Characterization and Preparation of Cross-Sectional Semiconductor IC Device Sample for Microscopy Analysis by Gatan Inc.

Topics Covered

Background
Scanning Electron Microscopy (SEM) Characterization of Semiconductor IC Device
Preparation of Cross-Sectional Semiconductor IC Device
Ion Beam Etching by Gatan Precision Etching and Coating System (PECS^TM)
Analyzing Areas of Interest in Cross-Sectional Semiconductor IC Device
Mechanical Preparation of Cross-Sectional Semiconductor IC Device
Ion Beam Etching and Coating Requirements
Identifying Irregularities Between Boundaries of Compound
Summary

Background

Gatan Inc. is the world's leading manufacturer of instrumentation and software used to enhance and extend the operation and performance of electron microscopes.

Gatan's products, which are fully compatible with all brands of electron microscopes, cover the entire range of the analytical process from specimen preparation and manipulation to imaging and analysis. Our customer base spans the complete spectrum of end users of analytical instrumentation typically found in industrial, governmental and academic laboratories.

Scanning Electron Microscopy (SEM) Characterization of Semiconductor IC Device

SEM Characterization of semiconductor IC device packages is extremely essential for failure analysis and for evaluation of microelectronic package reliability. Observation in cross-section provides a wealth of information about the IC device such as layer thicknesses, layer structures, grain sizes of various crystals in the layers and the existence of voids and delaminations.

Preparation of Cross-Sectional Semiconductor IC Device

Preparation of cross-sections involves three broad steps: cutting, mechanical polishing and etching. Etching can be performed using chemical reagents or an ion beam. Not all materials have known chemical etchants, and dealing with chemicals can be hazardous. As a result, there has been an increased interest in ion beam etching. This article discusses one such application of ion beam etching to the preparation of an IC device sample.

Ion Beam Etching by Gatan Precision Etching and Coating System (PECS^TM)

Ion beam etching was accomplished using the Gatan Precision Etching and Coating System (PECS^TM).

Analyzing Areas of Interest in Cross-Sectional Semiconductor IC Device

An illustration of the sample in cross-section, cut along the desired cutting plane, is shown in Fig 1.

Figure 1. Schematic illustration of sample in cross-section

Also indicated in Fig 1 are seven analytical areas of interest:

• Boundary between the Copper lead and the silver spot, and between the silver spot and compound.
  
• Boundaries between Copper lead, Silver spot, Silver epoxy and Compound.
  
• Boundary between Passivation layer on the die and Compound.
  
• Boundary between Silver epoxy and compound.
  
• Boundary between the Gold ball and the Aluminum pad on die.
  
• Boundary between the Copper lead and the silver spot, and between the silver spot and silver epoxy.
  
• Boundary between the Die and silver epoxy.

Mechanical Preparation of Cross-Sectional Semiconductor IC Device

The bulk sample was cut along the desired cutting plane using a diamond saw. The cut sample was mechanically polished in the following order: 30 micron SiC paper, 15 micron SiC paper, 5 micron SiC paper, 1 micron Alumina lapping sheet, 0.25 micron Silica lapping sheet and 0.05 micron colloidal Silica suspension.

Ion Beam Etching and Coating Requirements

The sample was then etched in the Gatan PECS using these parameters:
1. 4.5 keV, 200 µA, 65o fixed angle, 20 min. (High angle polishing removes surface damage.)
2. 4.5 keV, 200 µA, 10o rock, 6 min. (Etching step at near-normal incidence)

The sample was immediately coated in the PECS with Pt using these parameters:
1. 6 keV, 200 µA, 30o rock, 2.5nm Pt.

Identifying Irregularities Between Boundaries of Compound

As seen in Fig. 2, a clear demarcation is achieved in Area 1 between the compound and Silver as well as between Silver and Copper. Irregularities at the boundary between Silver and Compound are revealed. No irregularities are seen at the boundary between Copper and Silver. The grain structure of both Copper and Silver is clearly visible as are the Porosity defects in Silver.

Figure 2. SEM micrograph of “Area 1”

Good differentiation is achieved in Area 2 among the Compound, Copper and Silver layers. Delamination between Silver and Compound is seen at the side (Fig. 3) but not at the top (Fig. 4).

Figure 3. SEM micrograph of “Area 2” (side region) showing delamination between Copper and Compound

Figure 4. SEM micrograph of “Area 2” (top region) showing no delamination between Copper and Compound

In Area 3 (Fig. 5) and Area 4 (Fig. 6), we see a regular and continuous boundary between the passivation layer and the Compound and between the Silver epoxy and compound respectively. The grain structure of Silver in the epoxy is visible as well.

Figure 5. SEM micrograph of “Area 3”.

Figure 6. SEM micrograph of “Area 4”.

The grain structure of the Gold ball, conforming to the flow pattern, and the boundary between the Gold ball and the passivation layer are clearly observed in Fig 7. The presence of cavities at the fore mentioned boundary is noted.

Figure 7. “SEM micrograph of “Area 5”.”

Area 6 and Area 7 revealed distinct, regular boundaries between Copper and Silver spot, Silver spot and Silver epoxy, and the Die and Silver epoxy.

Summary

Cross-sectional preparation of the IC device package was successfully achieved through etching with the PECS. Ion beam etching revealed discrete boundaries between the various layers in the sample as well as defects in some areas along the boundaries. Ion beam etching was also able to bring out the grain structures in all crystalline components of the sample. Thus, the sample preparation technique resulted in a high quality sample for SEM studies.

Source: Preparation of a cross-sectional semiconductor IC device sample for SEM observation

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