Reliability of BCB passivated InAlAs/InGaAs HEMTs under thermal stress

InP-based HEMTs have been regarded as promising devices for ultra-high speed applications [I]. For practical system applications, the device reliability is one of the most critical issues. Extensive works have been done on the reliability of AIGaAs/(In)GaAs pHEMTs and InP-based HEMTs [I-21. Recently, the device performance improvement of benzocyclobutene (BCB) passivated GaAs pHEMT has been reported demonstrating the advantages of its low dielectric constant and a low loss tangent of BCB [3]. But, up to date, there has been no report on the reliability study on BCB passivated InP-based HEMTs to our knowledge. In this study, the photosensitiveBCB layer was applied to InAlAsiInGaAs HEMTs for passivation and their thermal reliability characteristics were investigated and compared with the S i N x and polyimidepassivated devices for the first time. The designed HEMT layer structure is shown in Table I. For device fabrication, the mesa was defined by wet chemical etching. For ohmic contacts, AuGeiNiiAu were evaporated and alloyed resulting in a contact resistance of 0.085C2mm The thickness of Ni layer was optimized to have a minimum resistance variation during the thermal stress [4]. The conventional succinic acidHZOz solution was used for mesa-sidewall and gate recess etching. The Schottky gate was formed by evaporation of TUAu metal. The fabricated unpassivated-HEMTs with a 1.2pm gatelength showed good pinch-off characteristics with V, of -0.W and a maximum DC-transconductance g, of 398 mS/mm as shown in Fig. 1. The peak fr and f, measured at Vb=2.5V, estimated 60m -2OdB/decade extrapolation, were 25.7GHz and 53.1GH2, respectively. For fair comparison, the devices having I . , and g, variations within 5% were chosen and passivated by S N X (SiN-HEMT), polyimide (PI-HEMT) and BCB (BCB-HEMT). A 9Onm SiN. was deposited with Si”H,/He gas mixtures using an RPCVD for the SiN-HEMT. For the PIand BCB-HEMT, the polyimide and photosensitive-BCB were spin coated and cured at 200°C during 3Omin in nitrogen atmosphere, respectively. The devices were fmt characterized prior to thermal stressing, and during thermal stressing at 200°C for a period of 1006rs. The devices were not biased and were exposed to air in a closed oven during thermal stress. As shown in Fig. 1, g, is reduced by 5.6% in the BCB-HEMT and that of PI-HEMT is reduced by 4.5% after passivation. Compared to the PIand BCB-HEMT, the g, degadation in the SWHEMT (25%) at a high V, is significant due to the surface modification by the inherent CVD RF damage [5]. In all cases, some degradation of DC-characteristics was observed after passivation due to the increase of access resistance as shown in Fig. 3. Id and g, variation during thermal stress are shown in Fig. 2. Id and degradation of the SiN-HEMT were recovered by stabilition bake [I], whereas the PI-HEMT showed most severe degradation. The BCB-HEMT showed most stable DC-characteristics during thermal stress. In order to investigate the effect on thermal stability, parameter extraction was performed and analyzed based on the measured S-parameters. As shown in Fig. 3, the variation of increased ohmic resistance in the BCB-HEMT is minimal due to the low moisture uptakeand low barrier oxidationcharacteristics of BCB. Cutoff 6quencies of the fabricated devices were also characterized to determine the impact on the microwave performance as shown in Fig. 4. The fr of devices showed a similar behavior to that of &during thermal stress according to its relation to &. The f,, of the PI-HEMT is dramatically degraded after thermal stress, so polyimide is thought to be inadequate for passivation of InAlAslInGaAs HEMT. In an initial stage of thermal test, the BCBand SN-HEMT, having suppressed leakage path, showed enhanced f , , values resulting from the increased output resistance of rb by 33% and 43% for 24hrs’ test, respectively. After thermal stress of lOOhrs, degradation off& was observed for both devices and it was affected by g, rather than r&. In summary, JnAIAs/lnGaAs HEMTs passivated by different dielectric materials were fabricated and their performances were investigated prior to thermal stress and during thermal stress. The SiNxand BCB-passivated HEMTs show good device characteristics during reliability test. However, the BCB-passivated HEMTs have distinct advantages of much simpler fabrication process and reduced surface damages together with good thermal stability, sufficient for consideration as an attractive alternative to the conventional SiNx passivation technique.