In-line Cure of SOD low-k films
03/01/1999
In-line cure of SOD low-k films
Tom Batchelder, Wayne Cai, Fairchild Technologies, Fremont, California
Jeff Bremmer, Doug Gray, Dow Corning Corp., Auburn, Michigan
A controlled ambient hot-plate oven can optimally cure HSQ films in 1-3 min, replacing 30-60 min batch furnace cures. The move to an in-line cure allowed for greater control, reduced handling losses, and proved to be a more robust overall process. Oxygen in the nitrogen ambient must be <100 ppm to ensure optimal final film properties.
The cost effective application of low-k dielectric films is crucial (along with multilevel copper signal lines and vias) to the rapid evolution to higher speed circuitry. Interconnects capable of transmitting within-chip signals at higher speeds are particularly important for microprocessor and digital signal processing (DSP) integrated circuits where high speed number-crunching is critical to manipulate and display graphic images.
Spin coating a liquid precursor that can be baked to become a solid low-k dielectric thin film - spin-on dielectric (SOD) processing - is a particularly attractive deposition method. SOD processes use equipment that is much less expensive than competing CVD processes (which require vacuum pumps and load-locks); a single piece of SOD equipment can also be used to deposit a much wider variety of viable low-k materials than a single CVD tool [1]. The ability to deposit multiple materials with the same hardware set allows for a lower-risk investment in the current situation of uncertainty as to the low-k material of choice.
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A new process was developed to both spin-coat and cure hydrogen silsesquioxane (HSQ) films in a single production process cluster. A high temperature hot-plate (350-475?C) cure, carried out in a controlled nitrogen ambient (as low as 1 ppm of oxygen), replaces the batch furnace cure employed in current SOD process flows.
Batch curing has been the defacto standard for SOD deposition, not due to inherent advantages in quartz tube thermal processing, but because generic furnace tubes around the world were readily available for materials researchers` use. With basic materials development accomplished, industry researchers now focus on optimizing process steps for manufacturability.
In addition to the low dielectric constant (k = 2.5-2.9) of the HSQ films obtained in the new, in-line, single processing sequence, other benefits include high degrees of planarization, striation-free coatings, and controlled thickness on top of features. The elimination of batch furnace processing cuts a separate process step, reducing yield losses associated with handling. Also, the conversion of this step to a single-wafer rather than a batch process provides the advantage of higher throughput and individual wafer control, without the risk of misprocessing large numbers of wafers.
The degree of planarization and other materials properties of HSQ thin-films coated on silicon wafers in the open/closed spin ning bowl will be presented. The in-line cured film properties will be discussed over a range of processing parameters using Fourier Transform Infrared (FTIR) spectroscopy.
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Figure 1. The percent planarization of open/static bowl only and open/closed spinning bowl processes as a function of feature size; mean increase = 21%.
Experimental methods
The wafers evaluated in this paper were coated on a Fairchild Technologies` LK800 IMD processing system using the open/closed spinning bowl coater, low temperature soft bake module, and controlled ambient high temperature cure oven. FOx Flowable Oxide brand HSQ was spin coated, soft baked, and in-line cured on silicon wafers in a continuous processing sequence. The processing parameters are summarized in the table on p. 29. The degree of planarization of topographical features was evaluated using scanning electron microscope images and a Tencor P10 scanning profiler for larger features.
The open/closed spinning bowl coat process improves final layer planarity compared to a static bowl process. The planarization of small features (<5 ?m) is highly effective (90-100% planarization). However, the range over which this high degree of planarization is attained is dependent upon the thickness.
If a simple open chuck is used without the open/closed spin capability, the attainable planarization was reduced by about 20% for all features greater than 5 ?m (Fig. 1). While the degree of planarization is clearly very dependent on the thickness and material properties, the open/closed spinning bowl process enhances it for a given material.
Similarly, the thickness on top of the features is controllable using open/closed spinning bowl techniques. The optimized process includes a minimum amount of material on top of the features while maintaining a high degree of planarization.
In-line HSQ cure
After completing the coat process engineering, we investigated the feasibility of developing an in-line cure process. We performed a Box-Benken designed experiment using a high-temperature low-oxygen-ambient (<100 ppm) hot-plate oven over the following variable ranges: hot-plate temperature (375-475?C), time at temperature (1-3 min), and nitrogen purge rate (15-45 slpm).
Native oxide silicon wafers were coated with FOx 16 Flowable Oxide brand HSQ to a nominal thickness of 5000 ? using the coat/cure cluster. HSQ films on the wafer were characterized for inter-molecular band structure (by transmission FTIR spectroscopy), for refractive index, RI, (by optical reflectometry), and for thickness uniformity (by optical spectroscopy). For the FTIR data, the area under the SiH peak for a given cured film thickness was normalized to an as-spun (uncured) film of the same thickness to determine a normalized SiH bond density (Norm.[SiH]) remaining after cure.
HSQ is cured from a discrete cage-like structure to a three dimensional network that provides mechanical integrity to the film during subsequent wafer processing (Fig. 2). In addition to the desired rearrangement cure reaction, other reactions can occur at elevated temperatures which convert SiH bonds to silica. Excessive conversion to silica results in densification of the relatively loose HSQ molecular structure and possible formation of silanol species. These conversion reactions tend to increase the dielectric constant [2]. Thus the HSQ cure process must balance between promoting the favorable rearrangement reaction and inhibiting the SiH conversion reactions.
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Figure 2. Conceptual illustration of HSQ cure arrangement from a discrete cage-like structure to a three dimensional network that provides mechanical integrity to the film during subsequent wafer processing.
The FTIR spectra shown in Fig. 3 demonstrate that a similar degree of cure is obtained at 475?C in 1-3 min on the hot-plate as that obtained in a furnace tube at 400?C for 60 min. In addition to observing the conversion reactions by characterizing the remaining SiH bond density with the peak centered at 2250 cm-1, the SiO stretch peaks at about 1100 cm-1 are of utility to examine rearrangement from the cage structure to the network structure. The SiO peak at 1140 cm-1 is associated with a symmetric cage structure SiO stretch vibration mode, while the peak centered at
1080 cm-1 is associated with the asymmetric network structure. During cure, the bonds rearrange from a cage structure to a network structure as evidenced by the diminishing 1140 cm-1 peak and increasing 1080 cm-1 peak.
The cure at 475?C on the hot-plate was similar to the peak relationship obtained using 400?C for 1 hr in a furnace cure. In addition to the fact that the bonding structure and degree of cure are similar, the hot-plate cured HSQ appears to be more porous than the furnace cured HSQ, as indicated by the refractive index (RI) for the hot-plate cured wafers. This low RI corresponds to a HSQ film with a reduced k value. Previous work indicates that k values as low as 2.5 can be attained with hot-plate cure methods [3].
Analysis of the DOE results indicate that cure temperature was the most significant process parameter for all response variables characterized (i.e. Norm.[SiH], thickness uniformity, and RI). Other process parameters, hot-plate cure time, and nitrogen flow rate were insignificant. The only exception may be increased thickness nonuniformity at high nitrogen flow rates. It`s quite likely that the rearrangement cure or conversion reactions do not occur at a rate sufficient to distinguish changes within the short time frame studied. The data also imply that a nitrogen purge rate of 15 slpm is sufficient to provide an inert environment for inhibiting deleterious oxidation reactions during the 1-3 min cure times.
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Figure 3. Bonding structure observed by transmission FTIR of the 475?C hot-plate cured HSQ is nearly the same as the conventional furnace cured HSQ at 400?C for one hour. The 475?C hot-plate cure, however, produces lower RI and potentially lower HSQ k.
Conclusion
The benefits of spin-on low-k dielectric materials can be fully realized when these films are cured in real time in a single manufacturing cluster. HSQ films can be cured in an elevated temperature hot-plate module with a controlled low-oxygen ambient. The increased planarization using open/closed HSQ spin coating methods versus static bowl coating methods (up to 20% increase in planarization for features up to 80 ?m in width) was also shown.
The hot-plate temperature should be high during the cure process, and the nitrogen flow should be kept as low as possible while maintaining low oxygen levels. Cure times as short as 1 min achieve the desired dielectric constant and other film properties.n
Acknowledgment
FOx is a registered trademark of Dow Corning.
References
1. Tom Batchelder, OCG Interface, p. 156, 1996.
2. J. Bremmer, Y. Liu, K Grusynski, F. Dall, MRS Spring Meeting, April 1-4, 1997.
3. Y. Liu, et. al., Proceedings of DUMIC, February 16-20, 1998.
Contact Tom Batchelder at Fairchild Technologies, 47300 Kato Rd., Fremont, CA 94538; ph 510/360-3422; fax 510/623-5750.