One of the most prevalent failure modes of spot defects is the shorting of electrical nodes in the circuit. These failures are called bridges. Carafe classifies bridge faults into two categories: inter-layer and intra-layer. Inter-layer bridges occur when the layer of insulating material between areas of conducting material is compromised and allows the conducting areas to touch each other. Intra-layer bridge faults occur when extra conducting material is present between two regions of a given type of material causing them to become electrically connected together. Carafe makes no assumption about what layers can be bridged, and will only extract faults in or between layers listed in the bridge section of the technology file.
Carafe identifies bridge faults by the names of the nodes involved in the fault. Thus, failures that bridge the same set of nodes together are considered the same fault even though each failure may occur on different layers of material or in different areas of the circuit.
The bridge fault in Figure 4.2 is an example of an intra-layer bridge fault caused by a spot defect falling between Node 1, Node 2, and Node 3. This bridge fault would be reported as
(Node1 to Node2 to Node3)indicating a 3-way bridge fault.
Figure 4.2: A 3-way intra-layer bridge fault
Consider the inter-layer bridge fault shown in Figure 4.3 (a). Since inter-layer bridges are modeled as missing insulator, Node 1 and Node 2 would short together, Node 3 and Node 4 would short together, but no other bridging would result from this particular defect. Thus, this bridge fault would be reported as:
(Node1 to Node2) (Node3 to Node4)indicating the two separate bridges in the same fault. If Node 1 and Node 4 were actually the same node as in Figure 4.3 (b), this fault would be reported as:
(Node1 to Node2 to Node3)which is the same fault as in Figure 4.2.
Figure 4.3: (a) A 2 by 2 inter-layer bridge fault. (b) A 3-way inter-layer bridge fault.