Gate metal interdiffusion induced degradation in space-qualified GaAs PHEMTs

While gate metal sinking has been traditionally identified as the primary degradation mechanism in GaAs pseudomorphic high electron mobility transistors (PHEMTs), there is no physical demonstration of gate metal interdiffusion or understanding of the gate metal interdiffusion effect on reliability performance. This paper reviews our results on gate metal interdiffusion in 0.15-μm GaAs PHEMTs subjected to accelerated temperature lifetest. We used the techniques of focused ion beam (FIB), high-resolution energy-dispersive analysis with X-ray (EDX), and scanning transmission electron microscope (STEM). These results substantiate the observed d.c. and RF parametric evolution with respect to reverse gate leakage current (I_g), ideality factor, Schottky barrier height (Φ_BN), transconductance (G_m), I_dss, pinchoff voltage (V_po), S21, and provide insights into the effect of gate metal interdiffusion on reliability performance. The comprehensive understanding of gate metal interdiffusion induced degradation is essential for GaAs PHEMTs due to their widespread military/space applications.     

Correlating gate sinking and electrical performance of pseudomorphic high electron mobility transistors

Ti interdiffusion from the Ti/Pt/Au gate into the AlGaAs Schottky barrier layer (SBL) of 0.25-μm GaAs Pseudomorphic High Electron Mobility Transistors (PHEMTs) has been studied using the accelerated life testing technique. Based on measurements and modeling, analytical expressions for quantitative correlation between the positive pinch-off voltage (V_P) shift as well as the saturation drain current (I_Dsat) decrease and the physical damage occurring during gate sinking has been developed. It is suggested that the main cause for device failure is the growth of the TiAs phase leading to the decrease in the SBL thickness. Additionally, it is suggested that V_P may be used as a better indicator for device degradation than I_Dsat since it is linearly proportional to the degrading physical characteristic – the Schottky barrier layer thickness.     

Structural characterization of Ti and Pt thin films on GaAs(100) substrate

In an attempt to understand the Schottky barrier behavior of Ti/Pt/GaAs and Pt/Ti/ GaAs bimetal Schottky diodes, we have investigated the interfacial morphology of Ti and Pt thin films on GaAs(l00) substrate. The characterization was based on coverage profiling of Auger electron spectroscopy in conjunction with transmission electron microscopy. Emphasis was placed on film uniformity and atomic interdiffusion. The results showed Ga and As outdiffusion in Pt/GaAs interface and some oxygen incorporated in Ti film, but no evidence of clustering for both metal/GaAs systems.     

The effect of gate metal interdiffusion on reliability performance in GaAs PHEMTs

While Ti metal interdiffusion of Ti-Pt-Au gate metal stacks in GaAs pseudomorphic HEMT (PHEMTs) has been explored, the effect of Ti metal interdiffusion on the reliability performance is still lacking. We use a scanning transmission electron microscopy technique to correlate Ti-metal-InGaAs-channel-separation and Ti-sinking-depth with a threshold voltage V/sub T/. It has been found that Ti-sinking-depth is insensitive to V/sub T/. However, Ti metal interdiffusion reduces the separation of the gate metal and InGaAs channel, thus affecting the I/sub dss/ degradation rate. Accordingly, we observe the dependence of /spl Delta/I/sub dss/ on V/sub T/. Devices with less negative V/sub T/ exhibit inferior reliability performance to those devices with more negative V/sub T/. The results provide insight into a critical device parameter, V/sub T/, for optimizing reliability performance based on I/sub dss/ degradation.     

Hot-electron effects on AlGaAs/InGaAs/GaAs PHEMTs under accelerated DC stresses

The behavior of Schottky gate characteristics before and after hot-electron stress has been a relatively neglected topic. Thus, this paper discussed the effects of hot-electron accelerated stress on the DC characteristics of AlGaAs/InGaAs/GaAs PHEMTs as they relate to Schottky gate characteristics. It also presents studies of reverse Schottky gate characteristics before and after hot-electron stresses, as related to two major mechanisms: (1) the widening of the depletion region under the gate; and (2) the impact of the carriers trapped under the gate. The former induces a larger Schottky barrier height with a smaller reverse leakage current density than the latter, while the latter induces the opposite. Two hot-electron conditions are used to investigate the impact of the hot-electron stress on the gate leakage current. The gate leakage current decreases after a hot-electron stress, due the effect of hot-electron stress on the Schottky diode characteristics. Moreover, improvement in the noise performance is expected, due to the decrease in the gate leakage current. Both pre- and post-stress noise measurements have been done to demonstrate this.     

Copper-Airbridged Low-Noise GaAs PHEMT With$hboxTi/hboxWN_x/hboxTi$Diffusion Barrier for High-Frequency Applications

A GaAs pseudomorphic HEMT (PHEMT) with Cu-metallized interconnects was successfully developed. Sputtered$hboxWN_x$was used as the diffusion barrier and Ti was used as the adhesion layer to improve the adhesion between$hboxWN_x/hboxCu$interface in the thin-metal structure. After copper metallization, the PHEMTs were passivated with silicon nitride to avoid copper oxidation. The Cu-airbridged PHEMT showed the saturation$I_ DS$was 250 mA/mm and the$g_m$was 456 mS/mm. The Ti adhesion layer plays a significant role on the$g_m$and$V_p$uniformity of the Cu-metallized PHEMTs. The GaAs PHEMTs with$hboxTi/hboxWN_x/hboxTi/Cu$multilayer have better noise figure and associated gain than those of the devices without the Ti adhesion layer. The fabricated Cu-metallized GaAs PHEMT with$hboxTi/hboxWN_x/hboxTi/Cu$multilayer has a noise figure of 0.76 dB and an associated gain of 8.8 dB at 16 GHz. The cutoff frequency$(f_T)$is 70 GHz when biased at$V_ DS = hbox1.5 V$. These results show that the$hboxTi/hboxWN_x/hboxTi$multilayer can serve as a good diffusion barrier for Cu metallization process of airbridge interconnects on GaAs lownoise PHEMTs.     

Potential barrier formation at a metal-semiconductor contact using selective removal of atoms

The possibility of forming a potential profile in a semiconductor by forming a metal film on its surface via selective removal of oxygen atoms from a deposited metal oxide layer was studied. Selective removal of atoms (SRA) was performed using a beam of accelerated protons with an energy of about 1 keV. Epitaxially grown GaAs films with a thickness of ～100 nm and an electron concentration of 2×10¹⁷ cm^?3 were chosen as the semiconductor material, and W obtained from WO₃ was used as the metal. The potential profile appeared due to the formation of a Schottky barrier at the metal-semiconductor interface. It was found that the Schottky barrier formed at W/GaAs contacts made by the SRA method is noticeably higher (～1 eV) than the barrier formed at the contacts made by conventional metal deposition (0.8 eV for W/GaAs). The data presented indicate that there is no damaged layer in the gate region of the structures, which is most strongly affected by the proton irradiation. Specifically, it was shown that the electron mobility in this region equals the mobility in bulk GaAs with the same doping level.     

Failure mechanisms of Schottky gate contact degradation and deep traps creation in AlGaAs/InGaAs PM-HEMTs submitted to accelerated life tests

In this work we analyze degradation phenomena observed inpseudomorphic AlGaAs/InGaAs HEMTs with Al/Ti gate metallization, which have been submitted to accelerated tests at high drain-source voltage V_Ds and high power dissipation P_D. After these tests, we observe permanent degradation effects, consisting in electron trapping in the gate-drain access region, with consequent decrease in the longitudinal electric field and “breakdown walkout”, and in thermally-activated interdiffusion of the AI/Ti gate with decrease in the gate Schottky barrier height and increase in drain saturation current I_D. Rather than causing a degradation of therf characteristics of the device, these phenomena induce an increase in the associatedrf gain at 12 GHz, the other rf characteristics being almost unchanged. Overall, the most relevant failure mode observed is an increase of low-frequency transconductance.     

A low-gate-leakage-current GaAs MESFET with a thin epitaxialsilicon layer

An n-channel depletion-mode GaAs MESFET with an Al gate and a 6-A epitaxial Si layer between the metal and the GaAs, grown in situ by molecular beam epitaxy, is described. Its DC electrical characteristics are compared with a similar control structure grown without the Si layer. The gate leakage current in the Al/Si/GaAs MESFETs was three to four orders of magnitude lower than in the control structure, due to all increased barrier height in the Al/Si/n-GaAs Schottky gate of 1.04 eV, versus 0.78 eV for the Al/n-GaAs structure. The differences in threshold voltages, I-V characteristics, and transconductances between the two devices are consistent with an enhanced effective barrier height for the Al/Si/GaAs MESFET     

Effects of interfacial thin metal layer for high-performance Pt-Au-based Schottky contacts to AlGaN-GaN

Schottky diodes with Ni-Ti-Pt-Au Schottky electrodes on AlGaN-GaN heterostructures were fabricated and subjected to rapid thermal annealing. The electrical influence on them was investigated in terms of the existence of a thin Ni or Ti layer. The diodes of the Ni-Pt-Au system showed a drastic improvement in their electrical properties, such as an increase in the Schottky barrier height and a decrease in the leakage current, after the 600/spl deg/C treatment whereas the thermal annealing effect was found to be small in the Ti-Pt-Au and the Pt-Au systems. The Ni was considered to play a significant role in realizing a clean Pt contact to AlGaN and reducing surface traps, which were revealed from Auger electron spectroscopy measurement and frequency-dependent capacitance-voltage measurement, respectively. The thermally-treated Ni-Pt-Au gate electrode was concluded to be practicable for realizing high performance HEMTs.     

The device characteristics of Ir- and Ti-based Schottky gates AlSb/InAs high electron mobility transistors

In this work, the InAs/AlSb high electron mobility transistors (HEMTs) on GaAs semi-insulating substrate using refractory iridium (Ir) gate technology was proposed. The Ir-gate exhibited a superior metal work function which was beneficial for increasing Schottky barrier height of InAs/AlSb heterostructures to 0.58 eV. Compared to the Ti-gate HEMT, the Ir-gate HEMT shows higher threshold voltage and lower gate leakage current owing to its higher Schottky barrier height and higher melting point. Moreover, the Ir-gated HEMT also shows the manifest stability improvement of DC characteristics under hot carrier stress as the Ti and As diffusion is alleviated.     

Studies of aluminum Schottky barrier gate annealing on GaAs FET structures

Controlled annealing experiments on special GaAs FET structures have been used to assess the integrity of Al/nGaAs Schottky barrier gates. Metallurgical properties of the Al/nGaAs interface were analyzed using microspot Auger electron spectroscopy. Results indicate interdiffusion between Al and GaAs at the baseline annealing temperature of 275°C. At this temperature the three terminal static FET parameters were unaffected by the annealing, but the two terminal gate/source static characteristic was somewhat improved over unannealed control FETs. Annealing the Al gate up to 450°C does not appear to be harmful to the gate/source two terminal static characteristic although at this temperature three terminal static FET characteristics are severely degraded. The Al/nGaAs interface is assessed as being both metallurgically and electrically stable when employed in inert ambient environments up to 275°C for 24 hr.     

Control of threshold voltage of AlGaAs/GaAs 2DEG FET's through heat treatment

Threshold voltage controls of Ni/Ti/Au gate and Ti/Au gate 2DEG AlGaAs/GaAs FET's through only heat treatment are investigated. Ni/Ti/Au gate FET's vary over quite a wide range from a depletion mode to an enhancement mode without degradation of FET characteristics after heat treatment at 300°C. The same experiment is made for Ti/Au gate FET's, but the threshold voltage change is negligibly small. It is confirmed that Ni/Ti/Au can be used as the gate metal for E-FET and Ti/Au as the gate metal for D-FET under simultaneous heat treatment. In addition, a mechanism for penetrating the barrier metal into the underlying layer is discussed.     

Noise characteristics of InGaP-gated PHEMTs under high current and thermal accelerated stresses

The degradation mechanisms of the noise characteristics of InGaP-gated low-noise pseudomorphic high-electron mobility transistors (PHEMTs) under accelerated stresses through dc and thermal stresses are investigated. The devices used were metal-organic chemical vapor deposition-grown In/sub 0.49/Ga/sub 0.51/P/In/sub 0.15/Ga/sub 0.85/As/GaAs low noise PHEMT structures with the gate dimensions of 0.25/spl times/160 /spl mu/m/sup 2/. The key noise/effect parameters of devices including 1) related to the deep-trap behavior in device, 2) source/gate resistances, and 3) gate to source capacitance and intrinsic transconductance are discussed. Based on the dc characteristics under dc and thermal stresses, the variations of the current-voltage curve, the diode characteristics (Schottky gate) with related trapping/detrapping phenomena induced by impact ionization and the variation of the depletion in gate-drain region are also investigated. The high reliability of InGaP low noise PHEMTs is demonstrated by the extremely small variations of the minimum noise figure and the associated power gain at 12 GHz after dc and thermal stresses.     

Degradation mechanism of GaAs MESFETs

The reliability of the Au/Pt/Ti Schottky gate of low-high doped GaAs MESFETs has been investigated by thermal step stress and accelerated life tests and their degradation mechanisms were analyzed by means of Auger electron spectroscopy, X-ray diffractometry, cross-sectional transmission electron microscopy, current-voltage, and capacitance-voltage measurements. Electrical measurements showed that the failure of the GaAs MESFETs was mainly due to the degradation of the Au/Pt/Ti/GaAs Schottky contact. An activation energy of 1.3 eV and a lifetime of 2 × 10⁸ h at 125°C for Schottky contact were evaluated. At a temperature lower than 350°C, the degradation of the Schottky contact is attributed to the decrease of net electron concentration caused by outdiffusion of host Ga atoms of GaAs. The activation energy for the decrease of net electron concentration is determined to be 1.4 eV using the capacitance-voltage measurement, which is consistent with 1.3 eV obtained by the accelerated life tests. This suggests that the major thermal degradation mechanism at a temperature lower than 350°C is the outdiffusion of Ga atoms from the channel. Meanwhile, the effective channel thickness at a temperature higher than 350°C is reduced by the formation of TiAs at the Schottky interface, the activation energy of which is determined to be 1.74 eV.     

Reliability problems in TTL-LS devices

An analysis of the possible failure modes of TTL-LS was carried out, with particular emphasis on long term thermal stability of Schottky diodes, all realized by PtSi - Ti/W - Al metallization system.Only from one supplier were there diodes in which changes in the characteristics, due to metallization defects, were observed during accelerated thermal cycles. On the contrary, once its uniformity is guaranteed, the Ti/W barrier layer inhibits the Si-Al interdiffusion up to 550 °C, 1 hr thermal annealing.Sensitivity to negative input pulses and electrical overstresses were also investigated.Scanning electron microscope and microprobe, and voltage contrast techniques seem to be more suitable instruments to investigate the reliability of such devices than the conventional life tests.The technology employed today seems provide suitable reliability for TTL Low-Power Schottky devices.     

Effect of Gate Sinking on the Device Performance of the InGaP/AlGaAs/InGaAs Enhancement-Mode PHEMT

An enhancement-mode InGaP/AlGaAs/InGaAs pseudomorphic high-electron mobility transistor using platinum (Pt) as the Schottky contact metal was investigated for the first time. Following the Pt/Ti/Pt/Au gate metal deposition, the devices were thermally annealed at 325 degC for gate sinking. After the annealing, the device showed a positive threshold voltage (V_th) shift from 0.17 to 0.41 V and a very low drain leakage current from 1.56 to 0.16 muA/mm. These improvements are attributed to the Schottky barrier height increase and the decrease of the gate-to-channel distance as Pt sinks into the InGaP Schottky layer during gate-sinking process. The shift in the V_th was very uniform across a 4-in wafer and was reproducible from wafer to wafer. The device also showed excellent RF power performance after the gate-sinking process     

GaAs基InAlAs/InGaAsMHEMT直流特性研究

报道了用 MBE技术生长的 Ga As基 In Al As/In Ga As改变结构高电子迁移率晶体管 (MHEMT)的制作过程和器件的直流性能。对于栅长为 0 .8μm的器件 ,最大非本征跨导和饱和电流密度分别为 3 5 0 m S/mm和1 90 m A/mm。源漏击穿电压和栅反向击穿电压分别为 4V和 7.5 V。这些直流特性超过了相同的材料和工艺条件下 Ga As基 PHEMT的水平 ,与 In P基 In Al As/In Ga As HEMT的性能相当     

用于GaAs数字IC的难熔栅自对准技术研究

本文用直流磁控溅射方法在离子注入n型GaAs衬底上制备了WSi_xN_y难熔金属膜,研究了它的热稳定性、界面和势垒特性.同时对WSi_(0.6),W,WN等难熔栅金属膜也进行了研究.AES和SIMS分析表明,WSiN/GaAs的界面通过1000℃,10秒钟快速退火(RTA)或850℃,20分钟常规炉退火处理仍保持稳定,势垒高度达到0.8V,理想因子n=1.1.制作了WSiN栅自对准(SAG)增强和耗尽型MESFET.其跨导分别为154mS/mm和250mS/mm.用这一工艺制作的运放差分输入电路从直流到1千兆赫增益达29.5dB.     

Inverted gate GaAs MESFET by epitaxial liftoff

A GaAs film deposited on metal by epitaxial liftoff can form a Schottky barrier. This film is used to make a 1 mu m gate length inverted gate GaAs metal-semiconductor field-effect transistor (MESFET) that can be pinched off by the inverted gate.<>