Addicted To Nature

This was a paper I submitted for one of my General Education modules called GEK1509: Introduction to the Nanoworld which thoroughly sucked and gave me a B- for which I was anticipating at least an A- at least… but I sucked at the midterm MCQs, got below average, nothing could have saved me… not even this.. >.< .

Dated: October 2006

The Scanning Helium Ion Microscope

Advantages of a SHIM over a traditional SEM and FIB

SHIM technology has been widely coveted by many research industries, particularly those who want to “see” more. Nanotechnology, materials science, biological sciences, the semiconductor industry can all have massive benefits from the SHIM and its incredible resolution, high reduced brightness, small spot size and low energy spread. Below, I will discuss on the advantages of the SHIM topic by topic.

1. Superior Resolution

1.1 Wave-particle duality

The SEM, FIB machine, TEM, SHIM and all kinds of “matter” microscopes uses the property of matter waves. Quantum physics predicts the existence of a wave-particle duality of all known particles in this universe.

The de Broglie wavelength of particles is given by

The equation shows that if all other terms are constant, the wavelength and the mass is inversely proportional. As the mass increases, the matter wavelength would decrease, thus being able to see smaller features. This is explained in detail below.

1.2 Resolving power or resolution

The resolving power or resolution of any optical system is defined as the smallest distance between the centers of two points which can be distinguished as separate in the image.

For both a SEM and the SHIM, resolution can be affected by many things such as the optical system, the sample thickness, the nature of the sample etc. Assuming that all of them are optimum, this boils down to something called the ultimate resolving power. The ultimate resolving power is only affected by aberrations and diffraction of the beam. If we can further erase out the effects of aberrations such as using a monochromatic beam and other lens correctors, the diffraction limited resolution is now given by

where is the smallest distance between two distinguishable points, is some constant, is the spherical aberration and lambda the wavelength of the beam. Wondering where the diffraction part of the equation is? The spherical aberration can be made to be indefinitely small by decreasing the aperture angle. But there is a point where if the angle is decreased any further, the quality of the image is seriously destroyed due to diffraction. Therefore, there exist an optimum aperture angle where the diffraction and spherical aberration errors are about the same. This shows the dependence of the spherical aberration (one of the last two uncorrected properties) on the diffraction hence the term “diffraction limited resolution”. The above equation is derived for that particular angle.

The equation also shows that has very little effect as it powered to ¼. This shows that as the de Broglie wavelength decreases, the resolution is increased significantly as the mass of a helium ion is around 7000 times that of the electron. Theoretical calculations (mine) yields that the wavelength of the electron is 7000 times that of the electron and the resolving distance is around three to two orders of magnitude higher for that of a helium beam than that of an electron beam when their velocities are the same. This means that if the electron beam can differentiate an object to a meter, the helium beam can resolve up to a millimeter distance theoretically under optimum conditions.

1.3 Sample interaction and material contrast

Some SEMs have excellent resolution going as far as the angstrom scales. But the SEM lacks high material contrast, which is determined by the interaction of the beam with the sample. High material contrast enables to see critical features of the sample to a very high degree. A helium beam has shorter wavelength as described above and therefore can be focused to a smaller spot size versus a traditional SEM. ALIS corporation has claimed that the spot size could be focused to as small as 1nm. Some SEM microscopes can go as small as even 0.1nm. But aside from the optical performance, the SHIM’s other feature is that it has considerably less volume interaction than the SEM. The SHIM uses an ion beam which has shallower penetration depth than an electron beam. This enables the resolution of the SHIM to be directly proportionate to its spot size which is not always the case with the SEM. Furthermore, the SHIM can produce about 3-9 secondary electrons per ion compared to 1 per electron of the SEM. This also means that the SHIM can image properly with ion currents as low as 1 fA.

2. Staining and sputtering

One of the best features of the SHIM is that it will produce no appreciable staining whatsoever due to the relatively light mass of the helium ions compared to gallium. The largest artifacts with gallium FIB machines is staining or condensation and “sputtering” where the gallium could actually destroy some part of the sample.

In fact, the gallium FIB is used primarily for etching (making shapes by shooting at the sample). At higher beam currents, the FIB machine can be used as an atomic milling machine to perform precision cutting and etching with sub-micron accuracy. This precise milling can also be used for high accuracy precision micromachining from tens of millimeters to tens of nanometers. One of the most used applications for the FIB machine is in sample preparation for the TEM (transmission electron microscope). The SHIM will still be able to do this. The difference is that there will be no “undesired” staining unlike the gallium FIB.

3. Sample preparation and life science applications

In the TEM, tremendous amount of hard work has to be done for sample preparation. This is a very tedious job and requires much time and skill. There is an entirely separate research and industry going on for sample preparation alone. Even in the SEM, insulating materials has to be first sputter coated with a thin layer of gold so that it becomes conductive.

Another property of the SHIM requires no or very little sample preparation and the sample need not be conductive as well. The SHIM backscattered ion mode can produce images of more or less the same quality as the TEM with NO sample preparation at all as shown in the Fig2.3. The SHIM can collect backscattered helium ions like Rutherford Backscattered Spectroscopy (RBS). These backscattered ions can yield information on the material characterization of the sample to a very high detail.

Another issue is of depth of focus. In Biological sciences, large depth of focus is highly valued since high depth of focus can give high quality 3D images of biological samples showing more detail and finer features. ALIS claims that its SHIM has a depth of focus up to five times larger than the SEM.

Structure of the SHIM

The ALIS ‘LookingGlass LG-2′ Helium Ion Microscope is a noble gas source GFIS FIB machine. As explained in chapter 1, a GFIS machine can provide a high brightness, low energy spread ion beam many times better than a LMIS FIB machine. The exact details of how the ALIS SHIM works has still not been published as ALIS is not a research body but a corporation. However, they have published two papers one of which describes a small part of how the ion source functions.

The ion source

As described extensively in chapter 1 (which was in anticipation of this part), the ion source plays a critical role. The GFIS has actually been researched on for longer than the LMIS but until ALIS came into the picture, nobody could achieve the stability, reliability and properties needed for a suitable helium ion beam that could match the spot size of a SEM.

The exact nature of the source that ALIS uses could not be confirmed as I have mentioned, it is a corporate secret. I have also tried to search for patents and ALIS seems to have submitted none yet. I also wrote to ALIS but no reply came back. From one of their papers[4], the source is a needle type GFIS made of tungsten. An electric field of 3×10^10 V/m can be achieved at the sharpest point of the protrusion. Ionization can take place here just like in field ion microscopy. At this field strength, the gaseous helium atoms ionize near the “ionization discs” (Fig 2.7). They are accelerated away once they are ionized to form a beam and accelerated by the ion column. Increasing the pressure of the helium gas increases the ion current proportionately. The ion current can be regulated from 1fA to 100pA in this simple manner, by adjusting the pressure.

The technology which enables to create this highly stable beam is the carefully shaped tungsten tip. ALIS has not disclosed how this is achieved but stated that “the ability to consistently shape the end form of the emitter with atomic precision is an essential part of the helium ion microscope technology.”

In my opinion, this tip must certainly be a variation of the GFIS supertip mentioned in chapter 1 since ALIS mentioned of a “shaping” of the tip. There has been extensive research in supertip GFIS over the years and some of the problems are described below.

Challenges of the needle supertip ion source

As mentioned in chapter 1, in conventional literature, a needle tip GFIS or a FIM (field ion microscope) needs a low temperature (<77 K) coldfinger so that the ionization probability, which contributes to the ion current and the energy spread, is much higher. It is not desirable for industrial type applications to maintain such a low temperature. Whether the ALIS LG-2 uses a coldfinger or not cannot be confirmed. A needle type GFIS without a coldfinger (running at room temperature) reduces the ionization probability and hence reduces the brightness and ion current. ALIS says that the ionization process in its SHIM is similar to that of a traditional FIM. Therefore, even if its “tip-shaping” technology is incredibly advanced, some of the above mentioned problems must apply to it some way as well.

It is also known that in previous attempts to make a suitable ion source, the GFIS beam was very unstable. Tiny impurities in the helium gas could affect the beam current seriously. Tiny changes in pressure and temperature also affected the beam quality. It would be very interesting to know how ALIS overcame these problems. In fact, the supertip needle type is even more unstable than that of the normal needle type with coldfinger. If all the problems were actually solved by carefully shaping the tip as ALIS claims, it must have been a very advanced breakthrough for ion source technology.

Potential Applications

If I am not mistaken, this is clearly a serious and major breakthrough in “seeing” things. The applications of this technology are immense. Imagine, a microscope, with the resolution of a TEM, does not require sample preparation, provides higher voltage and material contrast information, inert ion beam with much less volume interaction leaving no artifacts. And to top it all off, it’s still the start whereas electron microscopy and focused ion beam microscopy has been under development for over 40 years.

ALIS plans to use its LG-2 for semiconductor applications first, especially in defect inspection in the wafer fabrication process due to the extreme image contrast it is capable of. Failure analysis is very important as the failed sample could be more “failed” due to heavy sample preparation in the traditional microscopes. The SHIM requires very simple sample preparation cutting time, defects and errors.

Again, due to the high material contrast of the SHIM, measurement applications are also very useful. The SEM has a very high resolution but has low material contrast and therefore it is harder to see the sites where critical features and changes take place.

One of the other features of the SHIM is its backscattered helium ions. These ions provide very accurate information of the atomic number of the substrate material. They provide no whatsoever topographical information. This is similar to Rutherford backscattered spectroscopy which has been in use as an analytical technique in materials sciences research. The backscattered ions can portray rich chemical composition with high contrast.

There are even more applications in life sciences as the non-sputtering SHIM will be able to preserve delicate samples with TEM resolution. Simpler sample preparation will also help in sample preservation. The high material contrast once again can contribute to the distinction of fine features. This is especially useful in biology as very minute and complex features are of the most interest and importance.

Carbon nanotubes are one of the hardest to image as they are made of rolled up graphene sheets which have large hexagonal holes in them. SEM electrons normally penetrate through these holes especially if the nanotubes is single walled. Researchers use TEM and AFM (atomic force microscope) which take ups much time and headache. The SHIM, as ALIS claims (I have no idea how this is plausible), will be able to have TEM resolution without the complicated sample preparation.

There are also potential applications in the photomask production process. Today’s methods use either electron beam lithography or gallium FIB machines to etch the substrate material. The gallium FIB machine, as repeated many times above, “stains” the sample which of course leads to complications in the process to reduce this staining. Electron beams cannot compete with an ion in etching since it is 1/1830 times the mass of a hydrogen atom. With the helium beam, which is relatively inert and lighter than gallium with much higher mass than an electron beam, the semiconductor industry can have enormous benefits with much more accurate and perhaps even up to picometer process photomasks.

The atomic precision shaping technique and technology of the tip as claimed by ALIS might also have other properties which can be applied to other fields of interest. This shaping technique could lead the way to not only helium ion beams but to other noble gas beams such as xenon as well. All sorts of gaseous noble gas ion beams might be applied to a myriad of applications.

Final say

The intricate details of how the ion source needle is shaped should probably be published in the near future. There are incredible prospects in this new field of microscopy which will touch many areas of research and solve many remaining problems due to inadequate resolution and qualities of today’s microscopes. Congratulations to ALIS Corporation for being able to contribute another large future to mankind.

References

1. V.N. Tondare, Quest for High Brightness, Monochromatic Noble Gas Ion Sources, J. Vac. Sci. Technol. A 23(6), 1498, 2005.
2. J. Notte*, R. Hill, S. McVey, L. Farkas, R. Percival, B. Ward, An Introduction to Helium Ion Microscopy, Microscopy & Microanalysis Vol.12, Supplement S02, 126, 2006. *ALIS Corporation, Peabody, MA, USA.
3. J. Notte, Bill Ward, Sample Interaction and Contrast Mechanisms of the Helium Ion Microscope, Scanning Vol.28, 000-000(2006).
4. Mark Wendman, Wendmans’ View on Nanotech.
5. Cecil E. Hall, Introduction to Electron Microscopy 2nd edition (McGraw-Hill 1966).
6. Erwin W. Muller & Tien Tzou Tsong, Field Ion Microscopy; Principles and Applications (Elsevier 1969).
7. Spence, John C. H., High-Resolution Electron Microscopy 3rd edition, (Oxford University Press 2003).
8. ALIS Corporation website, www.aliscorporation.com.

Acknowledgements

I would like to thank “Claude Bile” of Physics Forums for advising me on various matters on this post. You may find our conversation here.

Here are some comparisons of the SHIM with traditional SEMs

Fig 2.4 Left: SHIM image of a solder bump showing the difference in the lead and tin sections. Right: SEM image of the same sample. ALIS Corporation 2006.

Fig 2.5 Left: SHIM image of a sample. It is discernible that the material in the cross is different from the outside. Some of the dark specks are actually the material from the inside scattered on the outside. Right: SEM image of the same sample which leaves no room for comparison. ALIS Corporation 2006.

Fig 2.3 Left: ALIS SHIM backscattered image of Benthic Foraminifera. Right: Benthic Foraminifera. Note that both of these samples are insulators and no sample preparation was made. ALIS Corporation 2006.