Polysilicon Nanofilm Research Articles

Electrical Trimming Characteristics of Polysilicon Nanofilms with Different Doping Concentrations and Deposition Temperatures

Polysilicon nanofilm (PSNF) can provide a large gauge factor and good temperature stability, which promotes their application in piezoresistive sensing devices. Electrical trimming is necessary to further improve the stability and matching of piezoresistive resistors after sensor fabrication. The advantages of PSNF are realized by first preparing PSNF samples with different doping concentrations and deposition temperatures. By applying an incremental DC current that is higher than the threshold current of the PSNF resistors, the PSNF resistors are trimmed and the resistance changes are measured. The results of electrical trimming show that the threshold current, trimming rate, and trimming error are related to the doping concentration and deposition temperature. According to tunneling piezoresistive theory and the interstitial-vacancy pair model, the experimental results are expounded. These results are useful for the design and fabrication of PSNF piezoresistive sensors.

Theoretical relationship between p-type polysilicon thin film gauge factor and doping concentration

The polysilicon thin film piezoresistors are widely used in semiconductor pressure sensors. The polysilicon thin film has good piezoresistance properties, which are determined by the grain structure and doping concentration. The gauge factor of the polysilicon thin film is usually modified according to the relationship between gauge factor and doping concentration. The polysilicon thin films are classified into common polysilicon thin films and polysilicon nanofilms according to their thickness. The common polysilicon thin film thickness is more than 0.3 μm, which has good temperature characteristic, but its piezoresistance coefficient is small. However, the polysilicon nanofilm thickness is less than 0.1 μm, which has good temperature characteristic and high piezoresistance coefficient. The existing piezoresistance theory of the common polysilicon thin film cannot explain reasonably the experimental result of polysilicon nanofilm piezoresistance. Therefore, the tunneling piezoresistance model and an algorithm for the p-type polysilicon nanofilm piezoresistance coefficient were established in 2006. However, this algorithm presents an incomplete fundamental piezoresistance coefficient. In order to improve the polysilicon thin film piezoresistance theory, based on the tunneling piezoresistance model and the mechanism of silicon and the valence band hole conductivity mass with the change of stress, a novel algorithm for the piezoresistance coefficient of the p-type polysilicon thin film is presented. The theoretical formulas for three fundamental piezoresistance coefficients π11, π12 and π44 of the grain neutral and grain boundary regions, are presented respectively. With these formulas for the coefficients, the longitudinal and transverse piezoresistance coefficients for arbitrary crystal direction texture polysilicon can be obtained. According to the structure characteristics, the gauge factors of the p-type polysilicon nanofilm and the common polysilicon thin film are calculated, and then the longitudinal and transverse gauge factors are plotted each as a function of doping concentration, which are compared with the experimental results. According to the experimental results of the polysilicon nanofilm, the grain size is L=30 nm, the grain crystal directions are randomly distributed. The trap density in grain boundary region is Nt=1013 cm-2, the Young's modulus of elastic diaphragm is Y=1.69×1011 Pa, the Poisson ratio of elastic diaphragm is ν=0.062, the grain boundary width is δ=1 nm, and the thickness is 80 nm. The comparison indicates that the gauge factor average error between calculation and experiment is 0.5 times less than the average experimental difference between the maximum and the minimum for each doping concentration. For the common polysilicon thin film, according to the experimental results, its grain size L is 135 nm, thickness is 400 nm, the orientations of crystal grain neutral region are[311],[111] and[110] in the ratio of 49:31:20, i.e., 〈311〉:〈111〉:〈110〉=49:31:20, and the gauge factor calculated result is also good agreement with the experimental result. Therefore, the proposed algorithm is comprehensive and accurate, which is applicable to the p-type common polysilicon film and the polysilicon nanofilm.

The algorithm for the piezoresistance coefficients of p-type polysilicon**Project supported by the National Natural Science Foundation of China (No. 61372019).

In order to improve the piezoresistance theory of polysilicon, based on the tunneling piezoresistance model, using the mechanisms of approximate valence band equation and shifts of the hole transfer and hole conduction mass by stress, a novel algorithm for the piezoresistance coefficients of p-type polysilicon is presented. It proposes three fundamental piezoresistance coefficients π11, π12 and π44 of the grain neutral and grain boundary regions, separately. With those piezoresistance coefficients, the gauge factors of the p-type polysilicon nanofilm and the p-type common polysilicon film are calculated, and then the plots of the gauge factor as a function of doping concentration are given, which are consistent with the experimental results.

A surface micromachined pressure sensor based on polysilicon nanofilm piezoresistors

In order to effectively apply the piezoresistive properties of the polysilicon nanofilm and the surface micromachined technology, a surface micromachined pressure sensor based on polysilicon nanofilm piezoresistors and vacuum cavity is presented. The design of the sensor structure is carried out by finite element analysis for improving its performance. Residual stress relaxation in the polysilicon diaphragm and prevention of adhesion in the sensor cavity are carried out in the process of the sensor fabrication. The sensor sample is fabricated according to the structure designed. The sample's measurement results show that a full scale pressure of 2.5MPa, a sensitivity of 2.896×10−2mV/V/kPa at 25°C, an overpressure of 7 times higher than the full scale pressure at 25°C, a thermal zero drift of −0.01%FS/°C and a thermal sensitivity drift of −0.1%FS/°C in a temperature range of −40 to 200°C are achieved. The measurement performance is excellent and agrees with the predictions of the design method.

High Overload Pressure Sensor Construct With Polysilicon Nanofilm

A high overload pressure sensor is prepared by surface micromachined technology, and its full scale pressure is 2.5 MPa. The structure of the proposed sensor is based on a seal cavity made by sacrificial layer, a deposition of polysilicon nanofilm acts as piezoresistors on the diaphragm. The stress distribution of the diaphragm of the sensor is simulated by the large displacement static analysis and the nonlinear contact analysis. Due to polysilicon high tensile strength and appropriate cavity height, the sensor full scale output voltage and overload capacity is greatly improved. The measured results indicate that the overpressure of the sensor sample is seven times higher than its full-scale pressure, and its full scale output voltage is 362 mV with a supply voltage of 5 V.

Optimized Technical Characteristics of Polysilicon Nanofilm

In order to take good advantage of polysilicon nanofilm, optimized technical characteristics of the polysilicon nanofilm are very necessary to investigated. In this paper, the polysilicon nanofilms were prepared under different technical parameters, including thickness and doping concentration, which are very important for preparation of the nanofilm. The experimental results of piezoresistive and temperature characteristics show that the optimized technical characteristics are followed, the thickness is about 90nm, and the doping concentration is about 4.1×?1019cm-3 or between 2.0×?1020cm-3 and 4.1×?1020cm-3 from different point of view. The investigations of optimized technical characteristics are very useful for application of the polysilicon nanofilm to piezoresistive sensor.

Tunnelling piezoresistive effect of grain boundary in polysilicon nano-films

The experiment results indicate that the gauge factor of highly boron doped polysilicon nanofilm is bigger than that of monocrystalline silicon with the same doping concentration, and increases with the grain size decreasing. To apply the unique properties reasonably in the fabrication of piezoresistive devices, it was expounded based on the analysis of energy band structure that the properties were caused by the tunnel current which varies with the strain change forming a tunnelling piezoresistive effect. Finally, a calculation method of piezoresistance coefficients around grain boundaries was presented, and then the experiment results of polysilicon nanofilms were explained theoretically.

Polysilicon nanofilm pressure sensor

Utilizing 80 nm polysilicon nanofilm as piezoresistors, a pressure sensor with high performance is developed. The complete fabrication process is described. The pressure properties of the sensor were measured at the temperature from 0 to 200 °C. For 0.6 MPa full scale pressure, the sensitivity is 23.00 mV/V/MPa at 0 °C and 18.27 mV/V/MPa at 200 °C, the temperature coefficient of sensitivity (TCS) is about −0.098%/ °C without any compensation. The temperature coefficient of offset (TCO) is about −0.017%/ °C. Because of the good piezoresistive and temperature characteristics of polysilicon nanofilm, the pressure sensor demonstrates a better performance.

Piezoresistive Sensitivity, Linearity and Resistance Time Drift of Polysilicon Nanofilms with Different Deposition Temperatures

Our previous research work indicated that highly boron doped polysilicon nanofilms (≤100 nm in thickness) have higher gauge factor (the maximum is ∼34 for 80 nm-thick films) and better temperature stability than common polysilicon films (≥ 200nm in thickness) at the same doping levels. Therefore, in order to further analyze the influence of deposition temperature on the film structure and piezoresistance performance, the piezoresistive sensitivity, piezoresistive linearity (PRL) and resistance time drift (RTD) of 80 nm-thick highly boron doped polysilicon nanofilms (PSNFs) with different deposition temperatures were studied here. The tunneling piezoresistive model was established to explain the relationship between the measured gauge factors (GFs) and deposition temperature. It was seen that the piezoresistance coefficient (PRC) of composite grain boundaries is higher than that of grains and the magnitude of GF is dependent on the resistivity of grain boundary (GB) barriers and the weight of the resistivity of composite GBs in the film resistivity. In the investigations on PRL and RTD, the interstitial-vacancy (IV) model was established to model GBs as the accumulation of IV pairs. And the recrystallization of metastable IV pairs caused by material deformation or current excitation is considered as the prime reason for piezoresistive nonlinearity (PRNL) and RTD. Finally, the optimal deposition temperature for the improvement of film performance and reliability is about 620 °C and the high temperature annealing is not very effective in improving the piezoresistive performance of PSNFs deposited at lower temperatures.

Gauge Factor and Nonlinearity of P-Type Polysilicon Nanofilms

The gauge factor and nonlinearity of 80nm polysilicon nanofilms with different doping concentration were tested. The experimental results show that, from 8.1×1018cm-3 to 2.0×1020cm-3, the gauge factors first increase then decrease, which like the common polysilicon films (thickness is larger than 100nm). From 2.0×1020cm-3 to 7.1×1020cm-3, the gauge factors do not change with doping concentration almost, which can be explained by tunneling piezoresistive theory. When doping concentration is low than 4.1×1019cm-3, the nonlinearities are big, and the nonlinearities become small when doping concentration is high than 4.1×1019cm-3. The nonlinearity is related to the occupied condition of trapping states in grain boundary. The longitudinal gauge factor and nonlinearity are smaller than transverse ones. Take the gauge factor and nonlinearity both into consideration, the optimal doping concentration should be 4.1×1019cm-3. The conclusions are very useful for design and fabrication of polysilicon nanofilms piezoresistive sensor.

Electrical Trimming of Boron-Doped Polysilicon Nanofilm Resistors Deposited at Different Temperatures

The previous research showed that highly doped polysilicon nanofilms (PSNFs) have a high gauge factor and stable temperature characteristics, promoting their applications in piezoresistive sensors. For correcting the resistance deviation and improving the resistance matching of sensing elements, the electrical trimming (ET) characteristics of PSNFs deposited by LPCVD at different temperatures were studied. By the interstitial-vacancy (IV) model, the resistance falloff in trimming phenomenon is considered as a result of the decrease of scattering centers caused by the recombination of IV pairs. By modeling grain boundaries (GBs) as the accumulation of IV pairs (basic defects), it is believed that IV pairs can recombine layer-by-layer under the energy excitation of joule heat generated by high current conduction. The experiment results show that the threshold current density for ET of PSNFs is an order of magnitude lower than that of polysilicon common films, while the trimming accuracy and stability of directly deposited PSNFs are superior to the recrystallized ones. Finally, it is gained that reducing amorphous phases at GBs by optimizing deposition temperature can improve ET characteristics of PSNFs.

Analysis of Tunneling Piezoresistive Effect of P-Type Polysilicon Nanofilms

The polysilicon nanofilms have significant piezoresistive characteristics. In this paper, an analysis of tunneling piezoresistive effect of p-type polysilicon nanofilms is presented based on the experimental data. The analysis results show that the tunneling piezoresistive effect is much remarkable than piezoresistive effect of neutral region, and the former is about 1.3 to 1.5 times of the latter. The higher is doping concentration, the more remarkable tunneling piezoresistive effect is. This advantage can be utilized to improve the temperature characteristics of polysilicon piezoresistive sensor.