Fabrication of Smooth and Ultra-Sharp Diamond Knives of Lower than 60 nm Tip Diameter by Low Energy Oxygen Ion Beam Machining

Conventional grinding and polishing mechanisms for fabricating ultra-sharp diamond knives often cause subsurface damage and micro-chipping at the cutting edge. Compared to mechanical methods, ion beam machining (IBM) is considered more suitable for nano-scale machining of diamond materials. But the problems associated with IBM are facet formation, ripples formation and high irradiation damage. In order to overcome all theses problems, we proposed a 500 eV O/O+2 ion beam machining for the fabrication of smooth and ultra-sharp diamond knives from mechanically coarse finished samples. In our method, the knives are irradiated at tilted condition to avoid facet formation at the cutting edge. To suppress ripples formation and obtain isotropic smoothening of the surface, the stage is simultaneously rotated around the axis of ion beam incidence. To reduce the irradiation damage, a low energy reactive ion species is used in this method. A simulation model is developed to predict the profile change of knives at different tilted conditions and then compared with experimentally obtained results. We achieved sharpening down of the apex angle from 90◦ to 68◦ and from 60◦ to 48◦ by 5 hrs of machining. We also successfully reduced down the diameter of tip from 5 μm to lower than 60 nm with an average cutting edge irregularity 20-40 nm. [DOI: 10.1380/ejssnt.2012.467]


I. INTRODUCTION
Because of supreme hardness, durability and chemical inertness, diamond is the most logical material for fabricating ultra-sharp knives for ultra-microtomy.Ultra sharp diamond knives are also used as ophthalmic surgeon's instruments.Diamond knives are very expensive and usually cost several thousand dollars.A substantial percentage of this cost evolves from highly sophisticated and expensive manufacturing process.Conventionally high quality diamond knives are fabricated from natural diamond by mechanical grinding and polished using harder diamond fine powder and a lap-plate of soft materials.However, this mechanical grinding and polishing mechanism often causes subsurface damage and micro-chipping at the cutting edge [1,2].Thereby, the durability and quality of the ultra-fine polished diamond knives are lost.In comparison to mechanical methods, ion beam machining (IBM) is found more suitable for processing super hard and brittle materials like diamond.Miyamoto et al. [3] developed methods of sharpening of the diamond knives by Ar + ion beam and demonstrated the dependence of apex angle on ion energy.In their method, 10-20 keV Ar + beam machining was applied to sharpen diamond knives of apex angle 55 • .Sato et al. [4] also applied 7-10 keV Ar + beam machining for the sharpening of CVD diamond coated knife of apex angle 55 • to the order of 100 nm tip width without facet formation.However, ion beam physical sputtering induced irradiation damage and formation of ripple like structures on the diamond surface are the major shortcomings of their proposed methods.Nagase et al. [5] reported on roughening and forming of ripples on diamond surface by 1 keV Ar + beam at ion incidence angles of 60 • -80 • .Physical sputtering becomes more pronounced as the ion energy increases causing higher irradiation damage.Kawabata et al. [6] identified the irradiation damage of different ion beam etching processes by the shift of binding energy from 285.2 eV (diamond) to 284.2 eV (graphite) and broadening of the peak in the XPS profile.They reported that the reactive ion beam assisted etching (RIBAE) and reactive ion beam etching (RIBE) have lower tendency to form graphite than ion beam etching (IBE) and ion beam assisted etching (IBAE).Even though the damaged surface layer can be eliminated by chemical or mechanical polishing, it causes significant deviation from the desired shape profile of the cutting edge and hence the performance is degenerated.
In order to ensure least irradiation damage, we developed 500 eV O + /O + 2 ion beam machining for the fabrication of ultra-sharp and smooth finished diamond knives from mechanically pre-finished samples possessing hemicircular shaped tip.In our method, the knives were irradiated at tilted condition instead of normal incidence sputtering to avoid forming of facet at the cutting edge.The working stage was simultaneously rotated around the axis of ion beam incidence which aids in isotropic smoothening of the surface and suppression of ripple formation.By our proposed machining method, we obtained a sharp knife of apex angle lower than 50 • and tip width lower than 60 nm.Moreover, the surface of the processed knives appeared smooth and ripple-free.A simulation method was also developed to predict the profile changes of knife by the proposed machining method.Our experimental and simulation result are well coincident with each other.

II. EXPERIMENTAL SETUP
The machining of the knives was conducted in an electron cyclotron resonance (ECR) type ion beam apparatus which can generate broad ion beam in the energy range of 0.3-3 keV.The pressure inside the plasma generation chamber was set at 1×10 −4 Pa and ion current density was 0.85 mA/cm 2 .
Diamond knives of mainly two different shapes were used in our experiment: (a) knife of initial apex angle, α = 90 • and tip diameter, d = 5 µm and (b) knife of initial apex angle, α = 60 • and tip diameter, d = 5 µm.The frequency of rotation was fixed at 1 rpm.After the machining, the knives were treated with ultra-sonic bath.Due to broad ion beam facility, several knives can be processed at the same time, thereby reducing the total time and cost of manufacturing to a great extent.The simulation model was developed using Matlab software.The unprocessed and processed diamond knives were observed by a scanning electron microscope (SEM).

III. SIMULATION
Due to hemi-circular shape of the tip, the ion incidence angle on each segment of the cutting edge of the diamond knife will be different and hence the etching rate.As shown in Fig. 1(a), we consider an initial point P 0 (x 0 , y 0 ) on the curved surface of the tip where the ion incidence angle is ϕ 0 .When the knife is tilted at an angle β, the ion incidence angle at point P 0 would be ϕ = β − ϕ 0 .Figure 1(b) shows the direction of rotation of the samples.Due to ion beam machining for t hours, the point P 0 (x 0 , y 0 ) will move to a new point P (x, y) which can be expressed by the following equations according to Ducommun et al. [7]: where, V (ϕ) represents the etching rate corresponding to ϕ. ion beam machining at different tilted conditions.It is found from these figures that the knives may either become blunt or sharp depending on the tilt angle, β.However, in both cases it is apparent that the sharpness of the knives increases as the tilt angle increases.Therefore, the tilt angle for the desired level of sharpness (in terms of narrowing down the apex angle) can be predicted from our simulation method.from 90 • to 100 • .When β was increased to 60 • , the knife became sharp with a little reduction of the apex angle from 90 • to around 80 • as shown in Fig. 3(c).Therefore, we increased the tilt angle further to obtain low apex angle.When β was set at 80 • , the knife was sharpened down to an apex angle 68 • as shown in Fig. 3(d).

IV. EXPERIMENTAL OBSERVATION AND DISCUSSION
Figure 4(a) shows the SEM image of the diamond knife of apex angle 60 • with the tip diameter 5 µm.Figure 4(b) represents the SEM image of the processed knife after machining for 5 hrs at β = 90 • and it is obvious from this figure that the apex angle was shrunk down to around 48 • after the machining process.Comparing the experimental results as represented in Figs. 3 and 4 to Fig. 2, it can be said that the experimental results are in good agreement with the predictions of profile change obtained simulation.In order to observe the condition of the surface as well as measure the tip width before and after the machining, SEM images were taken from the top of the diamond knives as represented in Fig. 5. Figures 5(a 1 ) and (a 2 ) show the top view images of unprocessed and processed diamond knife of initial apex angle 90 • .As shown in the figures, initially the tip width was 5 µm with some cutting edge irregularities (CEI, defined by the deviation from the major tip width) as high as 1 µm.Some protrusions were also observed near the cutting edge.After 5 hours of 500 eV O + /O + 2 ion beam machining at β = 80 • , the tip width was reduced down to around 50 nm with an average CEI of 20 nm.The top view images of the knife of initial apex angle 60 • before and after the machining are shown in Figs.5(b 1 ) and (b 2 ) respectively.From these figures it is obvious that initially the surface condition of 60 • knife was very poor with many protrusions and irregularities present all over the surface.The CEI was as high as 5 µm i.e. almost equal to the defined tip width.After the 500 eV O + /O + 2 ion beam machining at β = 90 • for 5 hrs, the tip width was reduced down to around 60 nm with an average CEI of 40 nm.From the images of the processed knives it is apparent that no ripples were formed during the machining.The reason of suppressing ripples formation may be ascribed to rotation of the sample during machining.According to Bradley and Cirlin [8,9] when the sample is rotated, the smoothening effects of viscous flow and surface self-diffusion may prevail over the roughening effect of the curvature-dependent sputter yield and generate a smooth surface.Moreover, we found in all cases of 500 eV O + /O + 2 ion beam machining, most of the protrusions were completely etched out except for the big ones and the reason can be attributed to enhanced erosion of the surface protrusions at the grazing incidence [10][11][12][13].When the knives are processed at sharp tilted condition, the ion incidence angle near the cutting edge is close to grazing incidence.Usually at grazing incidence, the etching rate is very low due to increased amount of reflection of the incident ion from the surface.The situation is almost inverse in case of the protrusions near the surface steps as they are situated normally on the surface.Since the etching rate of 500 eV O + /O + 2 ion beam machining is highest at the normal incidence [14], the protrusions and irregularities around the cutting edge are easily etched out due to grazing incidence sputtering at the tip.Therefore, sharpening of the cutting edge and polishing of the surface were simultaneously achieved in our proposed method.Moreover, minimal irradiation damage is expected to occur in our method due to low energy reactive ion beam machining (RIBM) using oxygen.In RIBM, chemical sputtering takes a significant role and it becomes more pronounced towards lower ion energy [14].Therefore, the physical sputtering induced damage structure on the diamond surface is simultaneously etched out by the chemical sputtering creating volatile substances like CO 2 or CO.Hence, the diamond composed base layer is recovered mostly in case of 500 eV O + /O + 2 ion beam machining.

V. CONCLUSION
We proposed a 500 eV O + /O + 2 ion beam machining process for the fabrication of ultra-sharp diamond knives from mechanically coarse finished samples.To avoid facet formation at the cutting edge, the knives were processed at tilted condition and to suppress ripple formation, the stage was rotated simultaneously around the axis of ion beam irradiance.A simulation method was developed to predict the profile changes of knife by the proposed machining method.Our experimental and simulation results are well coincident with each other.We achieved sharpening down of the apex angle from 90 • to 68 • and 60 • to around 48 • for two different samples.In both cases, the tip width reduced down from 5 µm to lower than 60 nm with an average CEI 20-40 nm.The surface of the processed knives appeared smooth and ripple-free and the protrusions on the pristine surface were cleared out by enhanced erosion of the surface protrusions at the grazing incidence.Therefore, in our proposed method, knives of any size and shape can be sharpened and simultaneously polished with the feasibility of batch fabrication.Due to low energy RIBM, irradiation damage is expected to be minimal.

FIG. 1 :
FIG. 1: Schematic diagram of (a) ion beam machining a tilted condition (b) direction of sample rotation.

Figures 2 (
a) and 2(b) show the profile changes of diamond knives of apex angle 90 • and 60 • respectively with the tip diameter of 5 µm by 2 hrs of 500 eV O + /O + 2

Figure 3 (Fig. 4 FIG. 4 :
Figure 3(a) shows the SEM image of an unprocessed diamond knife with apex angle of 90 • and tip diameter of 5 µm.Figures 3(b)-(d) show the SEM images of the processed knives by 5 hrs of 500 eV O + /O + 2 ion beam machining at the tilt angles, β = 40 • , 60 • and 80 • respectively.As shown in Fig. 3(b), at β = 40 • , the knife became blunt along with broadening of the apex angle