Failure analysis of bond pad metal peeling using FIB and AFM

Abstract

Aluminum bond pads on semiconductor chips play an important role in chips functionality and reliability. Bond pad peeling during wire bonding process results in yield reduction. The failure mechanisms of the peeling must be identified so that potential reliability problem of poor bond pad adhesion can be avoided. In this work, FIB, SEM, EDX, and AFM are used to identify the root causes of the peeling. The possible root causes are found to be the presence of an extra layer of thickness of 0.14 m and the poly-silicon surface roughness asperity due to prolonged BOE etching time.

Process windows for low-temperature Au wire bonding

“…The bond pad with high surface roughness requires a higher energy to overcome the high surface asperity before a bond could be formed. 1,16 During the bonding process, the wire was highly deformed to allow the free electrons to move across the interface to form metallic bonds.…”

Role of process parameters on bondability and pad damage indicators in copper ball bonding

“…Due to its higher hardness compared to Au wire, higher normal and ultrasonic forces are often used in Cu ball bonding, resulting in %30% higher bonding stress [12] acting at the bond pad. The higher pad stress increases the likelihood of chip damage such as pad cracking [13,14], pad splash [15][16][17], silicon cratering [18][19][20]. This paper focuses on hardness related issues in Cu ball bonding to Al bond pad (Cu-Al process).…”

Effect of BOE etching time on wire bonding quality

“…The failure mechanisms are found to be either the surface morphology of the poly silicon underneath the bondpad metal or the extra metal layer on top of the poly silicon [2]. While the presence of extra metal layer can be eliminated by tight process control, it is found that the surface morphology of polycrystalline silicon (poly-silicon) depends on the buffered oxide etch (BOE) etch time before the bondpad metal deposition [2]. Hence, it is believed that BOE etch time will have an effect on wire bonding quality.…”

“…Although the RMS roughness has now almost gone out of practical usage [4], we include the parameter as the atomic force microscopy (AFM) measurement provides this parameter value, as well as it tends to be more sensitive than the average roughness to large deviation from the mean line [4]. Localization factor was introduced by the previous work [2], and its physical significant is the degree of localization of the high roughness peaks. The definitions of these parameters can be defined as [2], [4], [5] Average roughness where is the height of the surface measured from the mean line, and is the profile sample length in the -direction ( Fig.…”

“…Localization factor was introduced by the previous work [2], and its physical significant is the degree of localization of the high roughness peaks. The definitions of these parameters can be defined as [2], [4], [5] Average roughness where is the height of the surface measured from the mean line, and is the profile sample length in the -direction ( Fig. 1).…”