Design and simulation of integrated inductors on porous silicon in CMOS-compatible processes

We present a comprehensive approach of designing on-chip inductors using a CMOS-compatible technology on a porous silicon substrate. On-chip inductors realized on standard CMOS technology on bulk silicon suffer from mediocre Q-factor values partly because of the loss created by the Si substrate at higher frequencies, in addition to the metal losses. We examine the alternative of using porous Si as a thick layer isolating the Si substrate from the metallization in an otherwise standard CMOS technology. We present theoretical designs produced with full-wave Method-of-Moments simulations, verified by measurements in standard 0.18 μm CMOS technology using Al metallization. When porous Si is introduced in that technology, the same inductor metallization produced Q-factor enhancements of the order of 50%, compared to the same inductor on bulk crystalline silicon. We also produce optimized single-ended inductor designs using Cu on porous Si, in a 0.13 μm-compatible CMOS technology. The resulting Q-factors are enhanced by a factor of 2 and reach values of 30 or more in the 2–3 GHz frequency range. Even higher quality factors can be obtained in this technology when differential designs are used.     

Carrier transport in porous silicon

Carrier transport in porous silicon layers has been studied by the time-of-flight method in the strong injection mode at temperatures T=290–350 K and electric field strengths F=(1.5–7)×10⁴ V cm^?1. The electron and hole drift mobilities μ_e≈2×10^?3 cm² V^?1 s^?1 and μ_h≈6×10^?4 cm² V^?1 s^?1 were obtained at T=292 K and F=4×10⁴ V cm^?1. An exponential temperature dependence of drift mobility with activation energy of ～0.38 and ～0.41 eV for, respectively, electrons and holes was established. It is shown that the type of time dependences of the photocurrent associated with carrier drift and the superlinear dependence of the transit time on the reciprocal of the voltage applied to a sample allow use of the concept of space-charge-limited currents under the conditions of anomalous dispersive transport. The experimental data are accounted for in terms of the model of transport controlled by carrier trapping into localized states with energy distribution near the conduction and valence band edges described by an exponential function with a characteristic energy of ～0.03 eV.     

NH3／H2O2溶液腐蚀下多孔硅光致荧光

用光致荧光谱、傅里叶变换红外光谱（ＦＴＩＲ）和扫描电子显微镜（ＳＥＭ）对用阳极氧化法制成的多孔硅层在１％ＮＨ３／Ｈ２Ｏ２溶液中的腐蚀现象进行了研究。红外分析表明，Ｓｉ－Ｏ键和Ｈ－Ｏ键的强度随ＮＨ３／Ｈ２Ｏ２溶液的腐蚀时间的增加而增加，Ｓｉ－Ｈ键强主匠随腐蚀时间增加而减少。光致荧光谱的峰值在腐蚀开始时先下降后上升，半高宽变窄，谱峰的以边明显蓝移。分析研究表明，１％ＮＨ３／Ｈ２Ｏ２溶液对多孔硅层有腐蚀     

Radiation hardness of porous silicon

The effect of irradiation by 300-keV Ar⁺ ions on the properties of electrochemically produced porous silicon is studied at doses of 5×10¹⁴–1×10¹⁶ cm⁻². Raman scattering and photoluminescence data are used to show that the radiation hardness of porous silicon layers is substantially greater than that of single crystal silicon. Fiz. Tekh. Poluprovodn. 31, 1126–1129 (September 1997)     

Changes in properties of a 〈porous silicon〉/silicon system during gradual etching off of the porous silicon layer

Scanning electron microscopy has shown etching of a porous silicon layer in an HF solution to be irregular. The intensity of porous silicon photoluminescence significantly decreases during gradual etching off, and its peak initially shifts to shorter and then to longer wavelengths. Under red-light pulse excitation, photo-voltage measurements have shown that the boundary potential ?_s of the p-Si substrate is positive, and ?_s grows with the etching time and as the temperature decreases from 300 to 200 K. At T<230 K, the photomemory of ?_s caused by nonequilibrium electron capture by p-Si boundary traps is observed. The concentration of shallow traps and boundary electron states in p-Si increases as porous silicon is etched. At T<180 K, the system of boundary electron states is rearranged. Photovoltage measurements with white-light pulses have revealed electron capture at oxide traps of aged porous silicon.     

Transient photocurrent and photoluminescence in porous silicon

Transient photocurrent in porous silicon samples subjected to prolonged storage in air has been studied using the time-of-flight technique at temperatures in the range 300–350 K and electric field strengths of 10⁴–10⁵ V/cm. Photoluminescence has been studied in the same samples at T=300 K using time-resolved spectroscopy. A conclusion is made on the basis of comparison of the data obtained that localized states play important part in both the processes at characteristic times of these processes falling within the microsecond range.     

Isolation of silicon film grown on porous silicon layer

We have investigated a new technology for dielectric isolation of a Si film grown epitaxially on a porous silicon layer. After oxidation of the porous silicon layer, a Si on Ohcidized Porous Silicon(SOPS) structure can be obtained. It is proposed that micropores pinch off quickly in the interfacial region between the porous silicon layer and the epitaxial film. A minimum yield calculated from Rutherford backscattering spectra of the epitaxial silicon film is 5.3%, and an electron Hall mobility of 600cm²/V.s is obtained in the film with a carrier concentration of 1 x 10¹⁷ /cm³. MOSFETs were fabricated on the SOPS structure.     

Carrier drift mobility in porous silicon carbide

The time-of-flight technique was used to determine the drift mobilities of electrons and holes in porous silicon carbide produced by surface anodization of n-type 4H-SiC wafers. The electron and hole mobilities at 300 K in an electric field of 10⁴ V/cm were μ_e=6×10^?3 and μ_h=3×10^?3 cm² V^?1 s^?1, respectively. The low values of the mobilities are accounted for by carrier capture in localized states.     

Kinetics of crack formation in porous silicon

Crack formation in electrochemical porous silicon has been experimentally and theoretically studied. It is established that the kinetics of porous silicon cracking can be described by the S-shaped Weibull distribution. This feature appears to be of general character; it may be observed during crack formation in other porous solid materials.     

Optoelectronic study in porous silicon thin films

A photoconductivity (PC) study in as deposited porous silicon (PS) thin films is presented in this work. PS thin films were produced by the electrochemical anodizing method at different anodizing times. The films surfaces were characterized by SEM and porosity was determined by gravimetric methods. Photoluminescence and PC measurements were taken at room temperature. The maximum of the photoluminescence spectra are located around 650 nm, whereas those of PC are placed around 400 nm. The maximum of the photoluminescence signal shifts toward short wavelengths as the quantum dimension of the material skeleton diminishes, while any spectral displacement of the photocurrent signal as the porosity of the material increases is not observed. The spectral position of the PC signal does not change because it is strongly affected by the large quantity of defects present in the sample surface which diminishes the mean free path of the carriers to reach the electrodes. In all the samples photocurrent is small around 10^?1 μA and the intensity of the signal goes down as the porosity increases. Two mechanisms exist that compete with one another, the carrier generation and recombination through light emission centers which diminish the photocurrent.     

Drift mobility of carriers in porous silicon

The drift mobility of carriers in porous silicon has been studied in a wide temperature range (190–360 K) at electric field strengths of 2×10³–3×10⁴ V/cm. An exponential temperature dependence of the hole drift mobility with an activation energy of d ～ 0.14 eV was established. The density of localized states controlling the transport is evaluated.     

High-performance inductors integrated on porous silicon

To study the substrate effect on inductor performance, several types of spiral inductors were fabricated on porous silicon (PS), p/sup -/ and p/sup +/ silicon substrate. /spl pi/-network analysis results show that the use of PS effectively reduces the shunt conductance and capacitance. The analysis further shows that the use of PS significantly reduces the eddy current portion of series resistance of inductor, leading to slower increase of the apparent series resistance with increasing frequency. Higher Q-factor and resonant frequency (f/sub r/) result from the reduced shunt conductance, shunt capacitance, and frequency dependence of series resistance. Inductors fabricated on PS regions are subjected to a much less stringent set of constraints than those on bulk Si substrate, allowing for much higher inductance to be achieved without severe sacrifice in Q-factor and f/sub r/. Similarly, much higher Q-factor can be obtained for reasonable inductance and f/sub r/.     

Surface chemistry of porous silicon carbide

The anodization reaction of SiC using HF solution makes a porous silicon carbide (PSC) layer develop. The luminescence behavior of PSC, however, is somewhat different from that of porous Si in that the so-called blue shift is not observed. Though the quantum confinement effect is said to be responsible for light emission in porous SiC, the surface state of PSC plays an important role. The effects of thermal annealing under various atmospheres on the luminescence properties were studied. Some spectroscopic analyses were adopted to elucidate the surface chemistry of PSC. The surface of PSC, which seems to be an origin of the luminescence, had C-H termination but Si-H or Si-O bonds were not detected. X-ray photoelectron spectroscopy analysis also showed that the Si-O bond that usually exists on the surface of bulk SiC was depressed and a strong peak assigned to -CH- appeared. The oxidation treatment reconstructed the Si-O bonds on the PSC surface, and this surface depressed the luminescence. Two other thermal treatments also depressed the PL spectra from the higher energy region, which is due to alternation from C-H to C-C on the surface.     

Free-standing luminescent layers of porous silicon

Free-standing layers of porous silicon with a thickness ranging from 50 to 200 μm have been fabricated using an electrolyte composed of HF and acetic acid. Chemical aspects of the etching process associated with the evolution of gases that favor detachment of layers from substrates are considered. The layers exhibit stable photoluminescence in the visible spectral region observed from both of their sides.     

Negative differential resistance of porous silicon

A porous silicon Al Schottky barrier diode shows differential negative resistance. The thin wires in porous silicon have much lower electron mobility than that of thick wires, due to electron surface scattering from space confinement. The energy of carriers in thick wires increases with applied bias. Some carriers can overcome the conduction-band discontinuity and flow into the thin wires. The negative differential resistance comes from the mobility difference between thick wires and thin wires in porous silicon.     

Void transformation and dopant distribution in porous silicon

It is established by experiment that the densification of a porous-silicon (PSi) film starts at its surface, where the material is most discontinuous. The film-wafer system tends to thermal equilibrium by downward mass transport along voids to a depth of 50 μm. As a result, open voids less than 50 μm deep disappear completely, whereas deeper ones become isolated from the surface. These structural changes are manifested in doping profiles. Strong dependence is found of void transformation on diffusion temperature and time, wafer doping level, and original PSi-film porosity. Specifically, changing to higher diffusion temperatures, longer diffusion times, or higher wafer doping levels results in a reduced porosity throughout the PSi film. Maximum densification is observed in PSi films with an original porosity of 15–50%.     

Mechanism of electroluminescence of porous silicon in electrolytes

A generalized model for the appearance of visible-and infrared-range electroluminescence of porous silicon in contact with an oxidizing electrolyte is proposed. According to the model, visible-range electroluminescence arises as a result of bipolar injection of electrons and holes from the electrolyte into electrically insulated quantum-well silicon microcrystallites, while infrared-range electroluminescence is due to monopolar injection of holes from the electrolyte into macrocrystals. A mechanism of electron injection from the electrolyte is proposed. It is concluded that the character of the electroluminescence should not depend on the magnitude and even the type of conductivity of the silicon substrate. Fiz. Tekh. Poluprovodn. 31, 844–847 (July 1997)     

Conductivity relaxation in coated porous silicon after annealing

A study has been carried out of the effect of a brief anneal at 450–550 °C on the conductivity of porous silicon coated with a metallic film. Porous silicon, formed on p-and n-type substrates, had porosities of 16–40% and 5–10%, respectively. It has been shown that for anneals at 500 and 550 °C porous silicon on p-type Si is converted to the highly conducting state. The conductivity relaxation in coated porous silicon layers on p-Si after annealing is described. The results are analyzed from the point of view of a model of passivation of the impurity atoms by hydrogen. It is shown that after annealing an aluminum-〈porous silicon〉 junction possesses a rectifying property. The potential barrier parameters for Al-〈porous silicon〉 junctions on p-and n-type substrates are determined. Fiz. Tekh. Poluprovodn. 33, 476–480 (April 1999)     

Reverse recovery processes in silicon power rectifiers

The present review gives an account of a number of investigations that have been recently carried out in the Semiconductor Laboratory Pretzfeld, Siemens AG. The switching processes in power rectifiers from the forward into the reverse state differ greatly from the corresponding process predicted by low-level theory. This result is caused not only by the fact that the conditions are different for high injection, but in addition, the sweeping out of the charge carriers takes place from two sides, owing to the nearly uniform concentration distribution in the forward state. Because of the unequal electron and hole mobilities, the impurity distribution on the side of the p contact is of much greater importance than at the n contact; if there is no p-n junction on this more important side, then the stored change can be swept out without much voltage buildup (example: rectifiers from uniformly doped p material). If, on the other hand, a p-n junction lies before the p contact (example: rectifiers from uniformly doped n material) then tbe reverse recovery current decays soon but slowly, and the switching process takes a longer time. This fact also contributes to the relatively long turn-off time of the thyristors.     

Using the temperature-dependent photovoltage to investigate porous silicon/silicon structures

Structures based on porous silicon por-Si/p-Si, both freshly prepared by chemical etching and aged, exhibit a temperature-dependent photovoltage at high levels of electron-hole pair generation by pulse trains of red and white light. These structures are investigated by measuring this photovoltage, which is shown to consist of two components: a photovoltage generated in p-Si, and an oppositely directed photovoltage that appears in por-Si, characterized by trapping of nonequilibrium holes at the surface of por-Si nanocrystals during the period of illumination by the first pulse of white light. For aged structures capture of electrons in the oxide of the por-Si is also observed. The concentration of interface electronic states and electron traps at the interface of p-Si with por-Si is determined by measuring the photovoltage induced by pulses of red light. Fiz. Tekh. Poluprovodn. 33, 1330–1333 (November 1999)