Advanced Process Control in Megasonic-Enhanced Pre-Bonding Cleaning

Transcription

1 Advanced Process Control in Megasonic-Enhanced Pre-Bonding Cleaning D. Dussault a, F. Fournel b, and V. Dragoi c a ProSys Inc., 1745 Dell Av., Campbell, CA 95008, USA b CEA, LETI, MINATEC, F38054 Grenoble, France c EV Group, DI E Thallner Str. 1, 4782 St. Florian/Inn, Austria The increased complexity of microelectronic and sensor applications that have emerged over the past decade have driven wafer bonding to mature as a production technology for both MEMS and 3D stacked die manufacturing. The use of CMOS device wafers has restricted the types of viable bonding processes to low temperature fusion bonding, adhesive bonding and metal bonding. Due to the specific requirements of CMOS technology the allowed bonding processes had to be adapted in order to fulfill the specific CMOS-compatibility demands. This paper presents a novel single wafer Megasonic based cleaning method with enhanced process control capabilities. This cleaning process is integrated in low temperature fusion bonding process and enables higher bonding yields and a reduction in process fluid usage. Introduction Wafer bonding has been used successfully during past years for 3D interconnects and MEMS applications. The need for higher levels of integration is increasing the complexity of the bonded structures as electronics and mechanical or optical functionality layers have to be integrated by bonding. Electronics integration in particular is imposing significant limitations to the bonding process in terms of the two main process features: process temperature and bonding layers allowed. The temperature range is clearly determined by the metal interconnects (<400 C for Al interconnections) and the choice of the bonding layers is conditioned by the compatibility with CMOS technology contamination levels. Low temperature fusion wafer bonding is one of the CMOS compatible bonding processes. This type of process was already employed in the manufacture of engineered substrates (1, 2) as well as for manufacturing of CMOS-MEMS and BSI CMOS IS (Backside Illuminated CMOS image sensors) (3). In fusion bonding, the surface condition plays a crucial role for a successful process. From the mechanical perspective, surface preparation consists of producing the desired surface layer (e.g. oxide, nitride) and make sure the flatness and smoothness required for direct bonding are met (e.g. by using CMP - Chemical-Mechanical Polishing). After the post-cmp cleaning procedure, the wafers typically see an additional cleaning process right before being placed in contact (in production environment this step is performed inside the integrated wafer bonding equipment). With this process flow, the risk of particle contamination of the surfaces is significantly decreased. The use of CMOS wafers raised challenges to this single wafer cleaning step. Legacy cleaning methods (brush scrubbers and megasonic nozzles) started to reach their limits in

2 terms of surface damage, cross contamination and cleaning uniformity. In order to address these limitations, a new type of megasonic-enhanced cleaning was introduced: the MegPie (4). The transducer is placed in close proximity to the wafer surface and the gap between wafer surface and transducer surface are filled with process fluid. Due to its triangular shape, uniform radial distribution of the acoustic energy across wafer is assured. Particle neutrality and particle removal efficiency results of this cleaning process were reported elsewhere (5). This work reports on recent design developments in process fluid management that result in improved process control and reduction of process fluid required for pre-bonding cleaning based on MegPie. Theory Enhancement of cleaning processes through the application of megasonic energy has been an accepted technology in IC and MEMS manufacturing for years. The useful effects of both the directional streaming forces and Megasonic induced cavitation in a wet process have been well documented, but limitations have arisen in various methods of application. Initial work on megasonic-enhanced wet processing was accomplished in immersion type configurations in which the substrate to be processed was submerged in a tank filled with the process fluid. Acoustic energy was introduced to the fluid either indirectly through a coupling fluid layer, or directly with a resonator in direct contact with the process fluid. Such configurations exhibited significant overall improvements in process time and efficiency over standard recirculated baths, but suffered from severe limitations in uniformity across the substrate (die to die). Efforts were then made to improve the acoustic transmission uniformity through various resonator designs and placements (position/angle) of substrate in relation to the resonator. These measures were limited by irregularities in field caused by acoustic wave reflections endemic to a tank type system with wafer present (6). This local area non-uniformity in the immersion configuration has several basic causes. First is the aforementioned acoustic field irregularity which effects both particle removal and chemical enhancement uniformity. Second is the difference in the replenishment rates of fresh process fluid. The process fluid exchange in the sub-boundary area caused by megasonic induced cavitation simply exchanges diffused fluid from the surface region with fluid from outside the boundary layer. When the fluid outside of the boundary layer has not been exchanged for fresh process fluid on the macro level due to flow restrictions, eddy currents, or depletion of the process fluid in the whole system, the local effectiveness of the megasonic enhancement is reduced considerably. In order to minimize the process irregularities and variances of the immersion method, the configuration was changed to that of a single substrate spin process. The substrate to be processed is held to a spinning chuck with the surface to be processed facing up and process fluid applied to the top surface only (figure 1). In this spin process system, megasonic energy is introduced to the process fluid via a wide area megasonic transducer, the MegPie. This transducer couples acoustic energy into the process fluid filled gap formed by the substrate and the transducer face. The centrifugal forces created by the spinning substrate expel the spent process fluid off of the wafer and provide for a constant refreshment of the process fluid from outside the boundary layer.

3 Figure 1. Horizontal spinner chuck system with basic fluid delivery and MegPie transducer. The uniform acoustic field of the MegPie resonator is shaped to provide radial uniformity (4). In a rotating substrate system, the outer portion of the substrate is moving faster than the inner portion when referenced to a fixed point. The form of the MegPie assures that every portion of the substrate receives the same amount of megasonic dosage with each substrate rotation. This dosage uniformity is assured without the requirement of mechanical scanning, the transducer remains at a fixed position and height above the substrate throughout the entire process. As with the aforementioned immersion configuration, control of process fluid characteristics in the Megasonic field is critical to the uniformity and density of cavitation created. Cavitation density in a given megasonic field is directly related to the gas content, temperature, and chemical composition of the process fluid and is measured through the detection of photons that are a byproduct of the imploding cavitation reaction (7) (fig. 2). Figure 2. Cavitation profile of typical clean process solution represented by photon count (sonoluminescence). The combination of an adjustable, uniform megasonic energy field and a fresh controlled supply of process fluid to that field provide the optimal conditions for controlled and reproducible cavitation densities and uniformity of process. Experimental Apparatus. The experimental setup consisted of an EVG 850LT industrial bonding equipment containing a single-wafer cleaning module equipped with sapphire surface MegPie similar to prior reported work (5).

4 The system was modified though the addition of a radial dispense manifold (fig. 3). In this configuration all of the process fluid is introduced through the manifold and directly into the acoustic field. The legacy method of introducing fluid to the wafer surface from an external source has been eliminated so that the manifold is the sole source. Figure 3. Cross section of MegPie with Radial Dispense Manifold. In addition to uniformity of process fluid delivery to the megasonic field, design goals for the radial dispense manifold included high speed and low fluid flow operation. High spin speed (>100 RPM) operation is desirable in order to eliminate fluid wrapping, where process fluid on the top of the wafer wraps around to the backside of the wafer. Fluid wrap can introduce contamination and watermark issues as well as exposing the outer edge of the backside to cavitation effects caused by sound energy transferring through the wafer into the residual fluid on the back side. Higher spin speeds also provide for higher centrifugal forces and help to accelerate the ejection of particles released in the Megasonic field from the wafer. The legacy fluid introduction method prevented reliable operation over 45 RPM. At higher RPM the fluid would separate in the gap between wafer and transducer. This air filled gap blocks the sound, increases the reflected energy and degrades the process performance. Reducing the amount of fluid required per wafer processed is an ongoing effort in semiconductor wet processes. The impact is not only in the production and purchasing costs of UPW and chemistry, but also the environmental issues and costs of chemical disposal. Process fluid requirement for the legacy fluid introduction method was ca. 1 liter per minute for process of a 200 mm wafer. The design goal for the radial dispense manifold is a 50% reduction in fluid use. Experiment. The MegPie fly height was adjusted to a gap of ca. 0.7 mm with UPW (Ultra Pure Water). Fine height adjustments were made to zero the reflected power (P ref ), indicating a fluid filled gap and optimum RF and acoustic conditions in resonance. The reflected power (P ref ) signal provided by the MegPie electronic controller was monitored during the subsequent experiments. The P ref signal is very sensitive to changes in the conditions within the gap, including introduction of air, or changes in the gap height. This signal is monitored by the control electronics continually during the process, actual values are updated every 5 ms.

5 Results First the MegPie with the mounted manifold was calibrated to determine the fluid flow with respect to gap values between transducer and wafer surfaces. The gap reference values were set mechanically for 0.5 mm and 0.75 mm. Fluid flow was varied along with the rotation speed in order to determine the minimum process fluid flow required to have the gap filled with water for each rotation speed. The flow values for the two different gaps are shown in fig. 4. Figure 4. Rotation speed vs. fluid flow at two reference gap settings Performance of the radial dispense manifold (fig. 5) using P ref as gap reference was very stable and predictable within the initial design parameters. Spin speed range was extended to 200 RPM at all fluid flow rates indicating a very safe process window in the RPM operating range. Fluid flow results were very encouraging, with stable operation up to 150 RPM at 0.2 l/min, representing an 80% reduction in process fluid flow requirement when compared to the legacy fluid introduction method. Figure 5. Reflected power readings at varied spin speeds and fluid flows.

6 Conclusion Megasonic enhanced processing has become a key technology in wafer scale pre-bond cleaning. While process results achieved with the sapphire MegPie have met current requirements, areas for improvement have been identified and addressed. In this work we have explained the impact of uncontrolled or non-uniform process fluid characteristics in the Megasonic field and introduced a radial uniform manifold that manages the flow of fluid from the system into the active gap between Transducer and Wafer. We have also verified the high RMP capabilities now available with the MegPie equipped with the radial dispense manifold. And further demonstrated large reduction (80%) in process fluid required and verified our design and initial test parameters with live testing using reflected power as a process monitor for the active process area. In ongoing work we plan further verification runs of particle neutrality and particle removal efficiency at the enhanced spin speeds and reduced, managed fluid flow. References 1. M.I. Current, in IEEE Int. Workshop on Junction Technol. 2002, IEEE Proceedings Series, doi: /IWJT , p. 97 (2002). 2. C.J. Tracy, P. Fejes, N.D. Theodore, P. Maniar, E. Johnson, A.J. Lamm, A.M. Paler, I.J. Malik and P.J. Ong, J. Electronic Mat.., 33, p. 886 (2004). 3. V. Dragoi, G. Mittendorfer, A. Filbert, and M. Wimplinger, doi: /PROC F08-06, MRS Online Proc. Library, vol. 1249, pp.1249-f08-06 (2010). 4. US Patent 6,791,242 (2004). 5. F. Fournel, L. Bally, D. Dussault, and V. Dragoi, ECS Trans. 33(4), 495 (2010). 6. A. Ting, Sandia Nat. Lab. Public Report, SAND , p. 18 (2002). 7. US Patent 7,443,079 (2008)