Kurt Wostyn

Surface Passivation for Si Solar Cells: A Combination of Advanced Surface Cleaning and Thermal Atomic Layer Deposition of Al2O3

Thermal atomic layer deposition (ALD) of Al2O3 provides an adequate level of surface passivation for both p-type and n-type Si solar cells. To obtain the most qualitative and uniform surface passivation advanced cleaning development is required. The studied pre-deposition treatments include an HF (Si-H) or oxidizing (Si-OH) last step and finish with simple hot-air drying or more sophisticated Marangoni drying. To examine the quality and uniformity of surface passivation - after cleaning and Al2O3 deposition - carrier density imaging (CDI) and quasi-steady-state photo-conductance (QSSPC) are applied. A hydrophilic surface clean that leads to improved surface passivation level is found. Si-H starting surfaces lead to equivalent passivation quality but worse passivation uniformity. The hydrophilic surface clean is preferred because it is thermodynamically stable, enables higher and more uniform ALD growth and consequently exhibits better surface passivation uniformity.

Effects of Interfacial Strength and Dimension of Structures on Physical Cleaning Window

Four different types of FINs; amorphous Si (a-Si), annealed a-Si, polycrystalline Si (poly-Si) and crystalline Si (c-Si) were used to investigate the effect of interfacial strength and the length of structures on the physical cleaning window by measuring their collapse forces by atomic force microscope (AFM). A transmission electron microscope (TEM) and a nanoneedle with a nanomanipulator in a scanning electron microscope (SEM) were employed in order to explain the different collapse behavior and their forces. Different fracture shapes and collapse forces of FINs could explain the influence of the interfacial strength on the pattern strength. Furthermore, the different lengths of a-Si FINs were prepared and their collapse forces were measured and the shorter length reduced their pattern strength. Strong adhesion at the interface resulted in a wider process window while smaller dimensions made the process window narrower.

Collapse behavior and forces of multistack nanolines

Two types of multistack nanolines (MNLs), Si-substrate (Si)/siliconoxynitride (SiON)/amorphous Si (a-Si)/ SiO(2) and Si/ SiO(2) /polycrystalline Si (poly-Si)/ SiO(2) were used to measure the collapse force and to investigate their collapse behavior by an atomic force microscope (AFM). The Si/SiON/a-Si/ SiO(2) MNL showed a larger length of fragment in the collapse patterns at a smaller collapse force. The Si/ SiO(2) /poly-Si/ SiO(2) MNL, however, demonstrated a smaller length of fragment at a higher applied collapse force. The collapse forces increased by the square of the linewidth in both Si/SiON/a-Si/ SiO(2) and Si/SiO(2) /poly-Si/ SiO(2) MNLs. Once an AFM tip touches an Si/SiON/a-Si/ SiO(2) line, which is a softer MNL, it was delaminated first at the Si/SiON interface. One end of the delaminated line was first broken and then the other end was bent until it was broken. A harder MNL, Si/ SiO(2) /poly-Si/ SiO(2), however, was broken at two ends simultaneously after the delamination occurred at the Si/ SiO(2) /poly-Si interface. The different collapse behaviors were attributed to the magnitude of adhesion forces at the stack material interfaces and the mechanical strength of MNLs.

Removal of Nano-Particles by Aerosol Spray: Effect of Droplet Size and Velocity on Cleaning Performance

As the dimensions of the structures of integrated circuits shrink, the influence of particles on device yield becomes increasingly important. According to the cleaning requirements of the International Technology Roadmap for Semiconductors (ITRS) in 2007, particles of 32 nm and larger are believed to be detrimental to devices and thus have to be removed. To remove nano-particles with minimal substrate loss and no damage requires very dilute chemistries and sufficiently gentle physical forces in a cleaning process. In this work the performance of an aerosol spray based cleaning technique is evaluated with regard to the removal efficiency of nano-particles as well as substrate loss and structural damage.

High Speed Imaging of 1 MHz Driven Microbubbles in Contact with a Rigid Wall

Since the introduction of megasonic cleaning in semiconductor industry a debate has been going on about which physical mechanism is responsible for the removal of particles. Because of the high frequency range it was believed that acoustic cavitation could not occur and cleaning was attributed to phenomena like Eckart and Schlichting streaming or pressure build-up on particles [1,2]. Recently it was shown however, that the removal of nanoparticles is closely related to the presence of acoustic cavitation in megasonic cleaning systems [3]. The dependence of particle removal efficiency on the concentration of dissolved gas and the presence of sonoluminescence are clear (but indirect) indications that the underlying mechanism is related to bubble dynamics. As the requirements for cleaning in semiconductor processing are ever more stringent, it becomes necessary to obtain a thorough understanding of the physical behavior of acoustically driven microbubbles in contact with a solid wall. In particular, the forces exerted thereby which might clean or damage a substrate are of interest. Here, a step in this direction is taken by visualization of both the removal of nanoparticles and the sub-microsecond timescale dynamics of the cavitation bubbles responsible thereof.

Wet Selective SiGe Etch to Enable Ge Nanowire Formation

For the Ge nanowire formation in a gate-all-around (GAA) integration scheme, a selective etch of Si0.5Ge0.5 or Si0.3Ge0.7 selective to Ge is considered. Two wet process approaches were evaluated: a boiling TMAH as a commodity chemistry is compared with a formulated chemistry using a multi-stack SiGe/Ge layer as a test vehicle. The boiling TMAH exhibits an anisotropic etch of the SiGe whereas the formulated semi-aqueous chemistry removes the sacrificial SiGe by an isotropic etch which makes the process suitable for a Ge nanowire release process.

Analyzing the Collapse Force of Narrow Lines Measured by Lateral Force AFM Using an Analytical Mechanical Model

When a physical cleaning technology, such as megasonic and high-velocity-liquid aerosol cleaning, is considered for the removal of particles or photo resist residues, damage addition is a major concern. After detection of defects in long gate stack lines by bright field inspection (KT2800), SEM imaging shows they extend over a length in the order of 1μm (Figure 1) [1].

Quantitative Measurement of Pattern Collapse Force and Particle Removal Force

The removal of particles from silicon wafers without pattern damage during fabrication process is extremely important for increasing the yield. Various cleaning techniques such as megasonic cleaning, jet spray cleaning, laser shock wave cleaning and so on are introduced to remove particles. However, most of these tools show pattern damage. One of main issues in the development of next generation cleaning process is how to clean wafers without pattern damages. As feature size continues to decrease, pattern can be easily damaged by cleaning itself which means no particle removal on the patterned wafers.. In order to secure effective cleaning process without pattern damages, the pattern collapse and particle removal force should be quantitatively understood and measured. Some groups tried to measure pattern collapse force of photoresist for improving adhesion [1, 2]. In this paper, the methods to measure the pattern collapse and particle adhesion forces were introduced and compared of interest to wafer cleaning. The pattern collapse and particle removal forces were measured by AFM (XE-100, PSIA, Korea) with XEL as nanolithography software to manipulate a cantilever and the signal access module to collect cantilever bending and torsion signals. These signals from AFM were acquired and processed by data acquisition board (PCI-6251, National Instrument, USA) and LabVIEW v8.2 software. Before measuring forces, the thickness and tip height of cantilever were measured by observing the first resonance frequency in normal direction and FESEM, respectively for theoretical calculation of a normal spring constant, kN and a torsional spring constant, kt. The angle conversion factor and the force conversion factor were calculated by a modified wedge calibration method with a calibration grating for quantifying lateral force (TGF11, Mikromash, USA). Measurements were performed in a class 100 cleanroom (R.H. 50%, Temp. 25oC). When a probe cantilever meets a structure, bending and torsion of a cantilever occur as shown in Fig. 1. If a known force is applied to the structure by a cantilever, the structure will move at a certain force as a removal force. At this moment, the torsion and bending moments will occur. If we know these moments, removal force can be obtained. In order to quantify these forces, the cantilever and AFM system should be calibrated. Especially, the contact height between the tip and pattern or particle should be known becasue the height directly relates to the torsion moment. The shape of a tip as shown in Fig. 2 is not symmetric. It is hard to define the contact point between tip and structures. However, this problem can be reduced by controlling z-feedback of AFM which controls the cantilever position during the scanning. Nanolithography software was used to control the cantilever in this purpose. As discussed previously the contact point and direction are key parameter to measure the pattern collapse force. For accurate measurements, these are carefully controlled and calibrated. When the cantilever was controlled precisely, the tip was pushed to the structure and collapsed it at a certain force as shown in Fig. 3. These signals were acquired as shown in Fig. 4. Angle conversion and force conversion factors were needed to quantify the torsion force as a removal force. To do this, structure which has known angle of slope, was used. Finally, only contact height as almost same as tip height and torsional spring constant removal force could be quantified. The force of pattern collapse and particle removal was successfully measured by AFM. These forces can provide a hand to understand removal behavior particles without pattern damages.

High Velocity Aerosol Cleaning with Organic Solvents: Particle Removal and Substrate Damage

High velocity aerosol cleaning using ultrapure water or dilute aqueous solutions (e.g. dilute ammonia) is common in semiconductor IC fabrication [1]. This process combines droplet impact forces with continuous liquid flow for improved cleaning efficiency of sub-100nm particles. As with any physically enhanced cleaning process, improved particle removal can be accompanied by increased substrate damage, especially to smaller (<80nm) features [2]. Solvents such as N-methylpyrrolidone (NMP) and tetrahydrofurfuryl alcohol (THFA) are used for resist strip applications [3]. It is possible, and sometimes useful, to deliver these solvents through the same spray nozzle normally used for aqueous spray cleaning. In this presentation we explore the particle removal and substrate damage performance of 2-ethoxyethanol (EGEE), NMP and THFA as used in a conventional aerosol spray cleaning system

'Just Clean Enough': Wet Cleaning for Solar Cell Manufacturing Applications

The cumulative installed solar power generation has been rising exponentially over the past decade. This has lead to a concomitant rise in production capabilities, leading eventually to excess production capabilities and rapid price declines per unit. In order to compete with the standard electricity generation the cost of solar panel production and installation needs to decrease even further. At the same time the solar panel and cell makers need to be able to keep a healthy margin. A crucial element in this exercise is a close control on the Cost of Ownership (CoO) of a solar cell / panel fabrication site.