<empty>
top row of product images<empty>ProSys logo photo of ProSys Quartz Megasonic System photo of ProSys Tabletop Megasonic System photo of ProSys Solar Bar photo of ProSys IMPulse RF photo of ProSys Sapphire MegPie <empty>
<empty>
  titleTechology
image of bubbles
  <empty>
Published Conference Papers

thin gray rule
<empty>
     
 

Acoustic Energy: a New Tool for MEMS Manufacturing
<empty>
D. Dussault (1) and V. Dragoi (2)
<empty>
(1) Product Systems Inc., 1745 Dell Ave., Campbell, California 95008, USA
(2) EV Group, DI E. Thallner Str. 1, 4782 - St. Florian/Inn, Austria
<empty>
<empty>
Abstract
<empty>
The use of non-standard materials (e.g. specific substrates shapes and dimensions or polymer materials) for MEMS applications imposed a requirement for the development of new techniques for even well-established processes. Acoustic energy in the MHz frequency range has been used in the semiconductor industry for various processes such as photoresist development, substrate cleaning and electroplating enhancement. The work presented here is focusing on single wafer cleaning and on photoresist development.
<empty>
The cleaning process developed addresses mainly wafer cleaning prior to wafer bonding processes, in which particle contamination is of crucial importance. The photoresist development process was developed mainly for thick resist layers development (few hundreds of µm) in order to improve definition of high aspect ratio features but was used as well as a significant process time reduction factor for development of regular thickness resists (few µm).
<empty>
Introduction
<empty>
MEMS-specific processes had to adapt standard IC manufacturing technologies in order to accommodate their custom requirements.
<empty>
Wafer cleaning is an important chapter in any semiconductor process flow as contamination typically has a negative impact on process results. The challenges of processes adapted for MEMS manufacturing demand the implementation of advanced cleaning techniques. One process requiring very strict contamination control is wafer bonding. Even extremely low levels of particle contamination prior to wafer bonding result in significant yield losses. For this reason, besides the standard batch cleaning process used to remove various types of contamination, wafer bonding also requires a single wafer cleaning step immediately prior bringing the wafers in contact in order to assure that any airborne particles that may have been added during transfer and handling are removed.
<empty>
Photolithography is a standard patterning technique which has had to adopt the use of thick photoresists (between few tens and up to few hundreds of µm). These thicknesses are required for needed for high aspect ratio features fabrication for and to adapt to the specific high volume manufacturing yield requirements demanded by developing new processing techniques.
<empty>
Thick resists processing require a very long cycle time due to some particular aspects as polymer coating (usually requiring edge bead removal), polymer slow baking for solvent removal after coating and development of high aspect ratio structures.
<empty>
A single wafer spin process station equipped with triangular-shaped megasonic transducer, the MegPie®, was used for both cleaning and photoresist processes. This transducer covers a sector of the wafer and is placed in proximity to substrate surface. The small gap between transducer and substrate is filled with the required process fluid. The shape and positioning of the transducer assure optimal acoustic energy transfer to the process fluid resulting in high dosage uniformity. The benefits of using megasonic energy to enhance the above processes will be presented with examples.
<empty>
Theory
<empty>
Enhancement of chemical and 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.
<empty>
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 [1].
<empty>
This die to die 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.
<empty>
In order to minimize or eliminate 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).
<empty>
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.
<empty>
The uniform acoustic field of the MegPie® resonator is shaped to provide radial uniformity [2]. 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. The combination of an adjustable uniform megasonic energy field and a fresh supply of process fluid provide optimal conditions for controlled and reproducible cavitation densities and uniformity.
<empty>
Figure 1
<empty>
Figure 1: Horizontal spinner chuck system with fluid delivery and MegPie® transducer.
<empty>
Experiment
<empty>
Single wafer cleaning. A radially uniform megasonic area transducer (MegPie®) was used in these experiments (Figure 1). This transducer couples acoustic energy into a fluid filled gap formed by the substrate and the transducer face. In Figure 2a, the MegPie® is in place but the acoustic (RF) power is turned OFF. In Figure 2b, the acoustic power is ON with the transducer coupled to a frequency around 1 MHz and with a power density of 1 W/cm². We can then see the acoustic waves generated inside the fluid.
<empty>
A MegPie® area megasonic transducer was integrated in the pre-bond cleaning station of an automated bonding system (EVG®850LT) in LETI clean room. The work was performed on 200 mm diameter Si wafers using a V3 dual zone MegPie® (for 200/300 mm wafer diameter) with a single crystal sapphire resonator and PEEK housing. For these experiments two types of process fluids have been used: high purity de-ionized water (DIW) and a diluted ammonia solution (2%). Prior to the MegPie® cleaning, all the silicon surface cleaning steps were achieved in a FSI Magellan® using 200mm wafers. On bare silicon wafers, the particle measurements were performed using a KLA Tencor SP2 system with a threshold of 90 nm.
<empty>
Figure 2aFigure 2b
<empty>
Figure 2: Large area megasonic transducer. a) The MegPie is in place but turned OFF. b) When turned ON, the waves generated inside the fluid can be observed on the water meniscus.
<empty>
Photoresist development. A high radial uniformity area megasonic transducer similar to the one described for single wafer cleaning was used in these experiments (Figure 1). The basic difference between this transducer and the one used for cleaning was the housing of the transducer, which in this case was made out of stainless steel. The patterning experiments were performed on SEMI standard 150 mm diameter silicon wafers. Before SU-8 coating all wafers were cleaned with acetone and isopropyl alcohol followed by a dehydration bake. The wafer coating was performed on EVG®101 semi-automated coating system. To achieve 470 µm thick layer, SU-8 100 material with a viscosity 51500 cSt at 25 °C was spin coated at 600 rpm for 60s. Following the coating, the wafers were soft baked on a flat-leveled hot plate in proximity mode at 105°C for 10 hours, with slow ramp up and cool down.
<empty>
Results
<empty>
Single wafer cleaning. The particle neutrality of this cleaning system was first investigated. Cleaning process employed used following process conditions: 3 W/cm² power density with DIW for 2 min followed by 1 W/cm² with NH4OH solution (2%) also for 2 min [3]. These process times were chosen very long in order to allow observation of any potential particle contamination on the surfaces after the MegPie® cleaning sequence. For most of the wafers, the MegPie® was particle neutral (it even removes a few particles, down to 5) or produces very few adders (+5 particles). Considering two wafers used as reference (wafers were not cleaned with MegPie®), it can be concluded that these adders do not come from the MegPie® itself but they may be airborne particles deposited on the surfaces during wafer handling for the reference particle measurement prior cleaning process.
<empty>
The MegPie® Particle Removal Efficiency (PRE) was also evaluated. Clean wafers were contaminated with around 3000 - 5000 nitride nano-particles per wafer ( ). After the megasonic cleaning process, subsequent particle measurement showed uniform removal and Particle-Removal Efficiency (PRE) values of > 95% and often > 100%.
<empty>

# Start After Nitride Particles MegPie PRE %PRE Delta Start
1 274 5709 237 -5472 100,68% -37
2 95 3068 127 -2941 98,92% 32
3 118 4468 578 -3890 89,43% 460
4 57 2918 316 -2602 90,95% 259
5 61 2839 68 -2771 99,75% 7
6 68 2991 197 -2794 95,59% 129
7 61 2952 51 -2901 100,35% -10
8 84 3069 61 -3008 100,77% -23
9 128 4642 122 -4520 100,13% -6
<empty> <empty> <empty> <empty> <empty> <empty>
<empty>
Table 1: Particle removal efficiency experimental results.
<empty>
The last column in table 1 shows the particles difference between the starting wafers (before intentional contamination) and the wafers after MegPie® cleaning. Figure 3 shows the post-cleaning particle size distribution on one of the wafers. It can be observed that most of the large particles are removed and especially the large size-ones (marked by "Sat") which typically lead to bonding defects.
<empty>
Figure 3
<empty>
Figure 3: Particle size distribution before and after the MegPie® cleaning of one wafer contaminated with nitride particles.
<empty>
Photoresist development. For UV exposure a flexible foil mask was used to compensate the edge bead and SU-8 topography. The use of a flexible mask brings several advantages over traditional glass mask: ability to compensate the wafer topography, easy release from the wafer surface after vacuum contact and significantly lower price. The vacuum contact exposure with flexible foil mask (from J.D. Photo Tools) was performed on EVG®6200 Infinity mask aligner [4]. The presence of i-line (365 nm) peak in exposure spectrum results in lines widening on the top part of the structures.
<empty>
The post exposure bake has been performed on the flat-leveled hotplate at 95°C for 20 min with slow ramping up and cooling down. Afterwards, the first set of wafers has been immersed into propylene glycol methyl ether acetate (PGMEA) bath without any agitation; the second set was puddle developed enhanced by the single wafer megasonic development. After development, wafers were rinsed with isopropanol, dried on a hotplate and inspected by SEM.
<empty>
Figure 4a<empty>Figure 4b

a<empty>b
Figure 4c<empty>Figure 4d
c<empty>d
<empty>
Figure 4: Free standing SU-8 structures: 470 µm high, 20 µm sidewall thickness, 1:23 aspect ratio. Structures a. and c. were developed in PGMEA bath for 240 min, structures b. and d. were processed with megasonic-enhanced development in 10 min.
<empty>
The megasonic-enhanced development has been performed with single wafer megasonic transducer, the MegPie® integrated on EVG®101 semi automated developer. For optimum operation, the distance between transducer and wafer surface has to be adjusted to minimize reflected power and so to maximize active forward power. The development time was increased for each following wafer in 2 min steps, and structures were inspected for residuals. For the wafers developed in bath tank, development time was increased in 30 min steps for each following wafer.
<empty>
After rinsing and drying, wafers were inspected for pattern definition, residuals and delamination. Wafers developed in the bath showed still after 210 min residuals close to the structures base. Wafers developed with megasonic agitation were residuals-free after 10 min of development. No delamination or structural deformation was observed on features with 20 µm sidewall thickness. Figure 4 shows the results of both development techniques.
<empty>
Conclusions
<empty>
Enhancement of wet processing with megasonic energy brings significant improvements over a conventional immersion bath in both yield and throughput. Introducing the megasonic energy in a single wafer configuration with a radially uniform transducer eliminates the local non-uniformity issues endemic to the immersion implementation.
<empty>
High particle removal efficiency (PRE) and particle neutrality was demonstrated for an extremely critical pre-bond silicon clean step using a single crystal sapphire resonator MegPie®.
<empty>
Process time was significantly reduced by megasonic-enhanced development. Open space SU-8 structures with vertical sidewalls and 1:23 aspect ratio were fabricated by two development techniques: in conventional bath for 240 min of development, with megasonic-enhanced single wafer process in 10 min for equivalent results. Structures were obtained in 470 µm thick SU-8 resist by using the foil mask exposure with filtered UV light.
<empty>
Acknowledgments
<empty>
The authors would like to thank Dr. Frank Fournel from LETI for performing the cleaning experiments and Mrs. Johanna Bartel for her contribution to the SU-8 development process.
<empty>
References
<empty>
[1] A. Ting, "Pressure waves induced by megasonic agitation in a LIGA development tank", Sandia Nat. Lab. Public Report, SAND2002-8333, p. 18, 2002.
<empty>
[2] US patent 6,791,242 (2004).
<empty>
[3] F. Fournel, L. Bally, D. Dussault, and V. Dragoi, "Innovative megasonic cleaning technology evaluated through direct wafer bonding", ECS Trans.¸ 33 (4), pp. 495, 2010.
<empty>
[4] V. Dragoi, J. Bartel and D. Dussault, "Novel photolithography yield-enhancement technique: megasonic-enhanced development", ECS Trans.¸ 33 (8), pp. 175, 2010.
<empty>
Download PDF of this conference paper (11.8 MB)
<empty>
back to top

 
Printable PDF
<empty>
Download PDF
of this conference paper (11.8 MB)
<empty>
Related Topics
<empty>
Published Papers

<empty>
Technical Briefs
<empty>

What is megasonic cleaning?
<empty>
What is the boundary layer as it relate to megasonic cleaning?
<empty>
Patents Granted
<empty>
<empty>
Company Information
<empty>
Quick Response
<empty>
Contact us for additional information or a quote
<empty>
Press Releases

<empty>
About ProSys
<empty>
Corporate Headquarters
<empty>
ProSys Representatives
<empty>
Map & Directions
<empty>
back to top
<empty>
back to top
<empty>
back to top
<empty>
back to top
<empty>
back to top
<empty>
back to top
<empty>
back to top
<empty>
back to top
<empty>
back to top
<empty>
back to top







 
  <empty>    
 
  thin gray rule
© 2018 ProSys Inc.
     
<empty> <empty> <empty> <empty> <empty>
ProSys logo