, an etch rate change for a particular substrate and from one substrate to the next is less than about 10%.
Process stability is affected by various methods and techniques commonly used in the industry. For example, a substantial change in the etch rate is observed after a cleaning process. Cleaning processes are periodically necessary to remove deposits of byproducts which form on the internal chamber surfaces during processing. Most volatile byproducts formed during etching are pumped out of the chamber. However, the byproducts can often react with various gas components used in silicon etching with a halogen plasma, such as oxygen additives, to form less volatile byproducts. In other cases, the byproducts themselves can be less volatile, depending on the material being etched and the etch chemistry. These less volatile species can deposit onto the chamber walls and other exposed surfaces enclosed within the chamber. Over time, the deposits can delaminate and flake off of the chamber surfaces creating a major source of particulate contamination. The particulate often become lodged in the mask or on the substrate surface and produce defective devices. As the size of the etched features become smaller, the effects of particulate become more pronounced.
Thus, in order to control the contamination buildup, the chamber surfaces are cleaned periodically. One method of cleaning a chamber, known as dry cleaning, involves placing a dummy substrate in the chamber and then igniting a plasma The plasma chemistry is selected to react both chemically and physically with the deposits on the chamber, thereby causing the deposits to form byproducts which can be pumped out of the chamber. However, a problem associated with chamber cleaning, is that the etching rates of the subsequent processes are adversely affected. In typical silicon plasma etching, etch rate drops in excess of 33% have been experienced following a chamber cleaning run. Changes in the etch rate are undesirable because of the resulting loss of process reproducibility, or repeatability. Because etching processes are often timed according to pre-programmed recipes, the fluctuation in etch rates results in over-etched or under-etched substrates. Consequently, repeatability between substrates is lost.
In order to minimize the detrimental effects of etch rate variation after a cleaning process, current practice employs a seasoning cycle during which the chamber is conditioned following a cleaning run. Seasoning refers to the operation of the chamber to allow deposition of a film on the chamber surfaces. During a recovery period a seasoning coating is allowed to form on the chamber surfaces by striking a plasma in the chamber and depositing a film on the internal exposed surfaces in the chamber. The chamber seasoning is continued until the pre-clean etch rate is completely recovered. The recovery period is time consuming and non-productive because no substrates are processed. Thus, the throughput of the system is significantly reduced.
Another problem related to process stability occurs when high concentrations of corrosive chemicals are used. For example, a problem exists in cases where fluorine is the major etchant, such as in planarization and recess-etching. Fluorine is a highly corrosive etchant that attacks the chamber surfaces causing a change in the topography of the surface and/or resulting in deposition of a byproduct, such as AlF
x
in the case of an Al
2
O
2
chamber surface. Unlike deposition of SiO
x
which can be removed by non-intrusive cleaning methods such as plasma cleaning, as described above, the effects of fluorine etching must be treated by opening the chamber and refinishing the chamber surfaces. Thus, throughput is substantially affected. Additionally, following the cleaning process, fluctuations in the etch rate are observed which inhibit etch rate stability.
Sensitivity to chamber conditions was also observed by the inventors in situations other than post-cleaning. For example, when the same chamber is used to run different applications involving alternating chemistry, varying etch rates are observed. Thus, processes alternating between fluorine-based chemistry and non-fluorine-based chemistry experience volatility in etch rates between each cycle. As an example, a chamber may first be used to process a number of substrates during a fluorine-bas
Podlesnik Dragan
Qian Xueyu
Sun Zhi-wen
Xu Songlin
Bach Joseph
Goudreau George A.
Moser, Patterson & Sheridan L.L.P.
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