2-5-1 Deposition Method:
A variety of deposition method to form FSG films has been developed.
Plasma-enhanced chemical vapor deposition (PE-CVD) and atmospheric chemical vapor deposition (APCVD) processes have been extensively investigated [30, 31]. FSG
deposited by AP-CVD system below 450oC show tensile stress and tend to absorb moisture that resulting in increasing dielectric constant and have integration problem in ULSIs. On the other hand, two plasma deposition processes, conventional plasma source and high density plasma source with bias rf, have been mainly to produce more stable FSG films and commonly use in current integrating ULSIs.
A number of gas source have also been investigated [32]. Fluorine precursor can be divided into two types. One type has pre-existing Si-F bonds, such as silicon tetrafluoride (SiF4) or fluorotriethoxysilane (FTES) [33,34], and the other is a clean gas without Si-F bonds, such as NF3, CF4 or C2F6. The forming FSG films by the latter precursor absorbed more water than films deposited by the former source gas. SiF4 and FTES have Si-F bonds and help in reducing the number of dangling or weakly bound fluorine atoms in the FSG films.
Generally speaking, precursors used commonly for the manufacture of ULSI are TEOS or SiH4 in the presence of SiF4 or FTES for the conventional PE-CVD technique [35,36], and SiH4 in the presence of SiF4 (SiH4/SiF4/O2/Ar) for the HDP-CVD process [37, 38].
2-5-2 Infrared Spectra
The Si-F bonds absorption peak appear about 935 cm-1 in the infrared (IR) spectrum for FSG films by the addition of fluorine into silicon oxide films. Katsumate et al. [39]
have suggested that the absorbance band of the Si-F bond was deconvoluted to three Gaussian bands and that that the development of the wave number feature on the 988 cm-1 absorption is indicative of the antisymmetric bond-stretching vibration of the Si-F2
bonding group in the high fluorine concentration (>10 at. %). It was reported that Si-F2
bonds reacted readily with moisture to form Si-OH bonds and result in corrosion in the metal wirings.
On the other hand, Lucovsky et al. [40] deny the existence of the Si-F2 bonding group because there is no definitive IR spectroscopic evidence for the bonding around 935 cm-1 in the FSG films even fluorine concentration up to 12 at. %. Besides, the silicon monofluorine (Si-F) bond-stretching vibration (935 cm-1) is approximately at the same frequency as the absorped-moisture vibration (920 cm-1), since the similar equivalence of masses of F (19 amu) and OH (17 amu). As a result, it is difficult to conclude the absorption peak around 935 cm-1 is decomposed into the Si-F peak and the absorbed moisture peak for FSG films with higher fluorine concentration [41].
2-5-3 Moisture-Absorption
High resistance to the moisture absorption is crucial for choosing a dielectric material. Moisture absorption increases the dielectric constant of films, degrades the device performance and reduces the reliability. The main problem of FSG films is that Si-F bonds have a hygroscopic nature and not always very stable. Though higher fluorine concentration in the FSG film can reduce the dielectric constant, this FSG film readily absorbs moisture while stored in a clean room and results in a higher dielectric constant [42].
Miyajima et al. [43] have pointed out that ion bombardment, as well as gas-dissociation efficiency, is one of the key factors to obtain high resistance to moisture uptake. They have used TEOS/O2/CF4 gas to deposition FSG films by PE-CVD
technology, or helicon-wave- type HDP-CVD without rf biasing. They suggested that HDP produces ions such as O+ and O2+ with higher ion energy than PE-CVD, and these ions densify the Si-O network, reducing the carbon and hydrogen content.
Plasma gas treatment was also provided to improve the moisture resistance of FSG films. Takeishi et al. [44] have proposed an N2O-plasma treatment to stabilize the FSG films. They found that this N2O plasma treatment at 400oC is effective to block moisture absorption. Although this treatment rarely increase the dielectric constant of FSG films, it also have improved the adhesion of the FSG films with the subsequent depositions of foreign passivation films, such as silicon nitride, silicon oxynitride, etc, in addition to blocking moisture absorption.
2-5-4 HDP-CVD FSG Film
Since voids formed in the 0.35 um wide spacing with the aspect ratios of more than 1.75 in the case of using PE-CVD FSG films, HDP-CVD method with radio frequency rf biasing attract more attention to deposit IMD films in ULSIs [45,46] because it can provide gap-filling capacity. In this technique, the deposition and physical sputter-etching processes are carried out simultaneously. This rf biased HDP-CVD method utilizes a dependency of an Ar sputter-etch rate upon an incidence angle. The rf power supplied to the substrate generates a large self-bias voltage that accelerates the ionized Ar atoms to the substrate effectively. The sputter rate at the corner of the interconnect is higher than on any other surface and this effect is contributed to good filling of the narrow gaps.
The schematic drawing of HDP-CVD chamber is shown in Figure 2-5. An inductively coupled plasma (ICP) was used to produce high-density plasma source. The
wafer needs effective cooling to maintain temperature and uniformity across it, because the bias rf power pulls the energetic ions out of the plasma and directs then at the wafer surface. The electro-static chuck (ESC) with a cooling liquid circulating through it was used in this equipment, and substrate temperature during the deposition was controlled by regulating the He pressure between the substrate backside and the ESC surface. The reaction gases used for this biased HDP-CVD FSG films are SiF4, SiH4, O2 and Ar. The Fluorine concentration is mainly controlled by varying the ratio of SiF4/SiH4 ratio.