Ti, Pd, Au BASED OHMIC CONTACT FOR HIGH SPEED COMPOUND SEMICONDUCTORS

Ohmic contacts are an important integral part of semiconductor devices. Without them, proper connection to other circuit elements would not be possible.

Two commonly used contact schemes for InP-based devices are Titanium / Platinum / Gold (Ti/Pt/Au) for n-InGaAs and Platinum / Titanium / Platinum / Gold (Pt/Ti/Pt/Au) for p-InGaAs. We have found that the evaporation of Platinum (Pt) requires a high temperature over a prolonged duration owing to its high melting point (1769oC) and low vapour pressure (0.14 m mHg). These conditions can cause problems such as in-situ "out-gassing" of impurities within the evaporation chamber, and hardening of the photoresist. Possible implications of these are degradation in yield and reliability. We therefore investigated the feasibility of replacing Platinum (Pt) by Palladium (Pd) in the Ti/Pt/Au and Pt/Ti/Pt/Au contact systems. Both Pd and Pt are group VIII transition metals, and hence have similar properties. In particular, Pd and Pt possess similar resistivity (Pd: 9.93 m W -cm; Pt: 9.85 m W -cm) and workfunction (Pd: 4.99 eV; Pt: 5.32 eV). The advantages with Pd are a lower melting point (1552oC) and a much higher vapour pressure (26 m mHg), which translate into easier and faster evaporation at a lower temperature compared with Pt thus implying the above-mentioned predicaments can be alleviated.

The specific contact resistance, r c, of Ti/Pd, Ti/Pd/Au and Pd/Ti/Pd/Au contacts to p- and n-doped In0.53Ga0.47As are shown in Figure 1 as a function of the rapid thermal anneal (RTA) temperature. The minimum r c’s obtained are comparable to those of the Pt based systems. A contacting Pd layer has been found to be crucial to the formation of a low resistance contact to p+- In0.53Ga0.47As while a Ti contacting layer is desirable in contacts to n+-In0.53Ga0.47As. Pd (100Å) / Ti (200Å) / Pd (200Å) / Au (2000Å) achieved the lowest r C of 9.61´ 10-6W -cm2 to p-In0.53Ga0.47As while the lowest r C of 2.54´ 10-7W -cm2 to n+-In0.53Ga0.47As was demonstrated by Ti (200Å) / Pd (200Å) / Au (2000Å).

Preliminary thermal ageing studies of Ti/Pd/Au and Pd/Ti/Pd/Au contacts have also been performed and have indicated a better stability over the Pt counterparts.

 paper1_image1.gif
Figure 1 Variation of specific contact resistance, r C, as a function of rapid thermal anneal (RTA) temperature for contacts to p-In0.53Ga0.47As (¾ ) and n- In0.53Ga0.47As (- - -): (a) l - Ti (700Å) / Pd (700Å), (b) s - Ti (200Å) / Pd (200Å) / Au (2000Å) and (c) t - Pd (100Å) / Ti (200Å) / Pd (200Å) / Au(2000Å). The anneal duration is 20 s.
 

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Source: Engineering Research News, National University of Singapore, Feburary 1998