Microfabrication of AFM tips using focused ion and electron beam techniques

Ultramicroscopy 42-44 (1992) 1526-1532 North-Holland

Microfabrication of AFM tips using focused ion and electron beam techniques H o n g y u X i m e n a n d Phillip E. Russell Precision Engineering Center, North Carolina State University, Raleigh, NC 27695-7918, USA Received 12 August 1991

Production of reproducible A F M tips with narrow and straight tip shanks is important for metrology and imaging of various submicron and high topography structures. Possibilities of micromachining commercial A F M pyramidal tips with focused ion beam (FIB) techniques have been investigated to improve the sharpness of the pyramidal tips. Also, the fabrication of electron-beam-induced microtips grown on the top of the pyramidal tips has been developed using a combination of focused ion and electron beam techniques. Microtips were reproducibly grown with tip shanks of 1.0 p.m length and 0.1 tzm diameter. The tip geometry was found to degrade only slightly after extensive A F M imaging, indicating minimal wear on the microtip during A F M scanning. The radius of curvature remained as sharp as 25 n m after tens of hours of A F M imaging in the contact mode. The microtips were found to be considerably stronger and more stable when they were grown on pyramids which had been heavily implanted with Ga ions.

1. Introduction

Recently, there has been great interest in various atomic force microscope [1] (AFM) tip-fabrication techniques, because of the great potential of direct profiling on both conducting and insulating surfaces with atomic resolution [2,3], as well as the inherent limitation of tip artifacts in actual AFM images. Tip shape is particularly important for critical dimension measurements of nonconducting lithographic patterns with high topography. The availability of assemblies [4] with both a force-sensing device (a cantilever) and a fine pyramidal tip as integrated AFM tip assemblies has made the efforts possible in improving the sharpness of the tips which use the same forcesensing devices. In this paper, two approaches to improving the sharpness of AFM tips are presented. One is a subtractive process, and the other is an additive process. By using direct electron beam contamination writing from the background vapors of an elec-

tron scanning microscope (SEM) chamber, electron-beam-deposited (EBD) tips with diameters of less than 0.1 /xm had been produced and tested as STM [5] and AFM [6] tips by others. From transmission electron microscopy and Auger studies, it had been concluded that the EBD tip was an amorphous material composed of carbon and oxygen. By using electron-beam-induced chemical-vapor-deposition (CVD) techniques [7-9], submicron needle-type protrusions have been produced and used as AFM tips [10]. The CVD fabrication process is conducted in a modified SEM with gas delivery systems. In this paper, a focused-ion-beam (FIB) process for micromachining of commercial Si3N 4 pyramidal tips has been explored, with various tip-fabrication concepts demonstrated. A method of growing microtips [11] using a combination of ion implantation and electron-beam-induced growth techniques is presented. These microtips are grown on top of commercial pyramidal AFM tips which have been implanted with Ga + ions. The result-

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H. Ximen, P.E. Russell / Microfabrication of AFM tips

(a)

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(b)

Fig. 1. (a, b) T h e FIB cutting strategies for initial and trimming cuts are illustrated. SEM micrographs of A F M cut tips are shown as (c) top view and (d) 45°-tilt view. The trimmed tip in final form is shown in (e) and (f). The cut-out section of the pyramid is still attached for clarity. (It must be completely detached for actual uses:)

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ing tips are shown to be capable of AFM imaging on soft (PMMA) and abrasive (CVD diamond films) substrates.

artifacts for AFM imaging. Fabrication concepts for such techniques are described in sections 2.1 and 2.2. 2.1. A F M cut tips using F I B M

2. Microtip-fabrication techniques Commercially available AFM tips (Nano Probe) are fabricated using thin-film lithography techniques [4] by which a pyramidal tip is created by depositing a Si3N 4 film of 0.7 txm thickness onto a Si surface which includes a pyramidal etch pit as a mold. These tips are highly symmetric with a 5.0 Izm by 5.0 /zm square base. The tip sidewalls have a slope of 55 °, and the tip radii are often less than 30 nm. For apparent geometry reasons, such pyramidal tips cannot image topographic features with nearly vertical sidewalls and relatively high depth-to-width aspect ratios. When the pyramidal tip is scanned over an edge of a deep groove, the actual force interaction point (contact point) is changed from the end of the AFM tip to the corner of the groove and a position which shifts up the side of the pyramid until the adjacent sidewall is reached. Thus, the AFM image is a convolution of the sample surface and the shape of the pyramidal tip. A long, straight and narrow tip would obviously reveal surface topography with reduced tip geometry

A digitized three-dimensional ion beam scan strategy [12] has been implemented in a FIB workstation [13] such that the focused ion beam could be scanned in a desired pattern to micromachine commercial AFM Si3N 4 pyramidal tips. The basic cutting strategy is shown in figs. la and lb. In the SEM micrographs of fig. 1, three sides of a pyramidal tip are shown to be cut off from the cantilever substrate. The three sides were left attached for the purpose of illustration. Fig. lc is a top view, and fig. ld was taken with the pyramid 45 ° tilted. Further trimming results in the tips are shown in figs. le and lf. The total cutting time required for this procedure is three minutes. Also, for special applications, such as scanning ion conductance microscopy, a pyramidal tip with a submicron hole opened on the tip apex might be desired. This type of structure was fabricated by locking the focused ion beam in the spot mode, as shown in the SEM micrograph of fig. 2 with top view (fig. 2a) and 45 ° tilt view (fig. 2b). In this case the hole diameter was 0.4 /zm, limited mainly by the ion beam diameter. FIB systems

Fig. 2. SEM micrograph of FIB machined special-purpose scanning probe microscopy (SPM) tips: (a) top view and (b) 45°-tilt view, where a 0.4 txm hole was opened on the top of a pyramidal A F M tip. This example shows the versatility of FIB micromachining for tip fabrications.

H. Ximen, P.E. Russell / Microfabrication of AFM tips a r e available with b e a m sizes of 50 nm a n d less, thus s m a l l e r a p e r t u r e s could b e f a b r i c a t e d .

2.2. Electron-beam-induced growth of microtips S y s t e m a t i c e x p e r i m e n t s have b e e n c o n d u c t e d to e x p l o r e the possibility o f p r o d u c i n g A F M microtips •by e l e c t r o n - b e a m - i n d u c e d g r o w t h techniques. Initial A F M i m a g e s that w e r e a c q u i r e d with t h e s e s u b m i c r o n p r o t r u s i o n s h a d d e m o n -

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s t r a t e d s o m e success. H o w e v e r , g o o d A F M images with high s i g n a l - t o - n o i s e r a t i o w e r e o b t a i n e d rather randomly. Both electron- and ion-beam-ind u c e d C V D m e t h o d s a r e well k n o w n as techniques for the g r o w t h o f m i c r o s t r u c t u r e s with a high m e t a l c o n t e n t . W e have f o u n d t h a t G a m e t a l can b e locally i m p l a n t e d into a tip r e g i o n a n d t h e n u s e d as a s o u r c e o f m e t a l for e l e c t r o n b e a m - i n d u c e d growth. C o m p a r i s o n o f growing m i c r o t i p s on t h e t o p of t h e S i a N 4 p y r a m i d s with

Fig. 3. SEM micrograph of electron-beam-induced growth microtips: (a) 45°-tilt view of a microtip after AFM imaging on PMMA for 10 h, (b) preliminary study of top length vs. electron be'am dwell time (grown on a flat region of a cantilever), and (c) 45°-tilt view of a specially grown microtip of 10° tilt off normal on the top of a pyramidal AFM tip.

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and without Ga + ion implantation revealed dramatically improved reproducibility and reliability for those with the Ga incorporation. The microtips are grown using a multistep process. First, the Si3N 4 pyramids are coated with gold to avoid beam-charging in subsequent process steps. These gold-coated tips are then placed in an FIB workstation with a liquid-metal ion source. A Ga + ion beam of 250 nm diameter operating at 20 keV is used to sputter away the gold coating at the pyramidal tip apex and then to implant Ga + ions into the tip region with a dose of 2.0 x 1017/cm 2 (requiring one minute of

beam time per tip). This results in a locally supersaturated region at the tip apex. These Si3N 4 tips supersaturated with Ga + ions are transferred into a field emission scanning electron microscope (SEM) with a specimen chamber pressure of 6 x 10 -7 Torr. A 10 keV, 10 pA electron beam, focused to 3 nm spot size, is then held in spot mode on the desired growth position for times of 1-5 min (depending on the length desired). Preliminary studies of microtip length vs. the electron dwell time was illustrated in the SEM micrograph of fig. 3b, where protrusions from the left to right were grown for different dwell times

Fig. 4. (a) Lithographic defined polycrystalline Si square pillars imaged with pyramidal AFM tips, (b) same pattern imaged with grown microtips, and (c) CVD diamond films, imaged with grown microtips.

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Fig. 4 (continued).

(10, 20, 30 s, 1, 2, 3, 4 and 5 min). The tip growth is initially rapid, but is seen to slow down after 1 min of growth. This could be due to a depletion of the Ga a n d / o r to diffusion limitations as the tip length increases. A microtip which was scanned over gold-coated P M M A lithographic gratings for 10 h in the contact mode with contact forces of approximately 30 nN is shown in fig. 3a. The tip radius after this period of scanning is seen to be 25 nm. This microtip of 0.1 /~m diameter and 1.0 /zm length was grown by locking the electro beam in the spot mode for 3.0 min. The sharpness of the microtip was reduced slightly due to the contact between the microtip and sample surfaces after the extensive AFM image acquisition. We have scanned over P M M A lithographic patterns for as long as 30 h with these microtips with no evidence of substantial degradation. In order to compensate for a 10° tilt of AFM cantilever holders used on Nano-Scope II, a spe-

cially grown microtip of 10° tilt off-normal is shown in the SEM micrograph of fig. 3c such that the AFM microtip will be straight down into the substrate provided that the AFM head is leveled, which is crucial for AFM imaging of square pillars with undercuts as shown in section 3.

3. AFM imaging results In order to verify the straightness of the microtips, objects with vertical side walls in two orthogonal directions are desired. Therefore, lithographically defined square pillars with undercuts were used to test the electron-beam-induced-grown microtips. Because of the undercut structure, the AFM images give a direct indication of the shape of the AFM tips. A series of AFM images of various substrate materials such as PMMA, polysilicon and CVD diamond films have been obtained with a Nano-

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Scope II AFM system using pyramidal tips with and without grown microtips in order to demonstrate the improvement. All AFM images have been obtained in air, with contact forces around 30 nN. In the 3D perspective view of fig. 4, AFM images of square pillars acquired with an unmodified pyramidal tip shows slanted side wails, whereas four straight side walls with a minimum of 70° slope (in fig. 4b) were obtained by AFM when using pyramids with the grown microtips. AFM images of CVD diamond films that were acquired with microtips are shown in a cross-sectional view of fig. 4c, where two opposite side walls in the x-direction have slopes of 66° and 76°. This demonstrates that the grown tips are suitable for use in the imaging of hard materials in the contact mode. Having been scanned over diamond films for 10 h, the degradation of the microtips was found to be of a similar degree of wear as shown in fig. 3a.

4. Conclusions Microfabrication procedures for producing AFM and other scanned probe tips have been developed. A micromachining method of cutting Si3N 4 pyramidal tips has been demonstrated. This technique could be further applied to micro-machining any AFM tips while attached to a cantilever. An additive process of growing submicron microtips on top of pyramidal AFM tips has been developed which reproducibly produces AFM tips which exhibit a long lifetime (30 h of AFM imaging PMMA) and good overall imaging performance. Details of the electron-beam-induced growth mechanism are currently under further investigation along with extensive characterization of the chemical and mechanical properties of the tips.

Acknowledgements This work was supported in part by the National Science Foundation Presidential Young Investigator Award Program (DMR 8657813) and by the Precision Engineering Center, North Carolina State University. We wish to thank Joe Griffith of A T & T and Jeff Glass of NCSU for providing samples used in this work.

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