Chemistry of inorganic compounds – Zeolite – Organic compound used to form zeolite
Reexamination Certificate
2002-01-11
2003-12-09
Sample, David (Department: 1755)
Chemistry of inorganic compounds
Zeolite
Organic compound used to form zeolite
C423SDIG002, C423S335000, C502S004000, C502S085000, C502S086000, C427S444000
Reexamination Certificate
active
06660245
ABSTRACT:
BACKGROUND
1. Technical Field
The present invention relates to colloidal silicalite and zeolite crystals and, more particularly, to methods of removing entrained organic template molecules from within the crystals, and to applications of the detemplated crystals to produce thin films or membranes of zeolites or silicalites on substrates. The thin films may be used as separation membranes, catalytic membranes or low dielectric constant insulators in microelectronic devices.
2. Description of Related Art
One recent innovation in the formation of porous membranes has been the development of colloidal zeolites and silicalites. Sub-micron sized particles suspended in a liquid matrix are easily converted to a thin film on a substrate by a variety of methods. While membranes formed in this manner may ultimately be very useful in chemical catalysis and purification, the descriptions and examples in this application will emphasize films used in integrated circuit manufacture.
Increasing the speed and performance of integrated circuits (“ICs”) typically calls for increasing the density of electronic components on the surface of a semiconductor wafer and increasing the speed at which the IC performs its functions. Increasing component density brings the charge-carrying circuit elements closer together, thereby increasing the capacitive coupling (crosstalk) between such circuit elements and delay in the propagation of signals through the conductors. Higher capacitance is detrimental to circuit performance, especially for high-frequency operation, as is typically encountered in telecommunication applications and elsewhere. One way of reducing the capacitive coupling between proximate circuit elements is to reduce the dielectric constant (“k”) of the insulator or insulating material(s) separating the coupled circuit elements.
It has been conventional in the fabrication of ICs to use dense materials as dielectrics, including silicon dioxide, silicon nitride and cured silsesquioxanes among others. The dielectric constant (k) of these materials typically lies in the range of approximately 3.0 to 7.0.
It is anticipated that the performance of future ICs is likely to be limited by resistive-capacitive (“RC”) delay occurring in the metallic interconnects of the IC, indicating that lower k dielectrics will be required for future ICs. As yet, the only fully dense materials with k less than about 2.4 are fluorinated polymers or fully aliphatic hydrocarbon polymers. However, such materials have not been shown to have sufficient thermal and mechanical stability to survive the thermal and mechanical stresses occurring during IC fabrication. In addition, these polymers typically have chemical properties that are similar in some respects to the chemical properties of photoresist materials commonly used in IC fabrication. Thus, chemical removal of photoresist layers without damaging dielectric layers becomes more difficult.
Several potential low k materials for IC dielectrics are materials that have a high degree of porosity. The open structure of such porous materials includes a significant amount of airspace. Therefore, the overall effective dielectric constant of the material lies between those of air and the fully dense material, typically significantly lower than that of the pure, solid material. Several general classes of porous materials have been described, including porous silicon dioxides.
Previous work by one of the present inventors relates to the use of colloidal silicalite crystals (“CSCs”) in forming spin-on dielectric coatings (interlayer dielectrics or “ILDs”) in the fabrication of ICs, as described in U.S. application Ser. No. 09/514,966, filed Feb. 29, 2000, incorporated herein by reference. Silicalites are porous crystalline forms of silica having the same crystal structure as zeolites, as described, for example, by Edith Flanigen and Robert Lyle Patton in U.S. Pat. Ser. No. 4,073,865. Colloidal suspensions of silicalite crystals are described, for example, by Jan-Erik Otterstedt and Dale A. Brandreth,
Small Particles Technology
(Plenum Press, 1998), especially Chapter 5. See also
The Synthesis of Discrete Colloidal Crystals of TPA-Silicalite
-1 by A. E. Persson et. al. appearing in
Zeolites
, September/October 1994, pp. 557-567. See also Li, Q., Creaser, D. and Sterte, J., “The Synthesis of Small Colloidal Crystals of TPA-silicalite-1 with Short Synthesis Times and High Yields”. in
Porous Materials in Environmentally Friendly Processes
, Ed. I. Kiricsi, G. Pál-Borbély, J. B. Nagy, H. G. Karge,
Stud Surf Sci. Catal
., 125, 133 (1999), available online as a Master's thesis at http://www.km.luth.se/kmt/theses/qlilic.pdf. CSCs offer the possibility of a porous, low k dielectric material that can easily be deposited on semiconductor wafers with standard wafer processing techniques and that can withstand subsequent etching, polishing and metallization steps.
However, CSCs are not suitable for film or membrane formation by themselves. A suitable binding agent must be used in cooperation with the CSC. That is, a CSC is typically deposited on the surface of a substrate along with a binding agent. Favored binding agents typically contain silicon and oxygen and crosslink at elevated temperatures, binding the CSCs into a porous ILD having adequate mechanical strength to withstand further processing. “Monolithic films” denote the films created by colloidal crystals having been bound together by a binding agent. For integrated circuits, binding agents based on silicon dioxide are desirable because of their proven compatibility with current IC processing steps, such as dielectric reactive ion etching and photoresist removal.
Silicalite crystals of an appropriate size for forming low dielectric constant films for integrated circuits are typically formed by stirring together a silica source, such as TEOS, and a so-called “structure directing agent,” or SDA, in water. For many colloidal zeolites and silicalites, the SDA is a quaternary ammonia base. (See, for example, Tsapatsis and Gavalis,
MRS Bulletin
, March 1999, p. 32.) The mixture is stirred at sufficient temperature and for sufficient time for crystals of the desired size to grow. The choice of SDA is the strongest determinant of the crystal structure obtained. For example, tetrapropylammonium hydroxide (a quaternary ammonia base) yields the MFI structure, while tetrabutylammonium hydroxide (another quaternary ammonia base) yields the MEL structure. (Structure nomenclature as used in this application follows International Zeolite Association guidelines.)
To a lesser extent, crystal growth temperature determines the crystal structure obtained. For example, there are some quaternary ammonia species which yield a range of different structures, the predominant structure in a given batch being determined by the growth temperature.
As the crystals form around the SDA's, the large SDA molecules eventually become entrained within the porous crystals. In order for the silicalite crystals to form an advantageous low dielectric constant film, the entrained molecules must be removed. This is because the SDA molecules themselves raise the dielectric constant, and also because they are often polar molecules. Polar molecules tend to attract water, which further increases the dielectric constant. Also, residual basic molecules in the film have the potential to cause unwanted reactions in deep-UV photoresist during subsequent processing. Similarly, to form a useful catalytic membrane or molecular sieve, the channels within the crystals must be cleared of obstructing molecules. The molecules have significantly larger diameter than any single channel in the crystal, in most cases, and so they cannot simply diffuse out. Thus, a need has been identified for a method to break the quaternary ammonia molecules into small, volatile byproducts that can diffuse out of the crystals.
The removal of entrained SDA molecules from zeolite or silicalite crystals may be referred to here as ‘detemplating’. Because most industrial uses of zeolites and silicalites
Gaynor Justin F.
Vancleemput Patrick
Novellus Systems Inc.
Sample David
Tso Roland
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