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Semiconductors

Understanding and characterizing nanostructures plays an essential role in the unprecedented technology developments in areas such as information processing, full color displays, and new sensor technologies, to name but a few. In this section, we show how different measurement solutions from Anton Paar are important for the technical progress of our time. The solutions include: characterization of particle size and the study of surface zeta potential to improve the chemical-mechanical polishing process and analysis of nanopatterned surfaces with grazing-incidence small-angle X-ray scattering (GISAXS).

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Characterization of nanoparticles to improve the CMP process

Introduction

Chemical mechanical polishing (or planarization) is the most popular technique for removing the surface irregularities of silicon wafers. In the CMP process, the use of slurries with abrasive particles (e.g. silica) allows achievement of highly smooth and planar material surfaces by combining chemical and mechanical actions. The role of the nanoparticles in the CMP process is to remove impurities without adhering to the silicon wafer. Moreover, the particle size has to be monitored because it affects the rate of removal and wafer defects. The particle size, as well as the electrostatic repulsion between slurry particles and the wafer surface, thus drive the success of a CMP process.

Experimental

Sample: A commercial slurry of silica particles was used. The pH dependence of the particle size and zeta potential was automatically recorded by using the automatic titration system of the Litesizer 500. For the surface zeta potential measurements, a 6” silicon wafer with a 1000 Å thick silicon oxide (SiO2) layer was cut into pieces of 20 mm x 10 mm and mounted on sample holders of the adjustable gap cell. Streaming current and an aqueous solution of KCl at various ionic strengths (0.001 mol/l and 0.023 mol/l) were selected for the measurements

Characterization techniques: The particle size and zeta potential of the CMP slurry were measured with the Litesizer 500. The streaming current was used to measure zeta potential via SurPASS 3.

Results and discussion

For the estimation of the electrostatic interaction between the SiO2 wafer surface and the silica particles of the CMP slurry, the zeta potential of particles and wafer was compared at the same salinity (0.023 mol/l KCl). The SiO2 wafer exhibited a zeta potential of z » –50 mV, while the CMP slurry at pH 10.8 showed an average zeta potential of z = –45 mV and particle size of 155.4 nm. The example of a SiO2 wafer and a commercial CMP slurry presented in this report suggests the suitable range of pH 6.7-10.8 for an effective CMP process, because the electrostatic repulsion is maximized and this reduces the probability of particle adhesion to the wafer surface.

Figure 1: Comparison of the zeta potential (a) at various pH of an aqueous solution for slurry particles and a silicon oxide wafer. Particle size distribution at pH 10.8.

GISAXS characterization of nanostructured semiconductors

Introduction

Nanopatterned surfaces have wide-ranging applications in different fields. Examples include templates for the deposition of thin films, coating of magnetic films on patterned surfaces, and many others. The key feature of these applications is that the formed nanostructures significantly control macroscopic properties of a material. Ion bombardment can be used for surface cleaning/smoothing of semiconductors, as well as for implementation of dopant atoms. In particular, bombardment with low-energy ions can lead to formation of self-organized periodic patterns, in the size range from a few nanometers, up to µm. These structures can be perfectly analyzed by grazing-incidence small-angle X-ray scattering (GISAXS), which probes a large sample area of surface and surface-near structures and therefore provides averaged, representative results across the sample.

Experimental

GISAXS measurements of semiconductor wafers (germanium and indium arsenide) were performed with the Anton Paar SAXSpoint 5.0 system, which is equipped with a high-precision GISAXS stage. Prior to the measurement, the samples were nanostructured by applying low-energy noble gas ion-induced surface patterning. In addition, the semiconductors were heated to a temperature above the recrystallization temperature which preserves the crystalline structure.

Results and discussion

The scattering reflections originating from the nanostructured surfaces can be recognized from the 2-dimensional GISAXS patterns as displayed in Figure 1.  These patterns were integrated and converted into 1D scattering curves in order to reveal information on the structures, such as periodicity. The two systems studied have a different surface structure. This was further confirmed by atomic force microscopy (AFM) results: the germanium wafer exhibited one main reflection, which corresponded to a periodic structure with around 75 nm d-spacing; in contrast, the indium arsenide sample showed a series of reflections corresponding to a lamellar spacing with a repeating distance of around 110 nm. In summary, GISAXS provided valuable results describing the nanostructured surface created by the applied ion-beam patterning of the semiconductor wafers.

Figure 1. 2D GISAXS patterns, 1D in-plane integrated scattering profiles and AFM images of the germanium (left) and indium arsenide (right) semiconductor wafers.

Additional information

Instruments:

Application reports: