Lab Technology

When it comes to imaging at high magnifications, thetraditional optical microscope has given way to anumber of alternative technologies. The ScanningElectron Microscope (SEM) has been used forapproximately 50 years and, compared with its opticalancestors, offers high resolution images with largedepth of field. But advancement in image resolution ofthe SEM (~2nm in practice) has stagnated in recentyears, mostly due to electron diffraction and the physicsof the beam/sample interaction.

The transmission electron microscope (TEM) avoids theseproblems, but requires a very sophisticated samplepreparation procedure. In the special case of where it ispermissible to be in contact with the sample, there are a

number of scanning probe techniquesthat can be used such asAtomic Force Microscopy(AFM), Scanning TunnellingMicrosopy (STM) and Near-Field Scanning OpticalMicroscopy (NSOM). Whileoffering excellent resolution,

these images require some manipulation before the humaneye can intuitively interpret them.

One could reasonably argue that the last dramaticbreakthrough in the world of imaging happened in theearly 1980s with the discovery of the scanningtunnelling microscope by Gerd Binnig and HeinrichRohrer, for which they won the 1986 Nobel Prize inPhysics. More recently, the development of the HeliumIon Microscope by ALIS Corporation offers the promiseof a new and unique imaging technique as an alternativeto existing microscopes. The ORIONTM helium ionmicroscope (see Figure 1) is much like a traditionalSEM, but offers images with superior resolution andstronger contrast.

THE HELIUM ION MICROSCOPE

The ALIS helium ion source provides intense ion currentsfrom a volume no larger than a single atom. The ionsource consists of a sharp needle that is maintained underhigh vacuum and cryogenic temperatures. A high voltageis applied to the needle to produce an extremely intense

Helium Ion Microscopy: an IntroductionDevelopment of the Helium Ion Microscope offers the promise of a newand unique imaging technique as an alternative to existing microscopes

By Nicholas Economou, Bill Ward, John Morgan and John Notte at the ALIS Business Unit of Carl Zeiss SMT

Dr Nicholas P Economou is President of the ALIS Business Unit of Carl Zeiss SMT. Dr Economou began his career as a researcher at Bell Laboratories and MIT Lincoln Laboratory before becoming involved in his first start-up company in1984. Over the last 20 years, he has served as Chief Executive Officer for several successful venture-backed technologycompanies in the semiconductor and telecommunications industries. Nicholas also currently serves as a Director on theboards of a number of private and public technology companies. He received his BA in Physics from Dartmouth Collegeand his MA and PhD in Physics from Harvard University.

Bill Ward is the Chief Technology Officer of the ALIS Business Unit of Carl Zeiss SMT. Mr Ward has been on the foundingteam of many successful technology start-up companies. He started his career at Extrion, the first commerciallysuccessful ion implanter company, which is now Varian Semiconductor Equipment Associates. He was also one of the co-founders of Micrion Corporation – a manufacturer of focused ion beam systems. Bill was the founder of ALISCorporation, which was purchased by Carl Zeiss SMT. He holds numerous patents for his early breakthroughs insemiconductor-related technology and has lectured throughout the world.

John Morgan is Director, Product Marketing, of the ALIS Business Unit of Carl Zeiss SMT. Mr Morgan began his careerat GCA Corporation in the field of optical lithography. He has more than 20 years’ experience in charged particlebeams and has held several technical, marketing and sales positions at Micrion Corporation, FEI Company and AxcelisTechnologies prior to joining Carl Zeiss SMT. John has a BS in Chemical Engineering from Clarkson University.

Dr John Notte is Director of Research and Development of the ALIS Business Unit of Carl Zeiss SMT. Dr Notte taughtphysics, astronomy and mathematics at Bates College. After leaving academia, he worked at AMRAY, KLA-Tencor and FEICompany in the areas of electron optics, detector optimisation, system automation and image analysis. John received hisBS in Physics from Case Western Reserve University and his PhD in Experimental Electron Plasma Physics from theUniversity of California, Berkeley.

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Figure 1: The heliumion microscope is nowcommercially available as a complement (or alternative) to the traditional SEM

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electric field at its apex. The field is sufficiently strong thatany nearby helium gas atom will have an appreciableprobability of having an electron tunnel out. Theresulting positive ion will then be accelerated away fromthe needle. When the needle is appropriately shaped (seeFigure 2), the ionisation process happens in the vicinity ofa single atom at the apex of the needle, and the resultingion beam appears to be emanating from a region that isless than one Angstrom in size. Consequently, theresulting ion beam has a remarkable brightness of 4 x 109

A/cm2sr. The high brightness is a necessary condition forfocusing the beam to a small probe size (1).

The helium ion beam is then steered down a column thatincludes a series of focusing, alignment and scanningelements. The resulting beam can be brought to afocused probe size of just 0.75 nm. Much like a SEM,the beam is rastered across the sample, pixel by pixel. Thenumber of detected secondary electrons is used todetermine the grey level of each particular pixel. Sincethe number of detected secondary electrons varies withmaterial composition and shape, the images provideexcellent topographic and compositional information.Some examples of the images that can be obtained witha helium ion microscope are shown in Figures 3 and 4.Figure 3 is an image of a white blood cell that shows bothtopographic and material details, and Figure 4 shows ahigh magnification image of a carbon nanotube.

Some of the advantages of the helium ion microscopearise from the properties of helium. Helium atoms aremuch more massive than electrons, so their deBrogliewavelength is short enough that the focused probe is notstrongly affected by diffraction effects. In contrast, mosttraditional SEMs are limited by the diffraction effectsarising from the electron’s wave-like properties. As thehelium ion beam penetrates into the sample, there islittle scattering, so the images do not suffer from thesub-surface blurring effects that are common in SEMimages (2). Therefore, the helium ion microscope canprovide images with higher resolution and more surfacespecific information.

In addition to images created through secondary electrondetection, the incident helium ion beam offers otheropportunities for imaging and analysis. A small fraction ofthe helium atoms will be scattered by heavier nuclei andricochet out of the sample. The probability of thisoccurring depends critically upon the atomic number ofthe scattering nucleus. So images based on the abundanceof scattered helium ions provide a strong elementalcontrast which can be used to distinguish differentmaterials. The image in Figure 5 (see page 30)

demonstrates the strong contrast between thechrome and quartz of a photomask. Anotheradvantage of this imaging mode is the strongimmunity to charging artifacts, since thescattered helium atoms are not appreciablydeflected by these electric fields.

HIGH RESOLUTION IMAGING

Scanning Electron Microscopes arecommonly used in the pharmaceuticalindustry for imaging drugs and for doingchemical analysis using Energy DispersiveX-Ray Spectroscopy (3). EnvironmentalSEMs (ESEMs) are typically theinstrument of choice for imaging as theycan mitigate the charging effects that areoften encountered when imaging non-conductive samples in a SEM. In theESEM, a small amount of gas, typically water vapour, isinjected into the vacuum chamber in close proximity tothe sample in order to help dissipate charge. The watervapour can and does react with the sample. This can bebeneficial if one wants to study the effect that water hason a particular sample (4). However, if one doesn’t wantthe sample to interact with a gas such as water, imagingnon-conductive samples can pose a problem with the SEM.

The Helium Ion Microscope does not suffer from thecharging issues commonly encountered with the SEM.The imaging beam is positively charged helium ions;these can induce a positive charge on the sample, but thischarge can be effectively neutralised using an electronfloodgun. In the multiplexed imaging mode, the electronfloodgun can be turned on and off rapidly during imageacquisition to effectively neutralise any charge induced

29Innovations in Pharmaceutical Technology

Figure 2: Helium atoms (green)become ionised in the high field

region (orange crescent). The resultingions (yellow) are accelerated away

Figure 3: A white blood cell as imagedusing the Orion helium ion microscope

Figure 4: High resolution image of a carbon nanotube

500nm 100nm

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by the helium ion beam. This image acquisitiontechnique has been used by focused ion beam tools in thesemiconductor industry for more than twenty years.Figure 6 shows an image of the cephalosporin antibioticcefuroxime (Ceftin, GlaxoSmithKline) acquired using multiplexed imaging on the Orion helium ion microscope. There is no evidence of charging in the image.

MATERIAL ANALYSIS

It is often desirable to analyse a sample to ascertainsome information about its chemical composition. Thiscan be accomplished with a SEM using EnergyDispersive X-Ray Spectroscopy (EDS). With thistechnique, an electron beam of sufficient energy togenerate X-rays is focused on the sample beinganalysed. The X-rays that are induced by the electronbeam are collected and analysed to determine theelemental composition of the sample. This techniquehas two shortcomings. Insulating samples can chargeup and deflect the beam from its intended location,giving false or misleading information. Also, theinteraction volume in the sample from which the X-rays are emitted can be relatively large, limiting thespatial resolution of this technique.

Since helium is so much heavier than an electron, lowand medium energy helium ions don’t produceappreciable X-rays. Fortunately, even in the absence of X-rays, helium ions can still be used to provide elementalidentification. The helium ions scatter off of heaviernuclei, and their scatter angle and energy can be used todetermine the mass of the scattering atom. The samemethod can also provide structural arrangement of thesurface atoms. Although this option is not presently

available in the commercialhelium ion microscope, thephysics is highly developed andwell understood. The sameprinciple of analysis is used with the established techniques of Medium Energy IonScattering (MEIS), Low EnergyIon Scattering (LEIS) andRutherford Back-Scattering(RBS) (5). But unlike thesetechniques, which use ion beamsmeasured in mm, the helium ionbeam can provide information atthe nanometer length scale.

CONCLUSION

The Orion Helium Ion Microscope represents abreakthrough in the field of microscopy and the firstcommercial shipments of the instrument have nowtaken place. Much investigation remains to be done tofind those applications for which this technology isbest suited. Several attributes of the technology – such as high resolution, strong material contrast,immunity to charging and the ability to performmaterial analysis at the nanometer scale – make it anintriguing option for the imaging needs of thepharmaceutical industry.

The author can be contacted at jmorgan@smt.zeiss.com

References

1. Orloff J, ed, Handbook of Charged Particle Optics,

CRC Press, New York, p353

2. Reimer L, Scanning Electron Microscopy, New York:

Springer, pp162-165, 1998

3. Lich B, SEM Provides Critical Process Information

for Pharmaceutical Applications, Microscopy Today,

pp12-15, Sept 2007

4. Bean S and Robinson K, Pharmaceutical Imaging

Solutions Offered by the Extended Pressure

Scanning Electron Microscope, Carl Zeiss

internal publication

5. Rabalais JW, Principles and Applications of Ion

Scattering Spectrometry, Wiley Interscience,

Hoboken, NJ, 2003

30 Innovations in Pharmaceutical Technology

Figure 5: This image of a photomask (chrome onquartz) shows the strong elemental information revealed when detecting scattered helium ions. Also note the absence of any charging artifacts

Figure 6: Secondary electron image of cefuroxime (Ceftin, GlaxoSmithKline) acquired using multiplexed imaging

with charge neutralisation

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