Polished Si Surface Research Articles

Effect and Mechanism of Dual-Official Group of Ethanolamines on the Chemical Mechanical Polishing of Monocrystalline Silicon

Chemical mechanical polishing (CMP) is a key step in semiconductor technology because it is crucial to produce a defect-free and flat enough surface for further processing of microelectronic devices. Silicon (Si) wafer is widely used in integrated circuit (IC) devices, high-density information storage devices, and other advanced applications. In this paper, the effect of different pH and three ethanolamines, such as monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA), on the removal rate of Si was studied. The influence mechanism of MEA and its concentration on Si removal rate and surface quality were mainly investigated. The results show that the removal rate increased first and then decreased with increasing pH value. Among the three ethanolamines, the effect of the removal rate of Si is MEA > DEA > TEA. It may be related to the denser passivation film formed on the Si surface by increased hydroxyl groups in ethanolamines. The removal rate first increased and then decreased slowly with the concentration of MEA increased. The removal rate reached the peak value (6800 Å·min−1) when the MEA concentration was 0.15 wt%. The changing trend of the removal rate of Si is mainly due to the Si-N bond being generated on the polished Si surface by MEA and the ionization properties of MEA, which are indicated through the X-ray photoelectron spectroscopy (XPS) and the Zeta potential measurements. Meanwhile, Si surfaces with low surface roughness and ultra-smooth with the increase of MEA concentration were obtained.

The Influence of Silicon Nanopillars Structure as the Substrate on the SnO2‐Based Gas Sensor

AbstractTin dioxide (SnO2) film is fabricated on the Silicon (Si) nanopillars surface with more than 5 aspect ratio via direct current (DC) magnetron sputtering for the first time. Form the XRD curves, SnO2 film become multi‐crystalline after annealing at 750 °C with 2 hours, and the nanopillars substrates have no influence on the crystallization of the SnO2 film. For the morphology, the SnO2 film on the polished Si surface becomes from planar to plicated after annealing, while there is no morphology change of the SnO2 film on Si nanopillars surface, which proves that the Si nanopillars surface is more suitable for heat and stress release during the annealing process. After testing the gas sensor properties of the SnO2 based gas sensors, the results reveal that the nanopillars based gas sensors have the higher gas response than the planar ones no matter for ethanol or acetone, with or without Au decoration. The Si nanopillars substrate with high aspect ratio makes the SnO2 film to have a larger surface to contact with air and target gas, and to have more defects to react with O2 and reducing gas molecules, which may be a promise method to fabricate the gas sensor with high response.

Surface Modification for High Photocurrent and pH Sensitivity in a Silicon-Based Light-Addressable Potentiometric Sensor

A device structure made from the Si substrate etched from the smooth surface by KOH wet etching is investigated for use as a light-addressable potentiometric sensor (LAPS). Stacked Si3N4/SiO2 layers were used as the sensing membrane on the top side, and a hard mask layer generated by KOH etching comprises the bottom side of the LAPS. A single-side polished P-type Si wafer with a thickness of $350~\mu \text{m}$ was reduced to $175~\mu \text{m}$ by KOH wet etching of the polished side. A high photocurrent was observed in an LAPS sample with increased surface roughness while under backside illumination. Additionally, the increased roughness of the sensing membrane on the top side generated by the unpolished Si surface in the same sample produced higher pH sensitivity than conventional polished Si surfaces. The pH sensitivity and linearity were 45.4 mV/pH and 99.7%, respectively, for pH values ranging from 2 to 12. A 4.4-fold higher photocurrent was achieved using the thin-Si LAPS with a thickness of $175~\mu \text{m}$ measured at a light-modulated frequency of 1 kHz compared with that using a Si substrate with a thickness of $550~\mu \text{m}$ . An acceptable photocurrent was obtained with a modulated illumination frequency up to 20 kHz, which is beneficial for 2-D images acquired by high-speed scanning. This simple device structure consisting of a thin-Si LAPS increases the photocurrent and pH sensitivity. The thickness of the LAPS Si substrate could be reduced in the future to optimize the process stability and sensing performance.

Spatial resolution and 2D chemical image of light-addressable potentiometric sensor improved by inductively coupled-plasma reactive-ion etching

Abstract Inductively coupled plasma reactive-ion etching (ICP-RIE) is first applied to decrease the thickness of a Si substrate for light-addressable potentiometric sensor (LAPS) characterization. A simple and reliable process flow with the self-aligned Al layer as a hard mask layer in ICP-RIE is used to fabricate a thin semiconductor substrate with Al back-side contact. A single-side polished P-type Si wafer with a thickness of 350 μm is decreased to 100 μm by ICP-RIE from the polished side. The thickness of the thin Si substrate and the dimensions of the designed patterns determined by ICP-RIE are verified using scanning electron microscopy (SEM) and an optical microscope (OM). A selective high-dielectric constant insulator, niobium oxide (NbOx), is directly deposited on the unpolished side of the thin Si substrate as the sensing membrane of a LAPS by reactive ratio frequency (rf) sputtering. A high photocurrent was found in the ICP-RIE LAPS sample under back-side illumination, particularly in high-frequency operation. Compared to an Si substrate without ICP-RIE, 3-fold and 6.5-fold higher photocurrents were achieved by the proposed LAPS with ICP-RIE measured at 1 and 50 kHz of light modulated frequency, respectively. The highest modulated frequency of illumination was 50 kHz for an acceptable photocurrent in the LAPS with ICP-RIE, which is sufficient for two-dimensional (2D) images acquired with high-speed scanning. The NbOx layer deposited on the unpolished Si surface could automatically generate higher pH sensitivity than conventional polished Si surfaces because of the higher surface roughness. The calculated pH sensitivity and linearity were 55.8 mV/pH and 99.6%, respectively, in a pH range from 2 to 12. No clear degradation of hysteresis was found for the LAPS with ICP-RIE. For the designed patterns of the Al hard mask layer, the spatial resolution of the 2D image of the LAPS with ICP-RIE could be obtained by the difference between dimensions observed by OM and the dimensions calculated from the photocurrent versus scanning length controlled by the X-Y stage. A pattern with a length of 250 μm generated by ICP-RIE was measured as 255 μm in the LAPS with scanning step of 5 μm. ICP-RIE is demonstrated to achieve a spatial resolution of 5 μm in the Si-based LAPS. Further reduction of the thickness of an Si substrate by ICP-RIE controlling and its ability to create a dynamic 2D image are suggested for future investigation in spatial resolution optimization.

In-situ Doping and Local Overcompensation of High Performance LPCVD Polysilicon Passivated Contacts as Approach to Industrial IBC Cells

In this work, we investigate polysilicon passivation contacts, to be used for high performance IBC cells. We demonstrate an LPCVD process for in-situ p-type doped polysilicon. The p-type polysilicon can be locally overcompensated to n-type polysilicon using phosphorous implantation or POCl3 diffusion. Both contacts show excellent surface passivation, with recombination current (Jo) of 12 fA/cm2 for p-poly on a polished Si surface, and less than 10 fA/cm2 for compensated n-poly both on textured and polished surfaces. These polysilicon layers are implemented in the fabrication of n-PERT cells with front and rear polysilicon contacts. The resulting textured half-fabricated cells have an implied Voc of 701 mV after hydrogenation via PECVD deposition of SiNx:H. Different metallization processes, relevant for IBC cells, are applied. A Voc of 685 mV is obtained for the best cell metallized with low-temperature metallization (ITO/low-temperature Ag paste), and 678 mV is obtained for the best cell with industrial screen-printed firing-through contacts. In these n-PERT cells the same thin oxide layer is used for p-type and n-type polysilicon contacts.

Pump-probe imaging of laser-induced periodic surface structures after ultrafast irradiation of Si

Ultrafast pump-probe microscopy has been used to investigate laser-induced periodic surface structure (LIPSS) formation on polished Si surfaces. A crater forms on the surface after irradiation by a 150 fs laser pulse, and a second, subsequent pulse forms LIPSS within the crater. Sequentially delayed images show that LIPSS with a periodicity slightly less than the fundamental laser wavelength of 780 nm appear on Si surfaces ∼50 ps after arrival of the second pump laser pulse, well after the onset of melting. LIPSS are observed on the same timescale as material removal, suggesting that their formation involves material ejection.

Size Dependent Gold Assisted ZnO Growth on Si Surface by Continuous Spray Pyrolysis Reactor for Light Suppression

Colloidal Au particles of sizes 15 and 40nm are spray deposited on polished Si surface and then the deposition of ZnO nanostructure layer using Continuous Spray Pyrolysis reactor is performed. XRD data suggests that the larger size Au nanoparticles act as seed particles for ZnO nanostructure formation, while smaller Au nanoparticles favour (002) oriented growth. SEM measurement confirms enhancement of ZnO nanorods growth in the case of 40nm Au assisted deposition. The reflection measurement shows higher surface plasmon resonance for larger size Au particles and hence significant light reduction up to 51% of existing value in 500-900nm wavelength region is achieved which may ultimately help in the increase of photocurrent in Si solar cells.

Effective surface passivation of Si surfaces by chemical deposition of (Al2O3)x(B2O3)1 − x thin layers

AbstractIn this work chemical liquid deposition from sol–gel solutions has been used to obtain thin (Al2O3)x(B2O3)1 − x dielectric films. Passivation of Si surfaces using these films has been studied. Morphological properties of thin films deposited on polished Si surfaces have been investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) technique. Their chemical composition, structural and optical features have been analysed by X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and ultraviolet‐visible (UV–VIS) spectroscopy. Results, obtained in this work, show that silicon surfaces can be effectively passivated by thin dielectric layers deposited from chemical solutions. The thin dielectric layers developed in his work, can be recommended as back side passivation coating for Si‐based solar cells.

Reduced conductivity in the terahertz skin-depth layer of metals

The terahertz conductivities of plates of Cu and Al were measured to remain the same at 295 and 77K using waveguide terahertz time domain spectroscopy (THz-TDS). This result was true for a variety of commercial alloys and surface preparations. Consequently, carrier scattering by lattice defects within the 100nm THz skin depth is much larger than scattering by phonons at room temperature. However, an exception was found to be the THz skin-depth layer of an evaporated 300nm Al film in contact with a polished Si surface. For this interface Al layer, the conductivity increased by a factor of 4 when cooled to 77K.

Nano crystalline porous silicon as large-area electrode for electrochemical synthesis of polypyrrole

Polypyrrole (pPy) films were electrochemically synthesized over the surface of nano-crystalline porous silicon (pSi), platinum (Pt) and polished silicon (Si) substrates. As evidenced by the SEM pictures and the potential time-transient characteristics the polymerized growth is relatively porous, smooth and faster over pSi substrates in comparison to the Pt and polished Si surfaces. The polymerization charge on pSi substrate is significantly enhanced and the high accessible surface area of nano-pSi/pPy structure resulted in an improved redox performance of pPy. Impedance study revealed no change in the diffusion control process for pSi/pPy and Pt/pPy electrodes whereas an increase in Faradaic resistance for pSi/pPy electrode is noticed. Redox properties of pPy on n- and p-type pSi structures were studied using cyclic voltametry and electrochemical impedance spectroscopy. FESEM was used to study the surface morphology and to further compliment the electrochemical performance of the electrode.

Self-Assembly of Gold Nanoparticles on Nanometre-PatternedSurface

The self-assembly processes of gold nanoparticles onnanometre-step-patterned Si surface and polished Si surface areinvestigated by the convective self-assembly method. The convectiveself-assembly method is used to deposit the colloids dispersed inbenzene onto the substrates. The SEM results show that theconfigurations of the gold arrays depend on the surface morphology ofthe substrates. On the nanometre-step-patterned Si surface, thenanoparticles self assemble into parallel lines, and the distancebetween the neighbouring lines is around 35 nm. On the polished Sisurface the nanoparticles form compact domains. In each domain theparticles are close-packed in a two-dimensional hexagonal superlatticeand are separated by uniform distances. The analysis shows that on thenanometre-step-patterned Si surface, the steps play critical roles inthe self-assembly process of gold nanoparticles. The capillary forcefrom the steps drives the particles to lines along the steps.Therefore, the particles tend to self-assemble into one-dimensionalline structures when the solvent evaporates. For the polished Sisubstrate there is a little difference that the particles formtwo-dimensional hexagonal superlattices without the directionalconfinement.

Scaling properties of a Si surface patterned by selective chemical etching

Fractal patterns have been observed on a Si{100} surface etched in HF:HNO 3. The microstructure of the surface was examined by scanning electron microscopy with consecutive magnifications in the range ×500 to 50 000. The fractal dimensions of the patterns were determined using the box counting method. The crystal structure of the surface was studied by means of reflection high energy electron diffraction. Electron diffraction patterns were compared with those obtained for a mirror polished Si surface. A model of the chemical etching at the atomic level was proposed. A patterned Si surface is addressed as shaped by crystallographic planes of high Miller indices. Such surfaces are proposed as promising surfaces for direct heteroepitaxy.

Porous Silicon for Chip Cooling Applications

Porous silicon (PSi) was prepared by electrochemical etching with electrolytes consisting of various concentrations of HF and dimethylsulphoxide (DMSO) to control the pore size in the range 0.4µm–8µm as measured by scanning electron microscopy. Pool boiling experiments employing resistive heating of samples with a PSi surface in the dielectric cooling liquid FC-72 was performed and compared to samples with a polished Si surface. It was observed that the PSi surface had favorable cooling behavior and yielding the lowest temperature overshoot upon warming up and the lowest surface temperature during boiling by promoting continuous bubble nucleation which again leads to the most efficient heat removal. The results indicate that PSi could favorably be used in cooling of electronic chips. We also present demonstration of the successful fabrication of a Si porous membrane by a onestep electrochemical etching procedure. The onestep etching technique could be used as an alternative method to the standard Si micro machining such as etched by KOH. The PSi membrane could be used for various closed loop two-phase coolers.

Ultrathin SiO2 on Si II. Issues in quantification of the oxide thickness

AbstractAn analysis is made of various quantification issues concerning the analysis of ultrathin layers of SiO2 on (100) and (111) polished Si surfaces. For analysis of the oxide thickness, a simple equation is generally used involving two parameters; the attenuation length of photoelectrons in the oxide and the ratio, Ro, of the intensities of the Si 2p peak from bulk thermal SiO2 and from pure Si. An analysis of previously reported measurements of the attenuation length gives an average value of only 6% less than the theoretical value. However, careful measurements of Ro, via two routes, indicate consistently that a value of 0.88 ± 0.03 should be used rather than the calculated value of 0.53 ± 0.05. This difference may arise through systematic uncertainties in the values for the relevant inelastic mean free paths, the silicon dioxide density and the shake‐up contributions. Previously reported experimental values of Ro range from 0.67 to 0.87. Uncertainties also arise from intensity variations caused by the crystal structure of the substrate. These are mapped and a position ‘A’ is found where further work is best conducted. For the (100) surface, A is 34° from the surface normal in an azimuth midway at 22.5° between the [010] and [011] azimuths. For the (111) surface at A is 25.5° from the surface normal in the [101] azimuth. Data for much of the present work are for the (100) surface at an angle of emission of 27° at position ‘B’ at 28.5° from the surface normal in the [110] azimuth, which is equivalently good but may degrade for spectrometers with high angular resolution. If the same equation is used for calculating the thickness, position B leads to a calculated thickness that is 4% less than that measured for an average orientation, whereas data acquired for normal emission lead to a value 18% lower, and those measured at A are 2% higher. Measurements of the carbonaceous contamination confirm earlier conclusions that the contamination is better described using data for an average polymer than for glassy carbon. © Crown copyright 2002. Published by John Wiley & Sons, Ltd.

Modification of spontaneous emission from CdSe/CdS quantum dots in the presence of a semiconductor interface

The spontaneous-emission lifetime of CdSe/CdS core-shell quantum dots was studied as a function of the distance between the dots and a polished Si surface. The experimental results reveal a significant modification of the spontaneous-emission rate of the quantum dots by the Si surface.

Photovoltaic Device Applications of Porous Silicon

ABSTRACTWe report on the results of our investigation of using porous Si to enhance the performance of crystalline silicon photovoltaic solar cells. Possible approaches include using the porous Si for (1) surface texturing to enhance light trapping, (2) front or back surface fields because of its wider bandgap, and (3) photon color conversion of blue light to longer wavelengths that have higher quantum efficiency in a Si solar cell. In our surface texturing study, a porous-Si-covered single-crystal Si wafer showed an integrated reflectance of only 1.4% at 500-nm wavelength compared to about 40% for a polished Si surface. For our solar cell study, we used a point-contact cell structure with diffused p+ and n+ point contacts on the back of the cell. This cell structure allows us to form the porous Si on the front surface after both the junction formation and the evaporation and alloying of metal contacts.

Selected-area deposition of diamond films

Selected-area deposition of diamond film has been accomplished on Si substrates prepared by two different methods: reactive-ion etching (RIE) and amorphous-Si masking (ASM). In the RIE method, a Si substrate polished by a diamond paste was patterned with a photoresist mask, and the unprotected areas were etched by RIE, followed by a complete removal of the photoresist films. The diamond deposition was done by electron-assisted chemical-vapor deposition (CVD), and diamond films grew only in the areas once covered with the photoresist films and not etched by RIE. In the ASM method, a polished Si substrate was also photolithographically masked with photoresist, followed by a uniform deposition of a hydrogenated amorphous silicon (a-Si:H) film. The photoresist film was then lifted off together with the overlay of deposited a-Si:H, leaving the polished Si surface patterned with an a-Si:H mask. In this case, the diamond deposition was done by microwave plasma CVD, and diamond films grew only in the areas not covered with a-Si:H. In both cases, well-defined diamond patterns with line/space dimensions of a few micrometers were formed on the substrates.