The atomic-scale surface roughness of Si(110) reconstructed via high-temperature Ar annealing is immediately increased by non uniform accidental oxidation during the unloading process (called reflow oxidation) during high-temperature Ar annealing. In particular, for a reconstructed Si(110) surface, characteristic line-shaped oxidation occurs at preferential oxidation sites appearing in pentagonal pairs in the directions of Si[-112] and/or [-11-2]. We previously reported that the roughness increase of reconstructed Si(110) due to reflow oxidation can be restrained by replacing Ar gas with H2 gas at 1000°C during the cooling to 100°C after high-temperature Ar annealing. It was speculated that preferential oxidation sites on reconstructed Si(110) were eliminated by H2 gas etching and hydrogen termination of dangling bonds. Thus, it is necessary to investigate the effect of H2 gas etching and hydrogen termination behavior on the reconstructed Si(110) surface structure. In this study, we evaluated in detail the relationship between the temperature at which the H2 gas replaces the Ar in high-temperature Ar annealing and the reconstructed Si(110) surface structure. The maximum height of the roughness on the reconstructed surface was the same as if Ar gas was used when the H2 gas introduction temperature was 200°C, although the amount of reflow oxidation was decreased to 70% by hydrogen termination. Furthermore, line-shaped oxidation still occurs when H2 gas replaces Ar at this low temperature. Therefore, we conclude that oxidation is caused by slight Si etching at low temperatures, and thus the preferential oxidation sites on the reconstructed structure must be eliminated by hydrogen etching in order to form an atomically smooth Si(110) surface.
Semiconductor integrated circuits are the most important hardware technology in today’s technologically advanced society. Improvements in the performance of semiconductor integrated circuits have been driven primarily by Si field-effect transistors (a type of metal-oxide-semiconductor field-effect transistor, MOSFET), which are the smallest constituent units of high-performance circuits. To improve the performance of semiconductor integrated circuits, it is essential to enhance the current-driving ability and reduce the power consumption of a transistor. Until now, these requirements have been met mainly by scaling the transistor structure. However, scaling will face limitations in near future [
Surface scattering at the Si/silicon dioxide (SiO2) interface in MOSFETs can reportedly be decreased by forming an atomically smooth Si surface. Moreover, such a smoothing process can also enhance the carrier mobility of MOSFETs [
It is well known that a smooth surface on Si wafers can be realized by wet hydrogen fluoride (HF) cleaning at room temperature or by high-temperature annealing [
In contrast, high-temperature annealing provides atomically smooth surfaces on both Si(100) and Si(110). Furthermore, high-temperature annealing in a furnace is a useful industrial process and can be used to process around 100 wafers in one batch. Kumagai et al. reported that the surface of a Si substrate annealed at 900˚C or above will contain well-developed terraces with monoatomic steps [
In particular, characteristic line-shaped oxidation occurs at preferential oxidation sites appearing in pentagonal pairs in the directions of Si[-2]">112] and/or [-11-2] on a reconstructed Si(110) surface. This line-shaped oxidation causes a height fluctuation of approximately two atomic layers (0.24 nm) in AFM measurements, which is an obstacle for decreasing the Si(110) surface roughness.
Thus, we have attempted to find other effective methods for restraining the influence of reflow oxidation. One such method is hydrogen termination (H-termination), i.e., terminating dangling bonds with hydrogen. This method provides a chemically inert surface that was found to restrain the growth of native oxides at room temperature [
We previously reported that the roughness increases of reconstructed Si(110) due to reflow oxidation can be restrained by replacing Ar gas with H2 gas at 1000˚C in the process of cooling the wafer to 100˚C after high-temperature Ar annealing [
Boron-doped Czochralski-grown Si(110) polished wafers (200 mm in diameter) with off-angles below 0.1˚ were used. Their electric resistivities were 15 Ω∙cm. The sample wafers were heat-treated at 1200˚C for 1 h in an Ar atmosphere by using a commercial vertical furnace. We chose the two temperature ranges, 1000˚C - 100˚C and 200˚C - 100˚C, in which the Ar in the furnace was replaced with H2 during the cooling process to obtain a better understanding of the changes in the surface structure due to the introduction of H2 gas. Under the former condition, which was used in our previous work [
H-termination on the Si surfaces was confirmed by Fourier transform infrared attenuated total reflection (FT-IR-ATR) spectroscopy carried out using a Bruker IFS-120HR system. The thickness of the oxide film on the surface was determined by X-ray photoelectron spectroscopy (XPS) carried out using a PHI Quantera SXM XPS spectrometer. The surface structure of the samples was observed using an atomic force microscope (Digital Instruments Nanoscope IIIa) operated in tapping mode. The measurement fields had areas of 3 × 3 and 0.2 × 0.2 μm2. The first measurement area was used to observe the reconstructed surface of an annealed wafer, which generally consists of a step/terrace structure, and the second measurement area was used to observe in detail a single terrace at a time.
We investigated the effect of changing the injected gas from Ar to H2 during the cooling process on the silicon oxide (SiO) layer formed on the surfaces of Si(110) wafers during high-temperature Ar annealing. The thickness of the oxide layer after the annealing process was measured by FT-IR-ATR and XPS.
In the spectra for the H2-cooled samples, three significant peaks were observed at 2089, 2013.7, and 2250 cm−1. The strong peaks at 2089 cm−1 and 2103.7 cm−1 were only observed for the samples cooled in H2 atmospheres, and were assumed to originate from Si-H and Si-H2 bonding, respectively [
FT-IR-ATR spectra of Si(110) surfaces (i) before Ar annealing (w/o Ar annealing); (ii) after Ar annealing using Ar gas during cooling process (Ar-cooled); (iii) after Ar annealing using H2 gas during the cooling process from 200˚C (H2-cooled_200˚C); and (iv) after Ar annealing using H2 gas during the cooling process from 1000˚C (H2-cooled_1000˚C)
The peak at 2250 cm−1 is known to be caused by oxidation, and thus if oxidation is enhanced during the annealing process, the height/intensity of the 2250 cm−1 peak should increase. However, we found that the intensity of this peak was lowest for the H2-cooled_1000˚C sample. Therefore, a stable atomic surface obtained after high-temperature annealing can be improved by replacing Ar with H2 at 1000˚C during cooling. On the other hand, the H2-cooled_200˚C sample showed no clear difference from the Ar-cooled sample in terms of the 2250 cm−1 peak intensity.
Finally, we investigated the atomic surface structures of the samples by AFM. The surface morphologies of typical Si(110) wafers measured by AFM in an area of 3 × 3 μm2 are shown in
The H2-cooled_1000˚C sample (
XPS spectra of Si(110) surfaces (i) before Ar annealing (w/o Ar annealing); (ii) after Ar annealing using Ar gas during the cooling process (Ar-cooled); (iii) after Ar annealing using H2 gas during the cooling process from 1000˚C (H2-cooled_1000˚C); and (iv) after Ar annealing using H2 gas during the cooling process from 200˚C (H2-cooled_200˚C)
Thicknesses of oxide layers on Si(110) wafers calculated from XPS spectra
Furthermore, we observed the roughness on the terrace in detail, as shown in the 0.2 × 0.2 μm2 view-field AFM images measured for one terrace on the Si(110) surface in
When H2 was used during the cooling process from 1000˚C [
On the basis of these results, the dependence of the atomic-scale Si(110) surface roughness on the hydrogen introduction temperature in high-temperature Ar annealing can be summarized as follows. Surface Si atoms migrate during high-temperature annealing at 1200˚C, and a reconstructed surface is considered to form during the cooling process below 800˚C [
Surface roughness and AFM images of polished and annealed Si(110) wafers (images were obtained for a 3 × 3 μm2 measurement area) (i) before Ar annealing (w/o Ar annealing); (ii) after Ar annealing using Ar gas during the cooling process (Ar-cooled); (iii) after Ar annealing using H2 gas during the cooling process from 1000˚C (H2-cooled_1000˚C); and (iv) after Ar annealing using H2 gas during the cooling process from 200˚C (H2-cooled_ 200˚C)
Surface roughness and AFM images of polished and annealed Si(110) wafers (images were obtained for a 0.2 × 0.2 μm2 measurement area) (i) before Ar annealing (w/o Ar annealing); (ii) after Ar annealing using Ar gas during the cooling process (Ar-cooled); (iii) after Ar annealing using H2 gas during the cooling process from 200˚C (H2-cooled_200˚C); and (iv) after Ar annealing using H2 gas during the cooling process from 1000˚C (H2-cooled_ 1000˚C)
is maintained when the sample is cooled to 100˚C in an Ar atmosphere. These reconstructed surfaces are atomically smooth but are chemically unstable. They are also unaffected by a reduction in the unloading temperature, but are easily oxidized by reflow oxidation during unloading. In particular, line-shaped oxidation occurs for Si(110) with a 16 × 2 surface structure after reconstruction because of preferential oxidation at the pentagonal pairs in the directions of Si[-112] and/or [-11-2], as described in [
When cooled in an H2 atmosphere, the reconstructed Si(110) surfaces are etched by H2 gas at a rate depending on the temperature. The etching rate of Si at 1000˚C is about 5 × 10−1 nm/min [
Introducing H2 at 1000˚C entirely eliminates the preferential oxidation sites, and results in such an atomically flat surface. This technology has been utilized for evaluations of the performance of actual devices. For example, S. Jeon et al. verified the effect of the controlled surface using a MOSFET [
We investigated the relationship between reflow oxidation and the structure of Si(110) surfaces by changing the H2 introduction temperature during the cooling process after high-temperature Ar annealing. The experimental results indicated that the maximum height of the roughness on reconstructed surface was not decreased at all when only slight Si etching occurred after hydrogen introduction at a lower temperature, even if H-termination occurred. The preferential oxidation sites need to be entirely eliminated in order to form atomically smooth Si(110) surfaces.
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