As the technology node keep on decreasing, the need for advanced metrology tool is essential to meet the challenges grown by this device miniaturization. The patterning of samples using Focused Ion Beam method is a very popular technique in which various researches are ongoing to improve the operation further. The current best outcome of the research is using Helium Ion Microscope (HIM) which has very high resolution, high material contrasts, good control over charge and high sensitivity for the surface. The valuable information obtained from HIM exceeds and faster from that of SEM and other famous microscopes. HIM provides SEM-like ease-of-use with TEM-like resolution. The HIM achieved a surface resolution of 0.24 nm as the lowest. Such a capability of HIM makes it possible failure analysis applications. This technique produces secondary electrons, backscattered ions and transmitted ions providing three different imaging options - secondary electrons imaging, backscattered ion imaging and transmission imaging. This combination of three imaging modes makes this microscope suitable for different analysis of the material under test.
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The Helium Ion Microscope (HIM) is an advanced instrumentation tool in the field of charged particle microscopes. The signals provided by helium ion beam are an essential thing in developing analytical value for the purpose of imaging. There are three imaging modes in HIM which gives we look in-depth about its ability in creating the image information. The Secondary Electron imaging has been discussed by looking into various qualities of the system. Then the Rutherford backscattering imaging (RBI) method has been discussed along with its limitations and features. Then transmission imaging has been discussed although this is least explored. Fig.1 shows schematic diagram of HIM with incident ion beam and imaging detectors. The basic setup is similar to that of FIB system.
Annular MCP Detector
Fig.1. Schematic diagram of incident ion beam and various detectors
Atomic Level Ion Source (ALIS) acts as ion gun which provides a stable He ion beam with very high brightness. This gives sub-nm probe size for the sample.
The scattered secondary electrons (SE) from the sample in the helium ion microscope are detected normally using an Everhart-Thornley (ET) detector. ET detector collects the ejected SE from the sample surface which provides high resolution images along with high signal to noise ratio. The backscattered helium detector collects the backscattered helium ions to produce an image. The incident ion beam through the silicon substrate produces a large number of electron-hole pairs after achieving lot of energy loss mechanisms. The number of electron-hole pairs is proportional to incident ions energy. Thus the detector detects this accumulated charge by using backscattered helium ion counts and this is the information for imaging.
1. Why go for Helium?
Helium ions have very high source brightness and short De Broglie wavelength (inversely proportional to their momentum). This makes it possible to get qualitative and quantitative information of the material ever achieved.
He ion beam produces both SE and backscattered ions.
SE yield is large.
Interaction volume of He ion beam is smaller near the surface.
Wavelength of He ions is smaller than electrons for same energy.
Faster operation than SEM.
Interaction volume: Electron beam with 1 keV will be having larger interaction volume. But 30 keV He beam well collimated below the SE escape depth and hence having a smaller interaction volume with the sample than that of low keV SEM. Also the resolution of HIM is better than that of SEM with high keV. This principle is shown in Fig.2.
Fig.2. He Ion Beam-Sample Interaction
The capability of HIM makes it to apply in Failure Analysis (FA). Some of the FA applications are:
Imaging of uncoated dielectric materials at maximum He ion energy and resolution.
Imaging of interface films and surface contaminations with very high resolution.
Imaging of thick dielectrics to localize underlying opens or shorts.
High resolution imaging to localize defects in high-aspect-ratio contacts, vias and trenches.
2. Different Imaging Modes
2.1. Secondary Electron Imaging
An incident He ion on a sample has both potential energy (PE) and kinetic energy (KE) in which KE normally varied from 5 to 45 keV whereas ionization potential energy is constant which is 24.6 eV. An incident ion beam will eject electrons and photons from the sample because of its PE and KE. The emission through PE is said to be potential emission, and emission through KE is said to be kinetic emission. Therefore the maximum value of energy from SE will be two thousand times lower than that of the energy of the ion beam. The energy of the SE which is emitted from the sample is to be 2eV, independent of the composition of the sample. This energy when compared with that of the SEMs electron KE which ranges few eV to several keV. In silicon, electron with 2eV energy has a range less than 1 nm. So information of HIM SE volume is in sub-nm. The probe size of HIM is also sub-nm range and hence the resolution of HIM SE is obviously below 1nm. The 2eV energy of SE indicates that HIM is more of surface sensitive. As indicated before SEs from the sample in HIM are detected normally using an Everhart-Thornley (ET) detector.
2.1.1. SE imaging - An example
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An image of aluminium fiducial mark taken by HIM SE imaging is shown in Fig.3. Here it is seen that contamination on the surface and the surface details of underlying barrier layer are clearly imaged. This information is not easily seen by SEM.
Al Oxide or Contamination
Fig.3. HIM-SE image of aluminum fiducial mark
2.2. Back Scattered Ion Imaging
Back scattering, here normally called as Rutherford backscattering phenomenon contains two relationships. They are,
The energy of backscattered He ion is related to the mass of the sample. So, if the energy of backscattered He ion is higher, it reflects the higher mass of the target.
The back scattering differential cross section rises with square of the atomic number of sample (Z2) and increases inversely with square of the ion energy.
Back scattering differential cross section is actually a measure of He ion which is scattered back from the sample by any detector. But for Rutherford backscattered imaging (RBI), HIM normally use annular MicroChannel plate (MCP) detector with negative bias as shown in Fig.1. MCP detector absorbs about 80% of the backscattered ions and rejects the SEs and forms an image. And as for second Rutherford relationship, the image from HIM RBI contains image contrast and video gray level which are dependent on atomic number of the sample.
2.2.1. Simultaneous Comparison of SE and RBI Images
HIM has a capability in capturing both SE and RBI images simultaneously with the help of two parallel detection channels. This capability can be used in failure analysis (FA) of materials. The Fig.4 shows sample of a tin-coated copper electrical connector in which it has been seen that tin whisker (defect indication) has been grown.
Fig.4. Image of Tin-coated copper electrical connector
(a) HIM SE image shows changes in contrast along the whisker
(b) HIM RBI image, showing little to no contrast changes
Transition is seen in the whisker at the point of Orange arrows (identical sites in each image)
These types of whiskers eventually grow over time and finally transit from one electrical connector to another. This is turn cause electronics to fail in operation. Lot of space satellites have been suffered by total or partial loss of its function since due to this phenomenon. The HIM images shown in Fig. 4 a, b shows complementary information from this type of whisker. The HIM SE image gives the topology of the sample. The different sections along the length of the connector are with different contrast levels which show some changes happened while the whiskers are growing. Wherever the whiskers have a kink or bend, these types of changes have been seen. The orange arrows as in Fig.4 point such type of areas.
The RBI in contrast shows that the material is uniform all over the length. This shows that the structure of the tin is changing physically or electronically in some other way and not by material variation. RBI image shows the indication of different elements at the transition points (orange arrow in Fig. 4) on the whisker. Here the two places indicated by arrows has been seen brighter contrast. This data provide a way for growth analysis. It should be noted down that most of the particles as seen in whisker of SE image are not seen in RBI image. This indicates that parts are most likely same as tin, like same as of whisker.
Fig.5. Simultaneous comparison of both SE and RBI images (a) SE image (b) RBI image
In the current trend of HIM instrument's configuration, there are two independent video signal chains which allow simultaneous imaging of both SE and RBI image. Fig. 5 shows a tin/lead solder sample with simultaneous comparison of both SE and an RBI image, where it has been shown that SE image shows surface contaminants (i.e. flux residue) and topography present in the tin and lead solder. But instead, RBI image (backscattered image) differentiates lead from that of tin. The data from both images will be useful in material analysis and fault localization.
1.3. Transmission Imaging
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Transmission imaging mode although least explored, this can be used in with different imaging modes simultaneously to get high resolution images. There is an option in obtaining image information from five different types of transmission signals in HIM. They are: 1. bright field transmitted ions, 2. dark field i.e., Rutherford scattered transmitted ions, 3. top-side SE signal, 4. bottom-side SE signal and 5. diffracted ion signal.
Let us take an example of thinned sample of gate region of a typical IC. Fig.6 (a) shows SE mode image as discussed previously. Here, the conductors are seen to be brighter and insulators, like gate spacers and interlayer dielectric are seen as darker. It should be noted that thin layers, like liner around the copper, imaged strongly due sensitivity of the surface and the material contrast of HIM SE imaging. Fig.6 (b) shows a bright field image of the same sample area. The bright field ion images normally give lower resolution. Fig.6 (c) shows bottom side transmitted SE image. This reveals gaps in the nitride, silicide contacts quality and identifies low density area of the tungsten studs in which via fill process has been choked off. Fig.6 (d) shows carbon black's dark field and the resolution here is 0.45nm.
Fig.6. Transmission Imaging (a) Top-side SE (b) Bright field (c) Bottom-side SE (d) Dark field
Thus the working principle, features and different modes of imaging of Helium Ion Microscope (HIM) have been studied. The applications especially relating to fault localization and failure analysis has been discussed along with imaging modes. The results from the high resolution images have been seen very useful in analyzing the material. It has been also seen that high resolution images allows atom level elemental analysis. Although SE imaging gives much of information of the sample, Rutherford and transmission imaging are also useful in some specific applications as seen before. The fault localization complexity keeps on increasing due device miniaturization which in turn requires advanced metrology tool. With the benefits of advanced HIM, there will be increase in use of HIM in semiconductor industries in the future.