Introductory Invited PaperGate metal interdiffusion induced degradation in space-qualified GaAs PHEMTs
Introduction
Recently, GaAs PHEMTs monolithic microwave integrated circuit (MMIC) amplifiers have been demonstrated with superior microwave and millimeter-wave performance (MMW) in order to meet the stringent performance requirements of present and future commercial, military and space applications. As compared to its counterpart—GaAs metal-semiconductor field-effect transistors (MESFETs), GaAs PHEMTs offer several advantages consisting of the improvement of electron mobility, cutoff frequency (Ft), noise figure (NF), output power (Pout), and power-added efficiency (PAE), etc. Accordingly, with the increasing maturity and manufacturability of GaAs HEMTs MMIC technology, GaAs PHEMT MMICs have been extensively inserted for both commercial [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] and military/space applications [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Commercial applications include home and office WLANs (wireless local area networks) [22], LMDSs (local multi-point distributed systems) at 28 GHz [9], [10], [11], cellular handsets [22], automotive radars at 77 GHz [23], [24], [25], [26], fiber-optic networks [22], satellite ground terminals [7] and so forth. Military/space GaAs PHEMT MMICs are mainly used as receivers and transmitters for satellite communications [12], [18], [19], [21], phased-array applications [13], [27], ground based military radar and ground based phased array and communications.
To ensure reliable operation of GaAs PHEMT MMICs in a variety of commercial, military and space applications, it is essential to explore the reliability performance with respect to the thermal reliability [28], [29], [30], [31], [32], [33], [34], [35], [36], hot-carrier-induced degradation (HCID) for power amplifiers under RF-overdrive [37], [38], [39], [40], [41], [42], [43], [44], [45], reliability under humid atmosphere [46], [47], [48], air sensitivity [49], and hydrogen reliability [50], [51], [52], [53], [54], [55]. As a result, it is important to identify the reliability constraint of GaAs PHEMTs depending on their respective applications. For military/space applications, due to the typical hermetic sealing of GaAs PHEMT MMICs in an inert gas environment and hydrogen getter implementation to alleviate the hydrogen reliability concern, the reliability investigation focus has been mainly on thermal reliability and hot carrier reliability. However, hot carrier reliability only becomes a concern when PHEMTs are subjected to significant RF-overdrive for power amplifier applications. Consequently, thermal reliability of GaAs PHEMTs subjected to accelerated temperature lifetest has become a primary emphasis in industry to assure the long-term reliability performance of GaAs PHEMTs over lifetime operation. As a result, in our investigation, the reliability performance of GaAs PHEMT under accelerated temperature lifetest was evaluated in order to assure that the reliability performance of GaAs PHEMTs could meet the stringent requirements in military/space applications.
While accelerated temperature lifetest was used to evaluate the reliability performance in both GaAs MESFETs and PHEMTs, the degradation mechanisms were primarily attributed to ohmic contact degradation [33], [56], [57], deterioration of epilayers [58], gate metal interaction with semiconductor [59], [60], gate metal sinking [61], [62], and others. Despite that most of degradation mechanisms could have been mitigated in the mature technologies of GaAs MESFETs and PHEMTs, gate-sinking-induced degradation (GSID) has still been reported as the major degradation mechanism in GaAs PHEMTs under accelerated temperature lifetest [31], [32], [63], [64], [65]. As a result, it is essential to have an in-depth understanding electrically and physically of GSID in GaAs PHEMTs.
Although gate metal sinking was reported in both GaAs MESFETs [61], [62] and PHEMTs [63], [64], [65], there is still no available information to substantiate the physical evidence of gate metal interdiffusion into semiconductors and how gate metal interdiffusion affects the reliability performance. In the previous investigation of gate metal sinking, backmetal etching, Scanning Electron Microscope (SEM), and Auger Electron Spectroscopy (AES) were used to substantiate the Au interdiffusion through the W and Ti into the GaAs region. Further study of GaAs MESFETs by Donzelli et al. [60] reported the likely formation of different compounds at the metal-GaAs interface without revealing the physical evidence of intermetallics owing to gate metal interdiffusion into the GaAs active layer. Additionally, the reported results from [60], [61], [62] only indicate the ending phenomena of the device failure after significant degradation. As a result, the conclusion of Au–GaAs interdiffusion in MESFETs might not provide the clear evolution of gate-metal-sinking-induced device degradation during the lifetesting in the preliminary, intermediate, and final steps. Investigation of the metal-GaAs interdiffusion [67], [68], [69], [70] that is induced by thermal annealing alone will not represent the true degradation mechanism in GaAs PHEMTs since the contribution from electric fields and channel temperature rise due to device bias is absent.
Nevertheless, the understanding of GSID in GaAs MESFETs has been inherently accepted as the phenomena seen in AlGaAs/GaAs HEMTs that are subjected to accelerated temperature lifetesting [31], [32], [63], [64], [65]. Chou et al. [66] also explained the experimental data of d.c. and RF evolution in GaAs PHEMTs subjected to accelerated temperature lifetest with gate metal sinking mechanism. However, the detailed development of gate metal interdiffusion into AlGaAs Schottky barrier layer in GaAs HEMTs starting from the initial phase of lifetest has still not been revealed. Until recently, Damman et al. [71] claimed the first time to positively identify the Pt diffusion into the semiconductor with a transmission electron microscope (TEM) technique in InP metamorphic HEMTs with Pt/Ti/Pt/Au gate metal. However, the comprehensive electrical/chemical analyses of intermetallic formation due to gate metal interdiffusion, its respective effect on d.c. and RF performance and impact of gate metal interdiffusion on reliability performance in GaAs PHEMTs are still lacking.
In this paper, the physical observation of Ti interdiffusion with Ti/Pt/Au gate metal stacks and its impact on d.c. and RF evolution during the accelerated temperature lifetest in GaAs PHEMTs [72] will be reviewed. Furthermore, the impact of gate metal interdiffusion on reliability performance [73] will be discussed. The results also provide insight into a critical device parameter, Vpo, for optimizing reliability performance.
Section snippets
0.15-μm GaAs PHEMT MMIC Technology
The standard 0.15-μm GaAs PHEMT technology at Northrop Grumman Space Technology (NGST), Redondo Beach, California, USA, was used to manufacture the two-stage Ka-band MMICs for accelerated temperature lifetest. The channel carriers are supplied by two silicon delta doping layers offering two-dimensional electron gas (2-DEG) in the channel with carrier density of 3.56 × 1012 cm−2 and Hall mobility of 4600 cm2/V-s at room temperature. A heavily-doped GaAs layer is used to facilitate the ohmic contact
Lifetest evaluation vehicle and lifetest conditions
A two-stage balanced Ka-band MMIC operating from 34 to 36 GHz was fabricated for the accelerated temperature lifetest. A photograph of this MMIC is shown in Fig. 2. As shown in Fig. 3, the MMIC exhibits a typical gain that is greater than 15 dB and has a noise figure that is approximately 4 dB. In the MMIC, there are four transistors with a total gate periphery of 400 μm. Each transistor has four gate fingers with a total gate periphery of 100 μm. The MMIC also contains passive interconnect,
Failure analysis with FIB/STEM/EDX
In addition to the typical d.c. and RF characterization of the lifetest samples, several parts were pulled out during intermediate lifetest intervals for cross-sectioning to uncover the physical evidence of the gate metal interdiffusion in GaAs PHEMTs. The focused ion beam equipment, FEI model 200 as shown in Fig. 4, using 30 kV Ga+ ions with a beam current of 5 pA (beam size ∼ 100 Å) was used to prepare the cross-section for the subsequent scanning transmission electron microscopy (STEM) imaging.
Evolution of d.c characteristics
Fig. 7 shows the representative evolution of forward and reverse diode characteristics of a 0.15-μm GaAs PHEMT subjected to lifetest at Tambient of 235 °C. It was observed that the reverse Ig at Vg = −6 V increases from the original −3.5 μA to −20 μA and −90 μA after 240 and 552 h, respectively. The amount of reverse Ig increase depends on the elapsed stress time in lifetest and lifetest temperature, which is related to the degree of gate metal interdiffusion. Also, it was noticed that the Schottky
Physical evidence of gate metal interdiffusion
The failure analysis was done on a degraded sample using FIB/STEM/high-resolution EDX techniques to examine the physical evidence of gate metal interdiffusion. In addition, experiments were performed on the virgin samples to measure Dgate-InGaAs-channel and confirm that there is no existing gate metal interdiffusion on samples prior to lifetest. As shown in Fig. 20, no obvious gate metal interdiffusion into AlGaAs Schottky barrier layer was detected on the virgin samples. There is a very thin
Effect of gate metal interdiffusion on reliability performance
With the physical identification of gate metal interdiffusion, we also explored the effect of gate metal interdiffusion on reliability performance in GaAs PHEMTs. To evaluate the effect of the effective Schottky layer thickness underneath the Ti/Pt/Au gate metal stack on reliability performance, we intentionally selected parts with similar recess width while having various Dgate-InGaAs-channel (effective separation between gate metal and InGaAs channel). In addition to the typical d.c. and RF
Reliability performance evaluation
The representative evolution of percentage ΔIdss degradation is shown in Fig. 29. The data shows the linear dependence of , suggesting that gate metal sinking is a diffusion-limited mechanism. The TTF is defined at the elapsed stress time of lifetested devices reaching ΔIdss of −20%. Accordingly, the cumulative failure distribution can be obtained as shown in Fig. 30, exhibiting log-normal characteristics with temperature-independent deviation (sigma) with a value of approximately 0.35. The
Conclusion
While gate metal sinking has been traditionally identified as the primary degradation mechanism in GaAs PHEMTs, there is no physical demonstration of gate metal interdiffusion or understanding of the gate metal interdiffusion effect on reliability performance. The data of gate metal interdiffusion in 0.15-μm GaAs PHEMTs subjected to accelerated temperature lifetest were illustrated in this paper. Electrical characterization and the techniques of FIB, high-resolution EDX, and STEM were used. The
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