Thermal Conductivity Characterization Of Ultra-thin Si/Ge Films Using Ultra-fast Transient Hot Strip Method And Its Impact On The Self-heating Effect In MOSFETs

Thermal conductivity is an important material parameter of silicon when studying the performance and reliability of devices or guiding the circuit design upon considering heat dissipation,especially when self-heating effect becomes prominent in ultra-scaled MOSFET devices.The cross-plane thermal conductivity of a thin silicon film is lacked due to the difficulty in sensing the high thermal conductivity in vertical direction.In this paper,a feasible method utilizing the ultra-fast electrical pulse within 20 ?s combining transient hot strip technique is adopted.To our knowledge,the cross-plane thermal conductivity of sub-50 nm(30/17/10 nm)silicon films on buried oxide are successfully extracted from the time dependent temperature response for the first time.The ratio of the extracted cross-plane thermal conductivity over the bulk value is only about 6.9%,4.3%and 3.8%at 300 K.As the size of the films is smaller than the phonon mean free path,the classical heat transport theory is failed to predict the heat dissipation in nanoscale transistors.Thus,the ballistic model,derived from the extended-irreversible-hydrodynamic(EIT)based heat transport equation,is used for further investigation,and the simulation results exhibit good consistencies with the experimental data.Germanium is a promising channel material candidate of MOSFETs due to its both high electron and hole mobilities.Therefore,the thermal conductivity of ultra-thin Ge films on insulator was also extracted by the same test method.The thermal conductivity of the 50/13/5 nm thick Ge films is 42.35%,12.3%and 2.7%over the bulk value at 300 K,respectively.Comparing the experimental data with the values calculated from the EIT model,they also shows good consistency.Condsidering that the surface roughness of Ge were more easily induced during processes than that of Si,systematic investigation of the impact of dry etching conditions on Ge surface morphology was also studied in this paper.By adjusting the dry etching parameters,including the applied power,gas flow rate,mask materials and so on,the Ge surfce morphology was characterized by SEM Adding 20%O2 into CF4 could smooth the Ge surface and reduce the surface roughness.Increasing or decreasing the proportion of O2 both lead to the Ge surface roughness increasion.By replacing photography resist with SiO2,large number of carbonaceous materials are reduced and the Ge surface roughness becomes small.As a consequence,dry etching may become a feasible technique of adjusting the Ge film thermal conductivity by optimizing the etching parametersBased on the experiment results,we investigate the impact of phonon-boundary scattering dominated thermal conductivity on self-heating effect of SOI and GeOI MOSFETs at different film thcknesses.For both Si and Ge thin films,the effect of phonon-boundary scattering on thermal conductivity becomes larger as the thickness decreases,which leads to heavier self-heating effect in MOSFETs.At the same Si film thickness,phonon-boundary scattering makes less impact on self-heating effect as the temperature increases.However,there is no such large change of self-heating effect as temperature increases due to the weaker correlation between Ge thermal conductivity ans temperature.Comparing the degradation degree of the on current(Ion_reduction)and the maximum temperature(Max T)in the transistors,the self-heating effect of Si MOSFET is sever than that of Ge MOSFET at low temperature.While as the temperature goes up or the thickness decreases,the distinction of self-heating effect on Si and Ge devices dimishes.Besides,the temperature distribution will largely be affected by phonon-boundary scattering.Therefore,irrespective of phonon-boundary scattering will greatly underestimate the self-heating effect in devices.At last,we investigated the impact of thermal conductivity on self-heating effect under the action of strain.Phase change materials(PCM)of GST,GeTe and Sb2Te3 were adopted as liner stessor.Compressive strain is induced in Si and Ge films by PCM contraction after low temperature annealing.Utilizing the finite elementary numerical method can be calculated.1%,1.5%and 2.8%compressive strain is induced in SOI substrate(TSi,=30 nm)by GST,GeTe and Sb2Te3,respectively.And the strain in GeOI substrate(TGe=13 nm)is 1.0%,2.1%and 3.0%.Then the phonon dispersions of Si and Ge under starin are recalculated by DFT method.Based on the phonon dispersion,thermal conductivity can be obtained according to the Holland model by utilizing the new phonon group velocity.With the increasion of compressive strain,the thermal conductivity of Si and Ge films increases.While the thermal conductivity decreases as the tensile strain increases.Finally,we studied the strain affected thermal conductivity impact on the self-heating effect in MOSFETs by TCAD tools.The results indicates that the thermal conductivity under strain has much less effect on self-heating effect in MOSFETs than that caused by carrier mobility promotion.However,strain technology can still be a feasible method to improve the thermoelectric figure of merit for thermoelectric materials.