Stress Control in Multi-Layer Backside

Modern trends in microelectronics require advanced backside metallization processes that work on thin and extremely thin substrates. This article describes some new approaches, using the Sputtered Films, Inc (SFI) ENDEAVOR AT cluster PVD tool. Our goal was to develop a dc,magnetron sputtering process which would allow the deposition of a low stress, three metal stack of titanium (3000 Å), nickel-vanadium (3000 Å), and silver (18500 Å) on the back sides of 200 mm diameter silicon wafers thinned to 200 mm. An inter-layer stress compensation approach was used. The stack, consisting of films strained in opposite directions, had residual stress (after self-relaxation period) of less than 2E8 dynes/cm2. This enabled high-quality metallization with very low wafer warpage.

The Semiconductor Industry needs to design new processes that can precisely control residual stresses in thin films. For example, stress control in backside and under bump metallurgy is very critical to reliable die attachment [1]. In applications on thinned wafers, low or even zero stress films are required to avoid “potato-chipping” of the wafers. With so many different web casinos out there it is often difficult to work through the clutter. This often confuses people and makes it difficult for the to decide which of these casinos they will play at.

The majority of vacuum deposited metal and alloy films have tensile stress unless special stress control methods are used. Highly efficient stress control methods developed in the last several years with magnetron sputtering systems allow creating plasma or low energy ion bombardment conditions on the substrate during film growth. Depending on the film material, different approaches are necessary to ensure an effective stress reduction. Some of the modern stress control methods are described in. The majority of the are safe online casinos but it can never hurt to be a little over cautious. You can always look at them by reading a couple of reviews before you start playing as this will give you a good indication of whether or not they are in fact safe online casinos. In backside metallization processes, multi-layer metal film stacks are typically employed, for instance, a stack of titanium (Ti), nickel-vanadium (Ni-V), and silver (Ag) films. Ti and Ni-V are materials in which tensile stress can be decreased significantly or even converted to compressive stress by means of deposition with a negative substrate bias. At the same time, magnetron
sputtered Ag is a very difficult material for stress control. It is possible to utilize the good stress controllability of some materials in a multi-layer stack, to compensate for the influence of other layers upon the total residual stress in the stack.

Therefore, we created a low stress sandwich consisting of films strained in opposite directions. This inter-layer stress compensation approach has been successfully employed in previous SFI work on 150 mm-diameter wafers. For larger size, thinned wafers, fine tuning the stress is more complicated. Here, one of the most important issues is the ability to ensure high stress uniformity. An erratic stress distribution in any layer can lead to the disturbance of the inter-layer stress compensation principle. As a result, the wafer can become essentially non-flat or even be curled up.
Our goal was to develop a dc magnetron sputtering process which would allow the deposition of a low stress, three metal stack of Ti (3000 Å), Ni-V (3000 Å), and Ag (18500 Å) on the back sides of 200 mm diameter silicon wafers thinned to 200 mm. All deposition experiments were done in the ENDEAVOR AT cluster PVD tool equipped with series IV S-GUNS [3]. The series IV S-GUN is a cylindrical dual concentric ring cathode sputter source. With so many safe online casinos around it is good to know that you needn’t worry. This makes it so much nicer as you can continue playing at the casinos and feel safe doing so. The two independently controlled cathodes (targets) and a central bias-able anode allow depositing highly uniform films. RF power can be applied to the substrate holder creating a negative bias on the wafer. By varying the substrate bias power it is possible to influence film properties, such as morphology and stress. We used standard 0.7 mm thick non-doped silicon (Si) wafers for stress investigation and process development. The process parameters were then adjusted for wafers thinned to 0.35 and 0.2 mm. Film thickness was measured using a Dektak-3ST Surface Profiler; thickness uniformity was evaluated with a CDE ResMap Automatic 4 Point Probe. For a quality evaluation of the stack adhesion a simple scratch test was employed. Wafer curvature radius and height measurements were taken by an FSM 128 Thin Film Stress and Flatness System. Sputtered Films, Inc. Stress Control in Mulit-Layer Backside Metallization of Thinned Wafers

Investigation results and discussion
Technology of pre-deposition surface treatment In accordance with specifications, the technological process included wafer etching, which removed 50 Å before metallization. RF plasma etching was performed using different levels of RF power (300-700W) and argon gas flow (5-15 sccm). It was found that the three-metal stacks had poor adhesion; at the same time, the titanium layer itself had good adhesion. An additional wafer degas (300C for 40 seconds) followed by etching did not prevent stack delamination in the scratch test. The most probable reason for this is the dilution of the substrate-coating bonds conditioned by residual tensile stresses in silver. We then employed a high-temperature wafer pre-heat followed immediately by Ti sputtering in the same process module to stimulate a diffusion interaction between first film and substrate material. Experiments have shown that for the standard 0.7-mm thick wafers, the thermal treatment at 400C ensured an excellent cohesion. Titanium films The process module for Ti deposition had an etch configuration which enabled extremely deep stress control possibilities for that material. Fig. 1 shows that Ti films become compressive with the application of a comparatively low RF bias power (40W) and can reach a stress level of about -1E10 dynes/cm2 with increasing power. Stress in the Ti film does not depend as much on argon gas flow and deposition temperature as some other materials. (For example, Ni-V films demonstrate significantly higher tensile stress for higher temperature and gas flow.) With a goal to ensure a certain compressive stress value, we split the Ti film deposition into two steps: (1) the “hot” step, which improved adhesion properties, and (2) the “cold” step, which gave an overall deeper compressive level. These two sub-layers, when deposited on standard 0.7- mm thick wafers, had the following properties: The “hot” layer, deposited on Si wafer with pre-heat 400 C for 50 seconds: stress = -8E8 dynes/cm2, bulk resistance = 75 mOhm-cm; Sputtered Films, Inc. Stress Control in Mulit-Layer Backside Metallization of Thinned Wafers.