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Ultrasharp tungsten tips—characterization and nondestructive cleaning
Ultramicroscopy 113 (2012) 152–157
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Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
Ultrasharp tungsten tips—characterization and nondestructive cleaning M. Setvı´n n, J. Javorsky´, D. Turcˇinkova´, I. Matolı´nova´, P. Sobotı´k, P. Koca´n, I. Oˇst’a´dal ´ch 2, Prague 18200 8, Czech Republic Charles University in Prague, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holeˇsovicˇka
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2011 Received in revised form 7 October 2011 Accepted 13 October 2011 Available online 9 November 2011
We study the treatment of ultrasharp tungsten tips used for applications in nanoscience and introduce a fast and simple method for estimation of the tip radius using a single measurement of the autoemission current. The method is based on a detailed investigation of the influence of an arrangement of electrodes on the electric field layout in close proximity of the tip apex. The electric field was calculated using Monte Carlo Floating Random Walk algorithm. The most frequently used cleaning procedures (heating the whole tip to high temperature, electron bombardment and selfsputtering) were investigated by electrical measurements and microscopy techniques (SEM, TEM) and the results of the particular methods are compared. We report on the effectiveness and limiting conditions of the cleaning methods with respect to the damage they cause to the tip apex. & 2011 Elsevier B.V. All rights reserved.
Keywords: Tungsten tips Field emission Electrostatics Tip cleaning In vivo STM
1. Introduction Sharp tungsten tips are important tools for probing and manipulating objects of nanometer size in current nanotechnology research. Methods for preparation of sharp tungsten tips are well known thanks to the long time they have been used in scanning tunneling microscopy (STM) under ultrahigh vacuum conditions (UHV). However, in the last decade new methods have emerged which require tips with a well defined profile and apex. Contacting single nanostructures (e.g. quantum wires, single molecules) [1,2] with interesting electrical properties and measuring their characteristics are challenging problems in nanotechnology. A possible solution is using a multiprobe STM [3] with 2, 3 or 4 probes. In that case, the curvature radius of the tip apex determines how close one can approach the probes and consequently how small structures can be probed. Another example is using the tip for controlled manipulation of nanoobjects (e.g. carbon nanotubes) when building nanodevices [4]. The controlled single atom and molecule manipulation on solid surfaces also requires tips that allow a defined local interaction. Conventional STM measurements on flat atomic terraces generally do not require a tip with a small curvature radius to obtain atomic resolution. However, there are specific STM measurements that impose very strict conditions on the size of the tip apex. For direct observation of adsorption and growth processes during deposition in vivo [5,6], the dimension of the tip apex must be
n
Corresponding author. E-mail address: [email protected] (M. Setvı´n).
0304-3991/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2011.10.005
smaller than the size of the scanned area, otherwise the tip screens off the scanned area from arriving particles. An example of the screening effect from a very sharp tip is in Fig. 1e. The tip screening effect is also important when measuring STM-induced luminescence [7] or stimulating the surface by laser radiation during the scanning. In UHV, tungsten tips are used almost exclusively. Preparation of sharp tungsten tips by electrochemical etching was studied in detail and the methods reproducibly providing tips with curvature radii of few nanometers are well known [8–10]. High mechanical resistance, stability and easy preparation are accompanied by the main disadvantage of tungsten – high reactivity with oxygen. It is necessary to remove the oxide layer after the tip is introduced into UHV. When the tip is used for electrical measurements (STM, STS, multiprobe measurements of electrical conductance) the tip metallicity is crucial. The cleaning process itself is quite challenging because the sharpest tips are extremely vulnerable. In this paper we briefly describe the procedure we used for reproducible fabrication of sharp tungsten tips. Then we introduce a simple and effective method for estimating a tip radius based on the field emission (autoemission) measurement. The methods using field emission current for this purpose are known [11,12] but the reported procedures are rather complicated and time consuming to be easily and routinely used. We did not find sufficient information how the tip shape and layout of the electrodes influence the field emission effect so we performed calculations of the electric field potential in close proximity of the tip apex using the MCFRW1 algorithm [13–16]. The obtained
1
Monte Carlo Floating Random Walk.
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Fig. 1. Images of tungsten tips taken few hours after etching. (a) Large-scale SEM image, the wire diameter is 0.3 mm; (b) SEM image showing microcrystalline structure of the tip; (c) and (d) SEM and TEM images of tips with curvature radius less than 5 nm. The tungsten core is covered by a thick oxide layer; (e) STM image of a surface area created during deposition of Ag on Si(111)-(7 7) surface at simultaneous scanning. The tip ‘shadow’ is visible on the surface.
results are discussed with respect to the field emission current. The layout of potential around a sharp tip apex is a frequently encountered problem and the presented results have wider applications. The main focus of this paper is to compare the effectiveness of several methods for tip cleaning in UHV. Results obtained by heating the whole tip to high temperature, by annealing the tip apex by electron bombardment and by means of self-sputtering are discussed. Finally we define conditions suitable for removing the contamination with minimum damage to the tip apex for each of the used methods.
used the depths ranging from 1.2 to 1.5 mm for the wire diameter of 0.3 mm. It is possible to optimize [10,18] the etching process by dividing it into two steps and obtain even sharper tips, but it is more difficult and time consuming. For the tip etching we tested several configurations of electrodes [8,19]. The arrangement of electrodes affected macroscopic shape of the tip, but the sharpness of the tip apex was comparable for all cases. We prefer the configuration similar to that used in [19] where the cathode consists of a stainless steel wire ring (diameter of few milimeters) placed just at the air-solution interface. This configuration is very resistant against vibrations and we found the etching process the most reproducible.
2. Experimental setup and tip preparation The shape of the prepared or further treated tungsten tips was investigated by means of a scanning electron microscope (SEM) Tescan MIRA and a transmission electron microscope (TEM) Tesla BS 613. Experiments focused on measuring the field emission and tip cleaning were performed in an UHV chamber under pressure in a range from 1 10 7 to 1 10 4 Pa. The chamber was evacuated by a titanium ion-sputtering pump. The noise of the autoemission current was measured by recording time series (4096 points) of voltage fluctuations on a 2 MO resistor connected in series with the tip. Low pass filtering was used with respect to a chosen sampling frequency. The power spectra were calculated using a fast Fourier transform (FFT). The tips were etched by DC current in a 2 M aqueous solution of NaOH from a polycrystalline tungsten wire (W005153 Goodfellow) with a diameter of 0.3 mm. We used the standard ‘‘drop-off’’ electrochemical etching method [17]. The etching proceeds fastest at the air–electrolyte interface giving a rise to an ever narrowing neck that ultimately breaks under the weight of the lower part of the wire. Two ‘‘opposite’’ tips are formed when the wire is etched through – we use the upper one. The etching process is stopped by a fast electronic circuit (adapted from [9]) measuring the time derivative of the etching current. The current depends on the area of the tungsten wire immersed into the etching solution. When the lower part of the wire is dropped off, the magnitude of time derivative of the current dramatically increases and that moment can be easily detected. Two crucial requirements have to be fulfilled for the preparation of sharp tips: (i) fast disconnection of the etching circuit immediately after the formation of the apex in order to prevent tip blunting due to continued etching, (ii) minimizing the mass of the lower part of the tungsten wire. About 90% of our tips had a curvature radius o 30 nm and 30% of the tips o 5 nm. We optimized the etching process by a precise measurement of the initial immersion depth of the wire into the etching solution—we
3. Tip radius estimation It is important to have a simple method to test a tip after its preparation. Microscopy techniques like SEM or TEM can provide excellent information on the tip shape and by means of a field ion microscope (FIM) [20,21] one can obtain atomic configuration of the tip apex. However, these methods are time consuming and routinely not available. Autoemission is commonly used for testing the tip quality before starting STM measurements – the autoemission current of hundreds of nA at a voltage of several hundreds of volts indicates very good sharpness (tip radius 10 nm) [22]. The radius can be estimated from the Fowler– Nordheim formula [11,12]: ! BF3=2 2 I ¼ AsF exp , ð1Þ F where I is the autoemission current, A and B are parameters derived from fundamental physical constants, F is the work function of tungsten, s is the size of the surface area emitting the electrons and F is electric field (an image potential correction was neglected). If the tip apex is modeled by a spherical termination of a shank, the electric field at the tip apex can be expressed as F¼
V , kR
ð2Þ
where V is the applied voltage, R is the tip radius and k is a field reduction factor. The factor k can be calculated analytically in special cases; k¼1 for a sphere in infinite space. For real tip shapes, 3o k o8 [12], but values around 20 were also reported [20]. Assuming that the area s R2 , the formula (1) can be expressed in a form: ! V2 BF3=2 kR I A 2 exp : ð3Þ V k
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The product F3=2 kR can be obtained by fitting a measured I–V characteristic by the function (3). However, both F and k may vary. The work function of tungsten depends on the surface plane (F100 ¼ 4:63 eV, F110 ¼ 5:25 eV, F111 ¼ 4:47 eV). An effective value F ¼ 4:5 eV was used – the emission comes from planes with the lowest work functions [23]. However, the effective value may be strongly affected by adsorbate. A thin layer of oxide can slightly increase or decrease the work function of metals [24]. When a new tip is introduced into UHV, it is covered by an oxide layer – Fig. 1c and d. Autoemission current of such a tip is strongly suppressed until the oxide is removed. The polarized dielectric layer effectively decreases the electric field at the tip apex. The electrons can also tunnel into conductance band of the oxide instead of vacuum. The autoemission current considerably increases after removing the oxide layer. It can be therefore used as a sensitive indicator of cleaning efficiency. On the other hand using autoemission to determine the tip radius requires careful tip cleaning, otherwise the work function is not well defined. The second not sufficiently defined parameter in the Fowler– Nordheim formula is the field reduction factor k. There are a few measurements that provide entirely different values [12,20,22] but there is only little knowledge on the quantitative meaning and the origin of the factor. Here we explain how the value of k depends on the shape of the tip and the configuration of the electrodes. We calculated the electric field potential in close proximity of the tip apex. The MCFRW algorithm was used to solve the Laplace equation and to obtain the electric field in close proximity of the tip apex. This method allows to calculate the potential in chosen representing points and is much more effective than the finite element method. The MCFRW method does not use a fixed lattice – one can calculate the electric potential close to the tip apex with subnanometer resolution while the opposite electrodes (representing boundary condition) are milimeters far. We tested the algorithm on electrode systems that can be solved analytically (e.g. two concentric spheres).
The field reduction factor was calculated for the electrode configuration shown in Fig. 2a. The tip potential is set to 1 and the tip is surrounded by grounded electrodes at certain distances from the apex. The system has a cylindrical symmetry with the tip in the main axis. The calculations show that the most important parameter for the potential layout around the tip apex is a ‘‘macroscopic’’ profile of the tip. The profile typical for tungsten tips is shown in Fig. 1a. The profile can be well described by an exponential function y ¼ R0 expðx=tÞ where 2R0 is a diameter of the tungsten wire and t is a parameter with an estimated value of 60 mm in our case (depends mainly on the cathode shape and wire immersion at the beginning of the etching process). Close to the tip apex the profile usually changes to a simple conical shape with a vertex angle of few degrees and the very end is terminated by a hemisphere. The calculations were performed for the following model shapes. We chose the exponential shape continuing to the very end terminated by a hemisphere of a defined radius. Then we tried, for comparison, conical profiles with various apex angles. The entirely conical shape is common for nickel or silver tips [19,25]. The layout of the calculated potential in direction of the tip axis x (the direction of the highest electric field) for various profiles is plotted in Fig. 2b. In the calculations we used a tip radius R¼10 nm and electrode distances dFRONT ¼3 mm, dREAR ¼7 mm and dSIDE ¼20 mm according to the real arrangement at the measurements. The field reduction factor was obtained from the formula (2) using the electric field intensity estimated from the linear change of the potential within a distance of 1 nm from the tip apex – see Fig. 2b. The exponential tip profile gives the lowest value: k¼3.0. The conical tip with a small apex angle of 61 provides a similar value (k¼3.5), but the value strongly increases with the apex angle. The angle of 901 corresponds to the value k¼52. The field reduction factor shows only weak dependence on the tip radius. A deviation is less than 10% in a range of radii from 5 to 100 nm which covers most of commonly used tips. The calculated values are in a very good agreement with our experiments and data reported in literature [12,20]. By measuring I–V characteristics of our tips and assuming the tungsten work function of 4.5 eV, we obtained values of k ranging from 3.3 to 4.0. Our calculations show that k does not generally depend on the layout of the electrodes except for several specific cases: Moving the front electrode very close to the tip apex results in a significant decrease of k. If the ‘rear electrode’ is at the same potential as the tip (corresponds to a massive tip holder) the value of k considerably increases. The calculated field reduction factors for several electrode configurations are summarized in Table 1. The data were obtained for the exponential tip profile while the side electrode distance was fixed to 20 mm. In summary – to keep the value of k well determined one should avoid using massive tip
Table 1 Values of the field reduction factor k calculated for various electrode configurations.
Fig. 2. (a) Arrangement with cylindrical symmetry used for calculation of electric potential in proximity of the tip apex. (b) Electric potential calculated as a function of distance x. The field reduction factor k is evaluated for different shapes of tip profiles. The listed apex angles are full angles.
Rear potential
dFRONT (mm)
0
100 10 3 1 0.1 0.01
7 7 7 7 7 7
3.1 3.1 3.0 2.7 2.1 1.7
1
3 3 3 3 3
20 10 5 2 1
3.1 3.5 4.0 5.8 8.9
dREAR (mm)
k
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sputtering’’ [27] caused by molecules of residual gases ionized and accelerated towards the tip apex by the high electric field near the apex. We will focus on finding the optimum conditions for removing undesirable contamination from the tip without damaging the tungsten core. The most often used methods are based on thermal decomposition and desorption of the contamination. On the other hand the increased temperature stimulates diffusion of metal atoms on the surface and grain interfaces, which may result in undesirable blunting of the tip apex. 4.1. Resistive heating of the tip Fig. 3. The dependence of tip radius on voltage necessary for autoemission current of 5 nA. Experimental data (black circles) are fitted by a dashed line. Dependences calculated from the Fowler–Nordheim formula for the field reduction factors 3.5 and 5.0 are included (upper and lower solid lines, respectively).
holders and the distance between the front electrode and tip apex has to be at least 10 larger than the tip wire diameter. The field reduction factor is a key parameter in the Fowler– Nordheim formula (3). Using the obtained results we can explain our simple method for estimating the tip radius. The value of the product F3=2 kR can be obtained by fitting the measured I–V characteristic using the formula (3), but we will show that the tip radius can be estimated from a single measurement with reasonable accuracy. The following experiment was performed for many tips: The tip was introduced into UHV and cleaned by one of the methods described further. Then we increased tip bias until a stable autoemission current of 5 nA appeared. The corresponding tip radius was measured ex situ by means of TEM. Results are in Fig. 3. The dependence of the tip radius on the bias for the emission current 5 nA shows that it is possible to obtain quite a good estimation of the radius only from a single measurement of the autoemission current. The diagram in Fig. 3 shows also values calculated from the Fowler–Nordheim formula (1), using a work function of F ¼ 4:5 eV and field reduction factors k¼ 3.5 and k¼5. Most of experimental points lie within the area limited by these two dependencies. The value of the emission current of 5 nA is suitable because it is high enough to be easily measured but it cannot damage the tip. The voltage corresponding to the value of 5 nA will be referred as the ‘‘emission threshold voltage’’.
To heat the whole tip to high temperature we brought the tip body into contact with a tungsten wire and used DC current for heating up the junction. Most of the power is dissipated at the junction and the small tip is uniformly heated. The temperature of the tip was measured by an optical pyrometer (error of the measurements was about 20 1C). We proceeded in the following way: The tip was annealed at a desired temperature for 10 s and then tested by the autoemission. Successively higher temperatures were used for the annealing. The dependence of the voltage necessary for an emission current of 5 nA (threshold voltage) on the annealing temperature well reflects changes of the tip apex. Results for 3 different tips are plotted in Fig. 4a. The initial decrease of the emission threshold at temperatures 500–800 1C corresponds to removing the WOx layer. Further annealing to higher temperatures results in an increase of the emission threshold voltage caused by blunting the tip apex. Two ways of damaging the apex were found. Profiles of the tips obtained by TEM after the annealing are in Fig. 4a. The tip 1 was originally sharp (radius 40 nm; field reduction factor was 4–5 due to presence of a contact electrode used for the resistive heating). The corresponding
4. Cleaning methods After finishing the etching procedure it is necessary to remove remains of the etching solution from a tungsten tip. Usually the tip is rinsed immediately in hot distilled water and ethanol – we used three beakers of ethanol and two of distilled water. To maximize the cleaning effect it is important to keep the temperature of all the bathes just below the boiling point, otherwise traces of contamination can be observed in TEM images of the tips. Tungsten tips are mostly etched in NaOH or KOH solutions. It was previously found that KOH cannot be washed out perfectly, therefore NaOH is used more frequently [26]. However, after the etching and rinsing, the tip is covered by an oxide layer. Removing the oxide layer and other contaminants under vacuum conditions in situ is rather a difficult task in case the tip apex must not be damaged. Fig. 1c and d shows SEM and TEM images of freshly etched tips after the initial washing. The tungsten core is covered by the oxide which is much thicker than the native oxide layer. There are several methods commonly used for in situ tip cleaning. We investigated the methods of heating the whole tip to high temperature, annealing the tip apex by electron bombardment and cleaning by means of ‘‘self-
Fig. 4. Cleaning the tip by direct heating to high temperature. (a) Dependence of voltage necessary for autoemission current 5 nA on annealing temperature. The tips were heated to gradually increasing temperature. Initial drop of the threshold voltage is caused by removing the oxide layer; following increase is a result of tip blunting. Profiles of tip 1 and 2 after the treatment are included (imaged by TEM). (b) Power spectra of autoemission current fluctuations before and after annealing the tip.
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plot contains jumps in the threshold voltage that indicate sudden changes of the tip apex. The TEM image shows that the tip apex was sharply ‘‘cut off’’. It can be explained by a polycrystalline structure of the tip—see Fig. 1b. The profile of the broken tip (Fig. 4a) looks as though the whole crystal grain was dropped off from the very end of the tip. This kind of damage could be probably avoided by etching the tip from a single-crystal tungsten. However, annealing at high temperature would anyway result in blunting the tip apex by thermally activated diffusion of the surface atoms (tip 2). The original apex radius of the tip 2 was about 20 nm and annealing to 1750 1C increased the radius to 50 nm. We tested and used another method to examine a state of the tip apex. The autoemission current from the just prepared tips is not very stable and contains significant fluctuations – flicker noise. The power spectra are of 1/f type which corresponds to a physical model of the tunneling transition [29–31]. The current fluctuations at low frequencies are basically caused by local changes of the tip work function due to the presence of an unstable adsorbate, by charging the dielectric surface layer and finally by random changes in the microscopic structure of the tip apex. The current noise therefore sensitively reflects the presence of unstable contamination on the stable tungsten core. Fig. 4b shows the power spectra of the autoemission current fluctuations just after tip introduction to the vacuum chamber and after annealing to 850 and 1150 1C, respectively. The mean emission current I was kept at 100 nA and the measurements were performed at a pressure of 1 10 6 Pa. The noise level significantly decreased after removal of the oxide layer (annealing to 850 1C). Further annealing to 1150 1C stabilized the tip even more. The power density of 1/f noise is far above the background level of shot noise 2eI ¼ 3 1026 A2 =Hz in Fig. 4b. According to our experience with this kind of treatment in STM, heating the tip to 800 1C leads to a significant improvement of tip behavior, but the cleaning is not perfect. After such a treatment the tips provide stable tunneling current and do not show diode-like behavior typical for the oxide-covered tips. However, the tips still frequently drop fragments of contamination onto the scanned surface. To obtain a perfectly clean and stable tip, it is necessary to flash it many times to 1800 1C. Unfortunately, it results in considerable blunting of the apex. For applications that require conservation of the original sharpness, the temperature should not exceed 800–1000 1C. Heating to higher temperatures improves the cleaning but causes blunting of the apex. The advantage of the method is that the tip temperature can be precisely controlled. 4.2. Heating by electron bombardment The electron source used for cleaning consisted of a resistively heated tungsten filament separated from the tip by a molybdenum diaphragms (at the same potential as the filament) with an aperture hole with a diameter of 2 mm. The distance between the aperture and the tip was 1 mm, a bias of þ1000 V was applied between the tip and the thermoemission filament. The tip was repeatedly heated by electron bombardment for 10 s by gradually increasing power and the autoemission threshold was tested after each flash. The results obtained for three tips are plotted in Fig. 5. TEM images show each tip after the whole treatment. The first tip had a curvature radius of about 10 nm and was cleaned at heating power 1 W. In case of the second tip, it is not clear which power exactly corresponds to optimum cleaning. However, blunting of the tip apex started at 100 mW and continued until the curvature radius of about 90 nm was reached at 1 W. The bubble-like termination is the most typical shape for tips damaged by electron bombardment. The third tip was cleaned at few milliwatts and the obtained emissivity remained constant up to 1 W when dramatically changed. The TEM image shows that the tip shape looks as if it was broken at a crystallite domain boundary.
Fig. 5. Cleaning the tips by electron bombardment. Evolution of autoemission threshold with increasing heating power (dashed lines are used to guide the eye). Profiles of the tips after the treatment were obtained by TEM.
Similar examination performed for other tips showed that the optimum power necessary for the cleaning varies in a large range from few mW to 1 W. The reason is that electrons bombard only an area very close to the apex and the exact size of the area depends on the tip shape and configuration of the electrodes. We did not find any correlation between the tip radius and the suitable heating power. In summary, the electron bombardment cleans the tip very effectively but it is impossible to predict optimal conditions. Finally, we do not recommend this kind of cleaning when the original tip shape has to be preserved. 4.3. Cleaning by controlled autoemission The field emission itself is a very effective method for tip cleaning. When an as-prepared tip is tested by the field emission, the autoemission threshold voltage gradually decreases until it is reduced usually to 1/2 or 1/3. The time necessary for such treatment strongly depends on the emission current and residual pressure in the chamber. The tip was at a distance of 3 mm from a planar counterelectrode in our experiments. At 100 nA and 106 Pa it takes tens of minutes to obtain the final value of the autoemission threshold. The origin of the cleaning effect is self-sputtering [28]. TEM images taken before and after the cleaning (not shown) reveal that the self-sputtering removes the oxide layer and does not damage the tip apex at all (if the autoemission current is kept under a certain safe value – see below). According to our experience the self-sputtering itself never results in changing the tip apex even after the cleaning for tens of minutes at a residual pressure of 5 103 Pa and autoemission current of 1 mA. Dependence of the autoemission current fluctuations on the self-sputtering time shows a significant reduction of the noise which indicates the cleaning and stabilization of the tip apex. Fig. 6a shows power spectra of the fluctuations for different sputtering times. The tip was gradually sputtered for longer time intervals. The number of the samples in the recorded time series was kept constant during each interval (4096), which results in different time scales of the data plotted in Fig. 6a. The total sputtering time is given for each data set. The tip was treated at a pressure of 3 10 6 Pa and 100 nA emission current. This method seems to be optimal for removing the oxide layer and preserving the apex sharpness. However, it is known that the tip can be destroyed during the field emission. We will try to describe the conditions resulting in the apex destruction. The destruction can be easily noticed because the autoemission current suddenly disappears. The TEM images of tips destroyed during the autoemission showed that the tip apexes were melted by the passing current [28]. There is a limiting value of the emission current for a safe tip treatment.
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of the MCFRW algorithm. Well defined shape of the tip and removed oxide layer are the crucial conditions for using this method. Three tip cleaning methods were compared—effectiveness and conditions necessary to avoid a destruction of the tip apex are described for each of them. Self-sputtering during the autoemission process can remove the oxide layer and causes minimum damage to the tungsten core of the tip apex. We described the safe conditions for such cleaning in dependence on the apex radius. Direct heating of the whole tip to high temperature is also effective and allows good control of the cleaning temperature. Heating the tip to 800 1C removes the oxide layer, but it is not sufficient to obtain a perfectly clean tip. Further heating to temperatures above 1000 1C blunts the tip due to thermally activated diffusion and increases the risk of breaking the tip apex at crystallite interfaces. Heating the tip apex by electron bombardment proved unsuitable because the safe heating power cannot be predicted.
Acknowledgements The work was partially supported by GD202/09/H041, GAUK ˇ P204/10/0952 and GACR ˇ P202/09/P033. 80010, GACR References
Fig. 6. (a) Cleaning the tip by self-sputtering. The cleaning is accompanied by decreasing noise power of autoemission current. (b) Maximum emission current which can be used for safe cleaning as a function of the tip radius (calculated).
According to our experiments, this value is related to the tip radius. We performed a simple computer simulation for a quantitative estimation: The current passing through the tip generates Joule heat. The heat is dissipated by convection and radiation. We investigated the dependence of the current that heats the tip apex to temperatures close to a melting point of tungsten. The results are plotted in Fig. 6b. The exponential profile of the tip was used in the calculations. The tips with the exponential shape are very vulnerable due to their small thermal conductance, therefore we consider the values plotted in Fig. 6b limiting and safe. The shape of most real tips in proximity of the apex is conic and it is more resistant to damage due to higher thermal conductance. For example, according to the calculation, the tip with an exponential profile and radius of 5 nm should be destroyed by 130 nA emission current. A conic tip with a vertex angle of 71 and the same radius has a limiting value of 1 mA. The dependence in Fig. 6b shows good qualitative agreement with our experiments. We note that when using field emission for tip cleaning, we always put a 1 GO resistor in the circuit to protect the tip. Removing the oxide layer results in a strong increase of the emission current that can melt the tip apex. This effect is very dangerous especially for the sharpest tips.
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5. Conclusions
[27]
We described a novel simple method that allows to estimate the tip radius from a single measurement of the autoemission current. To explain the method we presented calculations of the electric field potential in close proximity of the tip apex by means
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Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
Ultrasharp tungsten tips—characterization and nondestructive cleaning M. Setvı´n n, J. Javorsky´, D. Turcˇinkova´, I. Matolı´nova´, P. Sobotı´k, P. Koca´n, I. Oˇst’a´dal ´ch 2, Prague 18200 8, Czech Republic Charles University in Prague, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holeˇsovicˇka
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2011 Received in revised form 7 October 2011 Accepted 13 October 2011 Available online 9 November 2011
We study the treatment of ultrasharp tungsten tips used for applications in nanoscience and introduce a fast and simple method for estimation of the tip radius using a single measurement of the autoemission current. The method is based on a detailed investigation of the influence of an arrangement of electrodes on the electric field layout in close proximity of the tip apex. The electric field was calculated using Monte Carlo Floating Random Walk algorithm. The most frequently used cleaning procedures (heating the whole tip to high temperature, electron bombardment and selfsputtering) were investigated by electrical measurements and microscopy techniques (SEM, TEM) and the results of the particular methods are compared. We report on the effectiveness and limiting conditions of the cleaning methods with respect to the damage they cause to the tip apex. & 2011 Elsevier B.V. All rights reserved.
Keywords: Tungsten tips Field emission Electrostatics Tip cleaning In vivo STM
1. Introduction Sharp tungsten tips are important tools for probing and manipulating objects of nanometer size in current nanotechnology research. Methods for preparation of sharp tungsten tips are well known thanks to the long time they have been used in scanning tunneling microscopy (STM) under ultrahigh vacuum conditions (UHV). However, in the last decade new methods have emerged which require tips with a well defined profile and apex. Contacting single nanostructures (e.g. quantum wires, single molecules) [1,2] with interesting electrical properties and measuring their characteristics are challenging problems in nanotechnology. A possible solution is using a multiprobe STM [3] with 2, 3 or 4 probes. In that case, the curvature radius of the tip apex determines how close one can approach the probes and consequently how small structures can be probed. Another example is using the tip for controlled manipulation of nanoobjects (e.g. carbon nanotubes) when building nanodevices [4]. The controlled single atom and molecule manipulation on solid surfaces also requires tips that allow a defined local interaction. Conventional STM measurements on flat atomic terraces generally do not require a tip with a small curvature radius to obtain atomic resolution. However, there are specific STM measurements that impose very strict conditions on the size of the tip apex. For direct observation of adsorption and growth processes during deposition in vivo [5,6], the dimension of the tip apex must be
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Corresponding author. E-mail address: [email protected] (M. Setvı´n).
0304-3991/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2011.10.005
smaller than the size of the scanned area, otherwise the tip screens off the scanned area from arriving particles. An example of the screening effect from a very sharp tip is in Fig. 1e. The tip screening effect is also important when measuring STM-induced luminescence [7] or stimulating the surface by laser radiation during the scanning. In UHV, tungsten tips are used almost exclusively. Preparation of sharp tungsten tips by electrochemical etching was studied in detail and the methods reproducibly providing tips with curvature radii of few nanometers are well known [8–10]. High mechanical resistance, stability and easy preparation are accompanied by the main disadvantage of tungsten – high reactivity with oxygen. It is necessary to remove the oxide layer after the tip is introduced into UHV. When the tip is used for electrical measurements (STM, STS, multiprobe measurements of electrical conductance) the tip metallicity is crucial. The cleaning process itself is quite challenging because the sharpest tips are extremely vulnerable. In this paper we briefly describe the procedure we used for reproducible fabrication of sharp tungsten tips. Then we introduce a simple and effective method for estimating a tip radius based on the field emission (autoemission) measurement. The methods using field emission current for this purpose are known [11,12] but the reported procedures are rather complicated and time consuming to be easily and routinely used. We did not find sufficient information how the tip shape and layout of the electrodes influence the field emission effect so we performed calculations of the electric field potential in close proximity of the tip apex using the MCFRW1 algorithm [13–16]. The obtained
1
Monte Carlo Floating Random Walk.
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Fig. 1. Images of tungsten tips taken few hours after etching. (a) Large-scale SEM image, the wire diameter is 0.3 mm; (b) SEM image showing microcrystalline structure of the tip; (c) and (d) SEM and TEM images of tips with curvature radius less than 5 nm. The tungsten core is covered by a thick oxide layer; (e) STM image of a surface area created during deposition of Ag on Si(111)-(7 7) surface at simultaneous scanning. The tip ‘shadow’ is visible on the surface.
results are discussed with respect to the field emission current. The layout of potential around a sharp tip apex is a frequently encountered problem and the presented results have wider applications. The main focus of this paper is to compare the effectiveness of several methods for tip cleaning in UHV. Results obtained by heating the whole tip to high temperature, by annealing the tip apex by electron bombardment and by means of self-sputtering are discussed. Finally we define conditions suitable for removing the contamination with minimum damage to the tip apex for each of the used methods.
used the depths ranging from 1.2 to 1.5 mm for the wire diameter of 0.3 mm. It is possible to optimize [10,18] the etching process by dividing it into two steps and obtain even sharper tips, but it is more difficult and time consuming. For the tip etching we tested several configurations of electrodes [8,19]. The arrangement of electrodes affected macroscopic shape of the tip, but the sharpness of the tip apex was comparable for all cases. We prefer the configuration similar to that used in [19] where the cathode consists of a stainless steel wire ring (diameter of few milimeters) placed just at the air-solution interface. This configuration is very resistant against vibrations and we found the etching process the most reproducible.
2. Experimental setup and tip preparation The shape of the prepared or further treated tungsten tips was investigated by means of a scanning electron microscope (SEM) Tescan MIRA and a transmission electron microscope (TEM) Tesla BS 613. Experiments focused on measuring the field emission and tip cleaning were performed in an UHV chamber under pressure in a range from 1 10 7 to 1 10 4 Pa. The chamber was evacuated by a titanium ion-sputtering pump. The noise of the autoemission current was measured by recording time series (4096 points) of voltage fluctuations on a 2 MO resistor connected in series with the tip. Low pass filtering was used with respect to a chosen sampling frequency. The power spectra were calculated using a fast Fourier transform (FFT). The tips were etched by DC current in a 2 M aqueous solution of NaOH from a polycrystalline tungsten wire (W005153 Goodfellow) with a diameter of 0.3 mm. We used the standard ‘‘drop-off’’ electrochemical etching method [17]. The etching proceeds fastest at the air–electrolyte interface giving a rise to an ever narrowing neck that ultimately breaks under the weight of the lower part of the wire. Two ‘‘opposite’’ tips are formed when the wire is etched through – we use the upper one. The etching process is stopped by a fast electronic circuit (adapted from [9]) measuring the time derivative of the etching current. The current depends on the area of the tungsten wire immersed into the etching solution. When the lower part of the wire is dropped off, the magnitude of time derivative of the current dramatically increases and that moment can be easily detected. Two crucial requirements have to be fulfilled for the preparation of sharp tips: (i) fast disconnection of the etching circuit immediately after the formation of the apex in order to prevent tip blunting due to continued etching, (ii) minimizing the mass of the lower part of the tungsten wire. About 90% of our tips had a curvature radius o 30 nm and 30% of the tips o 5 nm. We optimized the etching process by a precise measurement of the initial immersion depth of the wire into the etching solution—we
3. Tip radius estimation It is important to have a simple method to test a tip after its preparation. Microscopy techniques like SEM or TEM can provide excellent information on the tip shape and by means of a field ion microscope (FIM) [20,21] one can obtain atomic configuration of the tip apex. However, these methods are time consuming and routinely not available. Autoemission is commonly used for testing the tip quality before starting STM measurements – the autoemission current of hundreds of nA at a voltage of several hundreds of volts indicates very good sharpness (tip radius 10 nm) [22]. The radius can be estimated from the Fowler– Nordheim formula [11,12]: ! BF3=2 2 I ¼ AsF exp , ð1Þ F where I is the autoemission current, A and B are parameters derived from fundamental physical constants, F is the work function of tungsten, s is the size of the surface area emitting the electrons and F is electric field (an image potential correction was neglected). If the tip apex is modeled by a spherical termination of a shank, the electric field at the tip apex can be expressed as F¼
V , kR
ð2Þ
where V is the applied voltage, R is the tip radius and k is a field reduction factor. The factor k can be calculated analytically in special cases; k¼1 for a sphere in infinite space. For real tip shapes, 3o k o8 [12], but values around 20 were also reported [20]. Assuming that the area s R2 , the formula (1) can be expressed in a form: ! V2 BF3=2 kR I A 2 exp : ð3Þ V k
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The product F3=2 kR can be obtained by fitting a measured I–V characteristic by the function (3). However, both F and k may vary. The work function of tungsten depends on the surface plane (F100 ¼ 4:63 eV, F110 ¼ 5:25 eV, F111 ¼ 4:47 eV). An effective value F ¼ 4:5 eV was used – the emission comes from planes with the lowest work functions [23]. However, the effective value may be strongly affected by adsorbate. A thin layer of oxide can slightly increase or decrease the work function of metals [24]. When a new tip is introduced into UHV, it is covered by an oxide layer – Fig. 1c and d. Autoemission current of such a tip is strongly suppressed until the oxide is removed. The polarized dielectric layer effectively decreases the electric field at the tip apex. The electrons can also tunnel into conductance band of the oxide instead of vacuum. The autoemission current considerably increases after removing the oxide layer. It can be therefore used as a sensitive indicator of cleaning efficiency. On the other hand using autoemission to determine the tip radius requires careful tip cleaning, otherwise the work function is not well defined. The second not sufficiently defined parameter in the Fowler– Nordheim formula is the field reduction factor k. There are a few measurements that provide entirely different values [12,20,22] but there is only little knowledge on the quantitative meaning and the origin of the factor. Here we explain how the value of k depends on the shape of the tip and the configuration of the electrodes. We calculated the electric field potential in close proximity of the tip apex. The MCFRW algorithm was used to solve the Laplace equation and to obtain the electric field in close proximity of the tip apex. This method allows to calculate the potential in chosen representing points and is much more effective than the finite element method. The MCFRW method does not use a fixed lattice – one can calculate the electric potential close to the tip apex with subnanometer resolution while the opposite electrodes (representing boundary condition) are milimeters far. We tested the algorithm on electrode systems that can be solved analytically (e.g. two concentric spheres).
The field reduction factor was calculated for the electrode configuration shown in Fig. 2a. The tip potential is set to 1 and the tip is surrounded by grounded electrodes at certain distances from the apex. The system has a cylindrical symmetry with the tip in the main axis. The calculations show that the most important parameter for the potential layout around the tip apex is a ‘‘macroscopic’’ profile of the tip. The profile typical for tungsten tips is shown in Fig. 1a. The profile can be well described by an exponential function y ¼ R0 expðx=tÞ where 2R0 is a diameter of the tungsten wire and t is a parameter with an estimated value of 60 mm in our case (depends mainly on the cathode shape and wire immersion at the beginning of the etching process). Close to the tip apex the profile usually changes to a simple conical shape with a vertex angle of few degrees and the very end is terminated by a hemisphere. The calculations were performed for the following model shapes. We chose the exponential shape continuing to the very end terminated by a hemisphere of a defined radius. Then we tried, for comparison, conical profiles with various apex angles. The entirely conical shape is common for nickel or silver tips [19,25]. The layout of the calculated potential in direction of the tip axis x (the direction of the highest electric field) for various profiles is plotted in Fig. 2b. In the calculations we used a tip radius R¼10 nm and electrode distances dFRONT ¼3 mm, dREAR ¼7 mm and dSIDE ¼20 mm according to the real arrangement at the measurements. The field reduction factor was obtained from the formula (2) using the electric field intensity estimated from the linear change of the potential within a distance of 1 nm from the tip apex – see Fig. 2b. The exponential tip profile gives the lowest value: k¼3.0. The conical tip with a small apex angle of 61 provides a similar value (k¼3.5), but the value strongly increases with the apex angle. The angle of 901 corresponds to the value k¼52. The field reduction factor shows only weak dependence on the tip radius. A deviation is less than 10% in a range of radii from 5 to 100 nm which covers most of commonly used tips. The calculated values are in a very good agreement with our experiments and data reported in literature [12,20]. By measuring I–V characteristics of our tips and assuming the tungsten work function of 4.5 eV, we obtained values of k ranging from 3.3 to 4.0. Our calculations show that k does not generally depend on the layout of the electrodes except for several specific cases: Moving the front electrode very close to the tip apex results in a significant decrease of k. If the ‘rear electrode’ is at the same potential as the tip (corresponds to a massive tip holder) the value of k considerably increases. The calculated field reduction factors for several electrode configurations are summarized in Table 1. The data were obtained for the exponential tip profile while the side electrode distance was fixed to 20 mm. In summary – to keep the value of k well determined one should avoid using massive tip
Table 1 Values of the field reduction factor k calculated for various electrode configurations.
Fig. 2. (a) Arrangement with cylindrical symmetry used for calculation of electric potential in proximity of the tip apex. (b) Electric potential calculated as a function of distance x. The field reduction factor k is evaluated for different shapes of tip profiles. The listed apex angles are full angles.
Rear potential
dFRONT (mm)
0
100 10 3 1 0.1 0.01
7 7 7 7 7 7
3.1 3.1 3.0 2.7 2.1 1.7
1
3 3 3 3 3
20 10 5 2 1
3.1 3.5 4.0 5.8 8.9
dREAR (mm)
k
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sputtering’’ [27] caused by molecules of residual gases ionized and accelerated towards the tip apex by the high electric field near the apex. We will focus on finding the optimum conditions for removing undesirable contamination from the tip without damaging the tungsten core. The most often used methods are based on thermal decomposition and desorption of the contamination. On the other hand the increased temperature stimulates diffusion of metal atoms on the surface and grain interfaces, which may result in undesirable blunting of the tip apex. 4.1. Resistive heating of the tip Fig. 3. The dependence of tip radius on voltage necessary for autoemission current of 5 nA. Experimental data (black circles) are fitted by a dashed line. Dependences calculated from the Fowler–Nordheim formula for the field reduction factors 3.5 and 5.0 are included (upper and lower solid lines, respectively).
holders and the distance between the front electrode and tip apex has to be at least 10 larger than the tip wire diameter. The field reduction factor is a key parameter in the Fowler– Nordheim formula (3). Using the obtained results we can explain our simple method for estimating the tip radius. The value of the product F3=2 kR can be obtained by fitting the measured I–V characteristic using the formula (3), but we will show that the tip radius can be estimated from a single measurement with reasonable accuracy. The following experiment was performed for many tips: The tip was introduced into UHV and cleaned by one of the methods described further. Then we increased tip bias until a stable autoemission current of 5 nA appeared. The corresponding tip radius was measured ex situ by means of TEM. Results are in Fig. 3. The dependence of the tip radius on the bias for the emission current 5 nA shows that it is possible to obtain quite a good estimation of the radius only from a single measurement of the autoemission current. The diagram in Fig. 3 shows also values calculated from the Fowler–Nordheim formula (1), using a work function of F ¼ 4:5 eV and field reduction factors k¼ 3.5 and k¼5. Most of experimental points lie within the area limited by these two dependencies. The value of the emission current of 5 nA is suitable because it is high enough to be easily measured but it cannot damage the tip. The voltage corresponding to the value of 5 nA will be referred as the ‘‘emission threshold voltage’’.
To heat the whole tip to high temperature we brought the tip body into contact with a tungsten wire and used DC current for heating up the junction. Most of the power is dissipated at the junction and the small tip is uniformly heated. The temperature of the tip was measured by an optical pyrometer (error of the measurements was about 20 1C). We proceeded in the following way: The tip was annealed at a desired temperature for 10 s and then tested by the autoemission. Successively higher temperatures were used for the annealing. The dependence of the voltage necessary for an emission current of 5 nA (threshold voltage) on the annealing temperature well reflects changes of the tip apex. Results for 3 different tips are plotted in Fig. 4a. The initial decrease of the emission threshold at temperatures 500–800 1C corresponds to removing the WOx layer. Further annealing to higher temperatures results in an increase of the emission threshold voltage caused by blunting the tip apex. Two ways of damaging the apex were found. Profiles of the tips obtained by TEM after the annealing are in Fig. 4a. The tip 1 was originally sharp (radius 40 nm; field reduction factor was 4–5 due to presence of a contact electrode used for the resistive heating). The corresponding
4. Cleaning methods After finishing the etching procedure it is necessary to remove remains of the etching solution from a tungsten tip. Usually the tip is rinsed immediately in hot distilled water and ethanol – we used three beakers of ethanol and two of distilled water. To maximize the cleaning effect it is important to keep the temperature of all the bathes just below the boiling point, otherwise traces of contamination can be observed in TEM images of the tips. Tungsten tips are mostly etched in NaOH or KOH solutions. It was previously found that KOH cannot be washed out perfectly, therefore NaOH is used more frequently [26]. However, after the etching and rinsing, the tip is covered by an oxide layer. Removing the oxide layer and other contaminants under vacuum conditions in situ is rather a difficult task in case the tip apex must not be damaged. Fig. 1c and d shows SEM and TEM images of freshly etched tips after the initial washing. The tungsten core is covered by the oxide which is much thicker than the native oxide layer. There are several methods commonly used for in situ tip cleaning. We investigated the methods of heating the whole tip to high temperature, annealing the tip apex by electron bombardment and cleaning by means of ‘‘self-
Fig. 4. Cleaning the tip by direct heating to high temperature. (a) Dependence of voltage necessary for autoemission current 5 nA on annealing temperature. The tips were heated to gradually increasing temperature. Initial drop of the threshold voltage is caused by removing the oxide layer; following increase is a result of tip blunting. Profiles of tip 1 and 2 after the treatment are included (imaged by TEM). (b) Power spectra of autoemission current fluctuations before and after annealing the tip.
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plot contains jumps in the threshold voltage that indicate sudden changes of the tip apex. The TEM image shows that the tip apex was sharply ‘‘cut off’’. It can be explained by a polycrystalline structure of the tip—see Fig. 1b. The profile of the broken tip (Fig. 4a) looks as though the whole crystal grain was dropped off from the very end of the tip. This kind of damage could be probably avoided by etching the tip from a single-crystal tungsten. However, annealing at high temperature would anyway result in blunting the tip apex by thermally activated diffusion of the surface atoms (tip 2). The original apex radius of the tip 2 was about 20 nm and annealing to 1750 1C increased the radius to 50 nm. We tested and used another method to examine a state of the tip apex. The autoemission current from the just prepared tips is not very stable and contains significant fluctuations – flicker noise. The power spectra are of 1/f type which corresponds to a physical model of the tunneling transition [29–31]. The current fluctuations at low frequencies are basically caused by local changes of the tip work function due to the presence of an unstable adsorbate, by charging the dielectric surface layer and finally by random changes in the microscopic structure of the tip apex. The current noise therefore sensitively reflects the presence of unstable contamination on the stable tungsten core. Fig. 4b shows the power spectra of the autoemission current fluctuations just after tip introduction to the vacuum chamber and after annealing to 850 and 1150 1C, respectively. The mean emission current I was kept at 100 nA and the measurements were performed at a pressure of 1 10 6 Pa. The noise level significantly decreased after removal of the oxide layer (annealing to 850 1C). Further annealing to 1150 1C stabilized the tip even more. The power density of 1/f noise is far above the background level of shot noise 2eI ¼ 3 1026 A2 =Hz in Fig. 4b. According to our experience with this kind of treatment in STM, heating the tip to 800 1C leads to a significant improvement of tip behavior, but the cleaning is not perfect. After such a treatment the tips provide stable tunneling current and do not show diode-like behavior typical for the oxide-covered tips. However, the tips still frequently drop fragments of contamination onto the scanned surface. To obtain a perfectly clean and stable tip, it is necessary to flash it many times to 1800 1C. Unfortunately, it results in considerable blunting of the apex. For applications that require conservation of the original sharpness, the temperature should not exceed 800–1000 1C. Heating to higher temperatures improves the cleaning but causes blunting of the apex. The advantage of the method is that the tip temperature can be precisely controlled. 4.2. Heating by electron bombardment The electron source used for cleaning consisted of a resistively heated tungsten filament separated from the tip by a molybdenum diaphragms (at the same potential as the filament) with an aperture hole with a diameter of 2 mm. The distance between the aperture and the tip was 1 mm, a bias of þ1000 V was applied between the tip and the thermoemission filament. The tip was repeatedly heated by electron bombardment for 10 s by gradually increasing power and the autoemission threshold was tested after each flash. The results obtained for three tips are plotted in Fig. 5. TEM images show each tip after the whole treatment. The first tip had a curvature radius of about 10 nm and was cleaned at heating power 1 W. In case of the second tip, it is not clear which power exactly corresponds to optimum cleaning. However, blunting of the tip apex started at 100 mW and continued until the curvature radius of about 90 nm was reached at 1 W. The bubble-like termination is the most typical shape for tips damaged by electron bombardment. The third tip was cleaned at few milliwatts and the obtained emissivity remained constant up to 1 W when dramatically changed. The TEM image shows that the tip shape looks as if it was broken at a crystallite domain boundary.
Fig. 5. Cleaning the tips by electron bombardment. Evolution of autoemission threshold with increasing heating power (dashed lines are used to guide the eye). Profiles of the tips after the treatment were obtained by TEM.
Similar examination performed for other tips showed that the optimum power necessary for the cleaning varies in a large range from few mW to 1 W. The reason is that electrons bombard only an area very close to the apex and the exact size of the area depends on the tip shape and configuration of the electrodes. We did not find any correlation between the tip radius and the suitable heating power. In summary, the electron bombardment cleans the tip very effectively but it is impossible to predict optimal conditions. Finally, we do not recommend this kind of cleaning when the original tip shape has to be preserved. 4.3. Cleaning by controlled autoemission The field emission itself is a very effective method for tip cleaning. When an as-prepared tip is tested by the field emission, the autoemission threshold voltage gradually decreases until it is reduced usually to 1/2 or 1/3. The time necessary for such treatment strongly depends on the emission current and residual pressure in the chamber. The tip was at a distance of 3 mm from a planar counterelectrode in our experiments. At 100 nA and 106 Pa it takes tens of minutes to obtain the final value of the autoemission threshold. The origin of the cleaning effect is self-sputtering [28]. TEM images taken before and after the cleaning (not shown) reveal that the self-sputtering removes the oxide layer and does not damage the tip apex at all (if the autoemission current is kept under a certain safe value – see below). According to our experience the self-sputtering itself never results in changing the tip apex even after the cleaning for tens of minutes at a residual pressure of 5 103 Pa and autoemission current of 1 mA. Dependence of the autoemission current fluctuations on the self-sputtering time shows a significant reduction of the noise which indicates the cleaning and stabilization of the tip apex. Fig. 6a shows power spectra of the fluctuations for different sputtering times. The tip was gradually sputtered for longer time intervals. The number of the samples in the recorded time series was kept constant during each interval (4096), which results in different time scales of the data plotted in Fig. 6a. The total sputtering time is given for each data set. The tip was treated at a pressure of 3 10 6 Pa and 100 nA emission current. This method seems to be optimal for removing the oxide layer and preserving the apex sharpness. However, it is known that the tip can be destroyed during the field emission. We will try to describe the conditions resulting in the apex destruction. The destruction can be easily noticed because the autoemission current suddenly disappears. The TEM images of tips destroyed during the autoemission showed that the tip apexes were melted by the passing current [28]. There is a limiting value of the emission current for a safe tip treatment.
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of the MCFRW algorithm. Well defined shape of the tip and removed oxide layer are the crucial conditions for using this method. Three tip cleaning methods were compared—effectiveness and conditions necessary to avoid a destruction of the tip apex are described for each of them. Self-sputtering during the autoemission process can remove the oxide layer and causes minimum damage to the tungsten core of the tip apex. We described the safe conditions for such cleaning in dependence on the apex radius. Direct heating of the whole tip to high temperature is also effective and allows good control of the cleaning temperature. Heating the tip to 800 1C removes the oxide layer, but it is not sufficient to obtain a perfectly clean tip. Further heating to temperatures above 1000 1C blunts the tip due to thermally activated diffusion and increases the risk of breaking the tip apex at crystallite interfaces. Heating the tip apex by electron bombardment proved unsuitable because the safe heating power cannot be predicted.
Acknowledgements The work was partially supported by GD202/09/H041, GAUK ˇ P204/10/0952 and GACR ˇ P202/09/P033. 80010, GACR References
Fig. 6. (a) Cleaning the tip by self-sputtering. The cleaning is accompanied by decreasing noise power of autoemission current. (b) Maximum emission current which can be used for safe cleaning as a function of the tip radius (calculated).
According to our experiments, this value is related to the tip radius. We performed a simple computer simulation for a quantitative estimation: The current passing through the tip generates Joule heat. The heat is dissipated by convection and radiation. We investigated the dependence of the current that heats the tip apex to temperatures close to a melting point of tungsten. The results are plotted in Fig. 6b. The exponential profile of the tip was used in the calculations. The tips with the exponential shape are very vulnerable due to their small thermal conductance, therefore we consider the values plotted in Fig. 6b limiting and safe. The shape of most real tips in proximity of the apex is conic and it is more resistant to damage due to higher thermal conductance. For example, according to the calculation, the tip with an exponential profile and radius of 5 nm should be destroyed by 130 nA emission current. A conic tip with a vertex angle of 71 and the same radius has a limiting value of 1 mA. The dependence in Fig. 6b shows good qualitative agreement with our experiments. We note that when using field emission for tip cleaning, we always put a 1 GO resistor in the circuit to protect the tip. Removing the oxide layer results in a strong increase of the emission current that can melt the tip apex. This effect is very dangerous especially for the sharpest tips.
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5. Conclusions
[27]
We described a novel simple method that allows to estimate the tip radius from a single measurement of the autoemission current. To explain the method we presented calculations of the electric field potential in close proximity of the tip apex by means
[28] [29] [30] [31]
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