Bump bonding employing solder bumps, such as flip chip, is used for electronic component mounting involving electrical connections between a pixel-type CdTe detector (radiation detector) and a read-out circuit, for example. It is known that UBM is formed on connecting electrodes in bump bonding (Patent Document 1, for example).
FIG. 1 schematically shows the bump bonding process between a CdTe (cadmium telluride) detector 1 provided with existing UBM, and an ASIC 2 which is a read-out circuit thereof. A pixel-type electrode 1a employing Al (aluminum), Ti (titanium) or Pt (platinum) etc. is formed by way of vapor deposition or plating etc. on the surface of the CdTe detector 1. A UBM layer 1b comprising an Au/Ni/Au (gold/nickel/gold) composite layer is deposited by way of vapor deposition or the like on the electrode 1a. The thickness of the UBM layer 1b in this process (the thickness of the whole composite layer) is set at around several hundred nanometers. A protective passivation layer 1c is formed on the surface of the CdTe detector 1 on which the UBM layer 1b was formed. The passivation layer 1c has an opening 1d for exposing the UBM layer 1b for the purposes of bump bonding. The ASIC 2 is also provided with a pixel-type electrode 2a corresponding to the electrode 1a, a UBM layer 2b and a passivation layer 2c, and a solder bump 3 is formed in the opening of the passivation layer 2c. 
The main aims in forming UBM are to form a connection between the solder bump and the electrode which is mechanically and electrically very stable, and to prevent diffusion of solder into the electrode. However, existing UBM structures do not adequately prevent solder diffusion. FIG. 2 is an SEM photograph showing the main parts in cross section of the UBM structure shown in FIG. 1 after bump bonding, and as can be seen, the UBM layer has been completely consumed and blended in with a solder bump 3, and ultimately the solder diffuses into the electrode 1a. Solder diffusion affects the performance of the CdTe detector and the reliability of the bump bonding connection.
[Patent Document 1] JP 2009-182741 A