PiFM is a viable tool for many semiconductor applications, including the visualization and analysis of area selective deposition (ASD), local strain, the cross-section of a trench, the cross-section of a multilayer stack, chemical mechanical polishing (CMP), buried conductive layers and defect analysis. Please note: for some examples, wavenumbers have been withheld.
Area Selective Deposition
PiFM works well with both organic and inorganic materials. In ASD, organic self-assembled monolayers (SAMs) act as masks to promote or inhibit the atomic layer deposition in selected regions.
In the example below, a SAM selectively covers the metal region and allows alumina to be deposited preferentially on the exposed oxide region. SiO2, SAM, and Al2O3 regions are imaged at 1103, 1471, and 972 cm-1 respectively. Silicon oxide displays well-defined, high contrast. The SAM and alumina are not as localized, though contrast can still be seen between areas of lower and higher concentration.
Visualizing Local Strain
When a material with IR active band (such as silicon oxide) is strained, its IR peak shifts from its unstrained wavenumber. In the example below, SiGe line patterns with varying line widths (from sub 40 nm up to 7500 nm) are generated via a SiGe FinFET process flow (25%). In between the SiGe lines are SiO2 regions, whose absorption peak will shift from an unstrained value of 1122 cm-1 to 1087 cm-1 with growing strain. The signal intensity on imaging the sample at 1122 cm-1 allows visualizing the relative amount of strain: the darker the contrast, the greater the strain.
In the sample below, the oxide experiences greater strain at the SiGe interface; in between the closely spaced SiGe lines, the oxide cannot relax to the unstrained state.
Analyzing Cross-section of a Trench
A filled-in trench in a semiconductor device is cross-sectioned (via cleaving) and imaged by PiFM. Two different materials are highlighted based on the unique IR absorption bands for each material. Twenty-five spectra are acquired at 10 nm intervals across the interface of the two materials. By tracking the intensity of the peak at 1100 cm-1, the transition from one material to the other is seen to take place between spectra 18 and 20.
PiFM provides local chemical information with sub-10 nm spatial resolution across the gradual transition between the two materials.
Cross-section of Multilayer Stack
A multilayer stack grown on silicon is cross-sectioned (via cleaving) and imaged by PiFM. In the three sample sites imaged below, three different materials are highlighted based on the unique IR absorption bands for each material. Excellent spatial resolution of PiFM is demonstrated for “material 1” where PiFM clearly highlights thin layers that are not recognizable in topography.
Visualizing CMP Sample Surface
A sample that has undergone chemical mechanical polishing (CMP) is imaged via PiFM. While metals are not IR active, they can be identified by their dielectric constant at different wavelengths. In the images below, PiFM distinguished between the metal, barrier metal, and oxide regions associated with topography.
Imaging Buried Conductive Layer
Silver nanowires underneath the protective layer are not visible in AFM topography and phase images. However, they are clearly visible in the PiFM image since the near-field from the tip is coupled effectively to the conductive nanowire, creating a strong attractive force even through a rather thick protective layer (> 100 nm). The same principle allows PiFM to image metal layers underneath a dielectric layer (not shown).