Results and discussion

3.3.1 SEPC result by primary electron energy adjustment

Two functional SRAM samples were polished to contact layer and the SEM image was acquired with 1 keV and 5 keV E_PE, respectively. Figure 3-2 shows the SEM image with 1 keV E_PE. In this image, the contrast of the contact can be classified into three levels. The contrast of the polysilicon contact, n⁺/p-well contact, and p⁺/n-well contact shows the low contrast, moderate contrast and high contrast, respectively. Figure 3-3 shows the SEM image obtained with 5 keV E_PE. Contrast in the image also has three levels, but differs trend from that of Fig 3-2. The contrast of the polysilicon contact, n⁺/p-well contact, and p⁺/n-well contact shows the high contrast, low contrast and moderate contrast, respectively. Contrast with different E_PE values behaves differently.

The SEPC arises from different surface potentials after primary electron (PE) irradiation.

The source of surface potential is the yield of the SE, which is not equal to that of the primary electrons. SE yield (δ) is the dividing of SE number by PE number. Figure 3-4 shows the tungsten SE yield (δ) as a function of EPE [24]. The surface potential will be positive charging when the SE yield is larger than 1, and negative charging when the SE yield is < 1. Based on Seiler’s study, the tungsten surface will be positive charging at E_PE=1 keV and negative charging at E_PE=5 keV [24].

Figure 3-5(a) is a schematic showing the contrast behavior when E_PE is 1 keV. According to the traditional SEPC effect, when a sample was exposed to the 1 keV electron beam, a positive charge was generated on the sample surface. On a floating contact, such as a polysilicon contact, the positive charge remained on the surface, and reduced the number of SEs collected by the detector. Thus, the polysilicon contact has low contrast in the SEM image. For a positive charge, the p⁺/n-well is forward biased, such that the positive charge can be discharged through the p⁺/n-well to the substrate. Therefore, the p⁺/n-well contact will be in a higher contrast. Conversely, the n⁺/p-well is reverse biased for the positive charge. Thus, positive charges are seldom discharged through the n⁺/p-well to the substrate and remain on the surface of the contact connected to the n⁺/p-well, such that the contact on n⁺/p-well will be lower contrast. For the grounded contact, the positive charge will be discharged to the substrate, and will not reduce the number of SEs collected by the detector. Thus, the grounded

contact is brighter than the floating contact in the SEM image.

Figure 3-5(b) shows a schematic explaining contrast behavior when E_PE is 5 keV. A negative charge will result on the sample surface (Fig. 3-3). Under this negative charging condition, the negative charge will be maintained on the polysilicon contact surface and the number of SEs collected by the detector will increase; the polysilicon contact is bright in the SEM image. For negative charging, the p⁺/n-well is reverse biased, and the negative charge cannot be discharged easily through the p⁺/n-well; thus, the p⁺/n-well contact will have high contrast. The n⁺/p-well contact is forward biased for the negative charge, such that the negative charge can be discharged through n⁺/p-well to the substrate. The n⁺/p-well contact will be low contrast in SEM image. For a grounded contact, the negative charge will be discharged to the ground and will not increase the number of SEs collected by the detector;

thus, the grounded contact is darker than the floating contact in the SEM image.

Table 3-1 summarizes the contrast behavior of contacts under the 1 keV and 5 keV E_PE conditions. According to table 3-1, identifying the defective contact is easy when SEM images were acquired under both 1 keV and 5 keV.

3.3.2 Application of primary electron energy adjustment in defect isolation

The sample is a 0.15-µm SRAM chip that suffers a single bit failure. The sample is planar polished to Metal 1 layer for SEPC inspection to find any abnormality in the Metal 1

layer. Figure 3-6 shows the SEM image under 1 keV E_PE. However, no abnormality was identified in the SEM micrograph. Thus, E_PE was increased to 5 keV and another SEM micrograph was acquired, as shown in Fig. 3-7. One C-shaped Metal 1, which acts as the storage node of SRAM, is significantly brighter than the other C-shaped Metal 1. Thus, a cross-sectional inspection is performed by FIB, which reveals a porous n⁺/p-well contact in the failing cell, as shown in Fig. 3-8.

The abnormal SEPC from this sample cannot be identified at E_PE=1 keV because three contacts are under Metal 1 layer: one connected to the p⁺/n-well another connected to the n⁺/p-well, and the last connected to polysilicon. When the sample is exposed to a 1 keV E_PE condition, positive charges were generated on the sample surface. According to the principle of SEPC described previously, positive charges can be discharged by the contact connected to the p⁺/n-well. Thus, each Metal 1 can discharge its positive charges via its normal contact to the p⁺/n-well and all Metal 1 SEPC would be normally bright. In this case, the defect was an open contact connected to the n⁺/p-well. Therefore, one cannot detect this defect by E_PE=1 keV. Conversely, negative charges will be generated on the sample surface when the sample is exposed to E_PE=5 keV. Negative charges will be discharged by the normal n⁺/p-well contact for all normal cells except the open contact. Thus, the negative charges were not discharged on abnormal cells, and would increase the number of SEs collected by the detector; thus, abnormal C-shaped Metal 1 was brighter than other metals. With the E_PE=5 keVcondition,

this defect may be identified because high resistance located on n⁺/p-well contact cannot be identified when E_PE=1 keV.