Masaaki Koganemaru

Residual stress evaluation in resin-molded IC chips using finite element analysis and piezoresistive gauges

The high residual stress in a resin-molded electronic package sometimes makes the electronic functions unstable. Therefore the residual stress in electronic packages, especially on the top surfaces of semiconductor chips, should be evaluated. The objective of this study is to present a simple method for evaluating residual stress in resin-molded semiconductor chips using a combination of experimental and numerical methods. The actual residual stress of the packaging process was measured by using test chips that included piezoresistive gauges. A linear thermoelastic finite element analysis was then carried out using a three-dimensional model. The finite element analysis was performed under a stress-free temperature determined by the temperature dependence of the residual stress, which was experimentally measured by using the piezoresistive test chips. The measured residual stress using the test chips agreed well with the results of the finite element analysis. It was therefore confirmed that the present evaluation method, combining experimental and numerical methods, is reliable and reasonable.

Evaluation of Stress Effects on Electrical Characteristics of N-Type MOSFETs: Variations of DC Characteristics During the Resin-Molding Process

Stress-induced changes in the electrical characteristics of a semiconductor device become a major concern in the production of semiconductor packages because the electrical characteristics are adversely affected by packaging (residual) stresses. The objective of our project is to evaluate the effects of stress on semiconductor devices. In this study, the shift of the DC characteristics of nMOSFETs during the resin-molding process was investigated experimentally. After a silicon chip including the n-type metal oxide semiconductor field effect transistors (nMOSFETs) was encapsulated in a quad flat package, the drain current variations and the transconductance shifts were measured. The drain current decreased during the resin-molding process while no significant shift in threshold voltage was observed. The experimental results were estimated adequately from the residual stress predicted by numerical and experimental analyses and from the stress-sensitivity of the nMOSFETs measured by the four-point bending method. Also, we tested the validity of an electron-mobility model that included the effect of stress. The electron-mobility model takes into account the variation in the relative occupancy of the electrons in each conduction-band energy valley. It was found that the effect of biaxial stress on the variation in electron-mobility can be qualitatively evaluated by the electron-mobility model but are quantitatively different from the experimental results. Several needed improvements to the electron-mobility model are proposed in this article.

Experimental Study of Uniaxial-Stress Effects on DC Characteristics of nMOSFETs

Stress-induced shifts of the direct current characteristics on n-type metal oxide semiconductor field effect transistors (nMOSFETs) were investigated experimentally. The stress sensitivities of nMOSFET characteristics were measured by the 4-point bending method, and the gate-length dependence of transconductance shifts caused by uniaxial stress was evaluated. As a result, it is shown that the gate-length dependence of transconductance shifts is attributed to parasitic resistance of the nMOSFETs. Also, this paper verified the electron-mobility model proposed in the previous study that includes stress effects in comparison with the experimental results. As a result, several improvements for the electron-mobility model are proposed in this paper. We describe the change of the conduction-band energy induced by the shear deformation of silicon. The shear deformation with a uniaxial stress along the direction of silicon should be considered in the change of the conduction-band energy.

Influence of uniaxial mechanical stress on the high frequency performance of metal-oxide-semiconductor field effect transistors on (100) Si wafer

The effects of uniaxial mechanical stress on the radio frequency performance of n- and p-metal-oxide-semiconductor field effect transistors (MOSFETs) are investigated up to 10 GHz. Under tensile stress, the gate transconductance (gm) increases in the n-MOSFETs while it decreases in the p-MOSFETs, whereas the results were vice versa for compressive stress. The total gate capacitance (CG) extracted from scattering parameters increases (decreases) under tensile (compressive) stress for both n- and p-MOSFETs, which is explained by the variation in the effective mass perpendicular to the Si/SiO2 interface. The cut-off frequency (fT) varies in inverse proportion to the CG variation.

Device Simulation for Evaluating Effects of Inplane Biaxial Mechanical Stress on n-Type Silicon Semiconductor Devices

This paper presents a practical method of drift-diffusion device simulation for evaluating the effects of mechanical stress on n-type silicon semiconductor devices. The device simulation incorporates an electron mobility model for considering the effects of stress. In this paper, we focus on stress effects that are induced by applying inplane biaxial stress to the devices. Therefore, two physical phenomena that are attributed to mechanical stress are modeled in the electron mobility model, i.e., the changes in relative population and the momentum relaxation time (intervalley scattering) of electrons in conduction-band valleys. Stress-induced variations of direct-current characteristics on n-type metal-oxide-semiconductor (MOS) field-effect transistors are evaluated using a device simulation including the proposed electron mobility model. Then, the electron mobility model and the simulation method are verified by comparing with experimental results. It is demonstrated that experimental results can be reasonably estimated using this device simulation method. From discussions regarding the electron mobility model, it is suggested that the comprehensive stress sensitivity of MOS devices is larger than that of lightly doped silicon.

Numerical study on impact of mechanical stress in the regions of high current density in N-Type MOS devices

This article shows numerical studies based on driftdiffusion device simulation for the effects of mechanical stress on n-type MOS devices. The device simulation incorporates an electron mobility model for illustrating the effects of stress. In this study, three numerical studies are conducted for the effects of mechanical stress on the electrical performance of an n-type MOS device: the effects of stress distribution in the device are simulated, dominant physical phenomena on electron mobility enhancement induced by stress are analyzed, and the impact of stress in the regions of high current density is examined. It is demonstrated that, quantitatively speaking, dominant physical phenomena is the change in the relative occupancy. It is shown that most of the stress effects result from stresses in the regions of high current density in the n-type MOS device.

Device simulation for evaluating effects of mechanical stress on semiconductor devices: Impact of stress-induced variation of electron effective mass

The effects of uniaxial loading on n-type metal-oxide-semiconductor field-effect transistors (nMOSFETs) were simulated by a drift-diffusion device simulation. The device simulation includes an electron mobility model for considering the effects of mechanical stress. The variations in the relative occupancy, the intervalley scattering and the effective mass of electrons were taken into account in the electron mobility model. In this study, the effects of uniaxial stress on nMOSFETs with gate lengths of 24µm and 0.8µm were evaluated; the stress-induced variations of the drain current and transconductance were simulated. Then, the simulation results were compared with experimental results obtained by the uniaxial loading of nMOSFETs. The simulation results obtained by considering the impact of the stress-induced variation of electron effective mass were in good qualitative agreement with those obtained by the experiments; the current simulation was able to qualitatively determine the uniaxial-load-direction dependence of the stress-induced variation of the electrical characteristics of nMOSFETs.

Evaluation of stress-induced effect on electronic characteristics of nMOSFETs using mechanical stress simulation and drift-diffusion device simulation

We evaluated the stress-induced effect on electronic characteristics of nMOSFETs using mechanical stress simulation and drift-diffusion device simulation. The simulation model includes the electron mobility model that takes the stress effects into consideration. We evaluated the variation in the electronic characteristics of nMOSFET during the actual resin-molding packaging process (QFP process). The stress distribution in the nMOSFET was considered in the device simulation. As a result, the experimental results were evaluated reasonably using the proposed simulation method. It is demonstrated that the device simulation is useful and versatile for evaluating the stress-induced effect on electronic characteristics of a semiconductor device.

Stress-Induced Effects in Electronic Characteristics of n-Type MOSFETs in Resin-Molded Packages

The shift of the DC characteristics of nMOSFETs during a resin-molding process was investigated experimentally. A silicon chip including the nMOSFETs was encapsulated in a quad flat package, and the drain current and transconductance shifts were measured. The drain current decreased during the resin-molding process, while no significant shift in threshold voltage was observed. The experimental results were estimated adequately from the residual stress predicted by numerical and experimental analysis, and from the stress sensitivity of the nMOSFETs measured by the four-point bending method. Also, we verified the validity of an electron mobility model that includes the effect of stress, used for drift-diffusion device simulation, by comparison with experimental results, and several improvements to the electron mobility model were found out.Copyright

Evaluation of Stress-Induced Effects in Electronic Characteristics of nMOSFETs

Stress-induced shifts in the DC characteristics of nMOSFETs were investigated experimentally by the 4-point bending method. We measured the device shape dependence and load direction dependence of the DC characteristic shifts. We also carried out drift-diffusion device simulation in order to evaluate the experimental results. The simulation model includes the electron mobility model that takes the stress effects into consideration. The conduction band energy change induced by the shear deformation of silicon is considered in the mobility model. The experimental results were evaluated reasonably using the proposed mobility model