Successful use of low-k films requires measuring Young's modulus
10/01/2003
IC manufacturers switching over to low-k interlevel dielectric (ILD) materials from SiO2 and fluorinated silica glass (FSG) have encountered a variety of problems, including delamination, peeling, and cracking. Many of these arise from degradation of mechanical properties and result in processing problems during etch, CMP, or packaging. However, greater knowledge and attention to the value of Young's modulus for ILD materials during manufacturing, which can be measured without contact and nondestructively, will likely result in more consistent mechanical performance.
Although the International Technology Roadmap for Semiconductors (ITRS) predicted the use of ILD materials with k values <3 at the 130nm node, most manufacturers selected an FSG with a somewhat lower 4.0 to 3.6 k value, thus maintaining most of the desirable film properties of SiO2. However, there is still general agreement that FSG cannot be used for the densely wired lower layers of 90nm designs. While a variety of different materials are being evaluated, each presents different integration challenges.
The two major contenders are carbon-doped oxides (e.g., Applied Materials' Black Diamond and Novellus' CORAL) and spin-on organic polymers (e.g., SiLK from the Dow Chemical). One major hurdle to integrating any of these materials is their reduced mechanical strength when compared to SiO2. With these materials, measuring the modulus of an ILD has been shown to be a good predictor of its ability to withstand mechanical stress and can be used to determine the down force that can be safely applied during CMP and wire-bonding.
Traditionally, modulus has been measured using a nano-indentor or bend test, which involve contacting the wafer and are relatively slow. Today, acoustic-based measurement technology provides a noncontact, nondestructive measure of Young's modulus, as well as the thickness of low-k ILDs.
null
The method
Briefly described, measurement begins with a 10-13sec laser pulse focused onto a 5¥7µm spot on the surface of a wafer. The same laser generates a probe pulse that measures the reflectivity of the sample. The pump pulse is transmitted through the transparent ILD to the underlying metal or silicon substrate. The metal film absorbs energy from the pump pulse, launching a strain pulse (sound wave) that travels up through the transparent ILD. The sound wave causes a local change in the index of refraction of the ILD, leading to constructive and destructive interference at the detector. Sound velocity is calculated from this signal. Then, with a separate measurement of index of refraction, Young's modulus can be calculated.
 |
null
Results with leading materials
In one example of our many tests with this method, the figure shows measured and best fit modeling data from a CORAL ILD film. From the interference oscillation period, we extract the sound velocity in the material. The phase change at ~200 psec is caused by the sound wave traveling through the film and reflecting from the top surface. It can be used to determine the thickness of the film.
The value here for fab processing is that the excellent correlation between measured and modeled data allows accurate and repeatable characterization of thickness and Young's modulus. The thickness of the CORAL film was measured to be 5003Å; the film's speed of sound was 25.9Å/ psec; and the modulus was 6.7GPa. Measurements such as this can be made in 2–5 sec/site with >1% accuracy and repeatability.
The table lists our data for samples of 3 popular ILDs. Our tests of a 5042Å-thick Black Diamond film revealed a Young's modulus of 7.2GPa. The tested SiO2 film was somewhat thicker at 7596Å with a Young's modulus of 73.2Gpa, which corresponds well with the expected value. The order-of-magnitude difference in modulus between the SiO2 and the low-k materials explains why low-k materials are prone to mechanical problems during processing. In the case of these carbon-doped oxides, the hydrocarbon groups added to the ILD reduce the dielectric constant, but also dramatically reduce their ability to withstand stress.
With the adaptation of existing metrology capabilities to quickly and accurately make these measurements, misprocessed films can be identified before additional processing steps are performed and resources wasted. The film's modulus can also be used as feed-forward information to tune CMP and other processes to a particular film's parameters.
Sean Leary, Guray Tas, Tim Johnson, Rudolph Technologies, Flanders, New Jersey
For more information, contact Sean Leary at Rudolph Technologies Inc., 1 Rudolph Rd., Flanders, NJ 07836; ph 973/448-4479, fax 973/691-5480, [email protected].