Nanotribological characterization of hydrogenated carbon films by scanning probe microscopy

ELSEVIER

Thin Solid Films 258 ( 1995) 75 -81

Nanotribological Zhaoguo

characterization of hydrogenated by scanning probe microscopy

Jiang,

C.-J. Lu, D.B. Bogy, C.S. Bhatia”,

Computer Mechanics Laboratory, Department of Mechanical Engineering,

carbon films

T. Miyamoto**

University of California at Berkeley, CA 94720, USA

Received 20 April 1994; accepted 14 September 1994

Abstract Mechanical and tribological properties of hydrogenated carbon films were evaluated using three newly developed nanotribological characterization techniques based on scanning probe microscopy, including nano-hardness tests and nano-wear tests using a point contact microscope and nano-friction tests using a friction force microscope. It was found that the nanoindentation hardness decreases with increase of hydrogen content in the films, and the substrate influence on the hardness values is not significant for indentation depths less than a quarter of the thickness of the carbon films. The critical load for wear on the friction force versus loading force curve also decreases with hydrogen concentration, whereas the nano-wear depth and the nano-friction coefficient increase. Our results show that the three characterization methods are very effective in determining mechanical or tribological properties of ultrathin films and optimizing the fabrication process parameters. Keywords:

Atomic

force microscopy; Coatings; Hydrocarbons; Tribology

1. Introduction

Due to their superior properties, amorphous carbon films have been widely used as protective films in the hard disk industry. These properties include high hardness, good wear resistivity, low static and dynamic friction coefficients, chemical inertness, low cost, process simplicity, good process control, and coating homogeneity [ 11. Carbon films used in magnetic media applications can be fabricated by several processes [2]. In every case, the physical properties of films change with process parameters, and it is important to understand the relationship. This has led to extensive studies over the last two decades. A variety of techniques have been used to identify microstructures of amorphous carbon films, including Raman spectroscopy [ 31, Fourier transform infrared spectroscopy [4], electron spectroscopy for chemical analysis [5], electron energy loss spectroscopy [6], nuclear magnetic resonance [7], *Storage Systems Division, IBM Corporations, USA. **On leave from the Kaneko Research Interdisciplinary Research Laboratories. 0040-6090/95/$9.50 d: 1995 SSDlOO40-6090(94)06376-I

Elsevier

San Jose, CA 95193,

Laboratory

at the

Science S.A. All rights

NTT

reserved

optical measurements [ 81, transmission electron microscopy [9], and X-ray photoemission spectroscopy [5]. The main concerns in magnetic media applications are the mechanical and tribological performances of the films. Several limited techniques for the mechanical and tribological characterization of amorphous carbon films have been reported. These include hardness measurements by microhardness indenters [lo], scratch tests [ 111, pin-on-disk tests [ 121, contact start-stop tests or drag tests [ 131, and chemical reactivity tests [ 141. However, the current development trend in the hard disk industry is to fabricate ultrathin carbon films with thicknesses only 50-400 A in order to reduce magnetic spacing and increase storage capacities. For such thin films, limitations of the above-reported methods are exposed. Nanomechanical and nanotribological characterizations of films are hence believed to be critical in determining tribological performance and optimizing the process parameters of the films. Recently, several nanotribological characterization methods for thin solid films have been developed based on scanning probe microscopy, such as nanoindentation hardness tests and nano-wear tests using a point contact microscope, and nano-friction tests using a friction force

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2. Jiang et al. / Thin Solid Films 258 (1995) 75-81

microscope. Lu et al. [ 15,161 developed hardness measurements using a scanning probe microscope called a point contact microscope. By this technique it was shown that hardnesses of films with 20- 100 nm thickness can be obtained. Nano-wear tests by use of a point contact microscope were successfully employed by Miyamoto et al. [ 171to investigate the microscopic wear resistance of an ion-implanted silicon. This method can be used to characterize wear properties of films as thin as only a few nanometers. Jiang et al. [ 181used a friction force microscope to measure the friction force versus normal loading force for sharp diamond tips sliding on solid films, and found that there are two distinct friction regimes on the friction curves. Relative nanotribological characteristics of films are obtained by comparisons of critical loads and friction coefficients in different regimes for different materials. In this research, we used these three nanotribological characterization techniques to determine the effect of hydrogen content on the microscopic tribological performance such as hardness, wear resistance and friction coefficient of the hydrogenated carbon films. Another purpose is to show the effectiveness and feasibility of the nanotribological characterization methods on evaluating ultrathin solid films.

2. Experimental apparatus and methods 2.1. Experimental

apparatus

Two types of scanning probe microscopes were used, a point contact microscope and a friction force microscope. Fig. 1 shows a schematic diagram of the point contact microscope (PCM) [ 19,201. Two micro-screw heads with 0.1 pm axial graduation are used to move the tube scanner and the optical head along the Z direction for coarse adjustment. The tube scanner holds the sample

by magnetic force and can move the sample in X, Y and Z directions with resolution better than 1 nm. With a sensitivity of 8 nm V- ‘, the Z displacement of the tube scanner is controlled by two d.c. power supplies with a combined range of + 200 V to produce a total Z range of about 3 pm. The cantilever tip assembly with a diamond tip is attached by magnetic force to the laminated PZT actuator with a sensitivity of about 150 nm V ‘. A d.c. power supply with a working range of O70 V controls the laminated PZT actuator, and is therefore capable of moving the tip assembly over a total range of about 10 urn. The optical head, as a displacement sensor with resolution better than 1 nm, measures the deflection of the cantilever tip. The PCM is connected through an interface circuit to the Digital Instruments Nanoscope II controller and uses the control and display software of this instrument. Using a very light loading force, the PCM can measure surface topography without causing any damage to the surface, just as the standard atomic force microscope can do. However, with the PCM, heavier loading forces with a wide range of values can be applied to the tip, so that nanoindentation hardness tests or nano-wear tests under various loading conditions can be conducted. There are two ways to change the value of the loading force on the tip. One is to change the laminated PZT power supply voltage and the other is to change the setpoint current of the PCM. Both methods have the effect of changing the deflection of the cantilever spring that carries the tip. After a nanoindentation or nano-wear test with a heavy load, the loading force can be reduced to a light load, and the PCM is controlled to scan the sample surface for surface image acquisition. Hence, the change resulting from nanoindentation or nanowear test is obtained. Fig. 2 shows a schematic diagram of the friction force microscope (FFM) with a double parallel-leaf spring

Sensor 1 Laminated PZT

Focusing-error-detemon

Double Parallel-leaf Spring Unit \

PZT tube scanner Fig. 1. Schematic diagram of the point contact microscope [ 19,201.

Fig. 2. Schematic diagram of the friction force microscope [IS].

II

Z. Jiang et al. 1 Thin Solid Films 258 (1995) 75-81

tip assembly [ 181. The basic working principle is similar to the PCM, except one more sensor is added to measure the friction force. A sample is attached on a PZT tube scanner by a magnetic force. The PZT tube scanner moves the sample along the X, Y and Z directions with nanometer resolution. A three-sided pyramid diamond tip is mounted on a double parallel-leaf spring unit. A focusing-error detection type optical head (sensor 1) is used to measure the vertical deflection of the tip (point A), which determines the normal loading force of the tip. Another optical head of the same type (sensor 2) is used to measure the horizontal deflection of the tip (point B), which determines the friction force between the sample and the tip. The resolution of both sensors is better than 1 nm. The FFM is also connected through an interface circuit to a controller (Digital Instruments Nanoscope III) and uses the control display software of this instrument. Both the surface topography and friction image can be obtained simultaneously. In addition, the friction signal is directed into a dynamic signal analyzer (HP3562A), so the friction loop can be recorded and processed. The detailed calibration method for this FFM was described by Lu et al. [21]. 2.2. Experimental

methods

Three methods were used in this research: nanoindentation hardness tests by the point contact microscope, nano-wear tests by the point contact microscope and nano-friction tests by the friction force microscope. 2.2.1. Nanoindentation hardness tests by the PCM Triangular diamond tips with tip radius less than 100 nm, which are attached on leaf springs of various lengths for providing different stiffnesses, were controlled by the point contact microscope to conduct both indentation and surface measurement [ 161.The indentation was performed by either moving the sample with the tube scanner or moving the tip with the laminated PZT. The loading force acting on the tip was calculated by multiplying the tip displacement with the spring constant. After the indentation, the surface image was acquired by tip scanning so the indentation mark was measured precisely. The hardness is defined as the ratio of the indentation force to the projected area of the residual indentation. Due to the good resolution of the instrument, hardness tests by the PCM can be conducted with an indentation depth in a range of 5-100 nm. 2.2.2. Nano-wear tests by the PCh4 The method was developed by Miyamoto et al. [ 171. Using the point contact microscope, the surface with an area of 5 urn x 5 urn was scanned with a light load to obtain the surface topography image using a diamond

tip. Then the scan size was reduced to 2 urn x 2 urn at the same location, and the loading force increased to a larger value. After each wear cycle, the loading force and scan size were reset to their original values and the surface topography is measured. The wear depth can be acquired from the cross-section of the wear mark. The wear depth curves versus either wear cycles or loading force are plotted from the results for comparing the wear resistance of different films. A wear depth as small as 1 nm can be detected [22]. 2.2.3. Nano-friction tests by the FFM The nano-friction tests were performed by the FFM with a double parallel-leaf spring tip assembly. The first step was to use the diamond tip with a very light load to measure the surface topography on a 6 urn x 6 urn area making sure no severe contamination or defects exist. Then on a 4ym x 4 urn area at the center of the above-measured region, the surface was scratched using the same tip with a gradually increased normal loading force. At the same time, the friction loop was measured and the average friction force corresponding to the normal loading force was obtained by taking half of the amplitude of the friction loop. When the diamond tip arrives at the upper end of the area, the normal loading force is immediately decreased to a very light value, and the surface topography (6 urn x 6 urn) is again obtained with the same tip scanning the surface. The surface wear or damage during the friction test was obtained by analyzing the surface image. Both the friction force and wear depth versus loading force can be measured by this method [ 181. 2.3. Tips and spring constants Three three-sided triangular diamond tips were used. The tip used to conduct indentation tests was attached on a single parallel-leaf spring.The nominal radius of the tip was 80 nm, and the spring constant was 105.1 N m ‘. The tip radius for nano-wear tests was 60 nm, and the spring constant was 17.0 N rn- ‘. The tip was attached on a single-leaf spring. For nano-friction tests, we made a tip with a double parallel-leaf spring. The nominal radius was 120 nm with a normal spring constant of 38.8 N m-’ and a lateral spring constant of 36.0 N m- ‘. 2.4. Hydrogenated

carbon samples

Amorphous carbon films of 25 nm thickness were deposited on Si( 100) substrates by magnetron sputtering with a graphite target. During the sputtering process, the flow rates of the argon and hydrogen gases were adjusted to control the hydrogen content. Three hydrogenated carbon films were obtained with different hydrogen concentrations of about 2%, 28% and 40% in the films, respectively.

Z. Jiang et al. / Thin Solid Films 258 (1995) 75-81

0’

I 2

6

Indentation Fig. 3. Hardness carbon films.

versus

14

10

indentation

Depth depth

18

O

for

the

10

0

(nm)

20

30

Hydrogen Concentration hydrogenated

Fig. 4. Hardness versus hydrogen concentration tion depth for the hydrogenated carbon films.

40

(96) with 5 nm indenta-

3. Results and discussions 3.1. Nanoindentation

tests

Fig. 3 shows the hardness versus indentation depth for three hydrogenated carbon films. An important observation is that the hardnesses for all samples increase with indentation depths, and tend to converge after about 10 nm indentation. This shows the effect of the silicon substrate on the measured hardness of the thin films, and it indicates that the indentation depth is a very critical factor in evaluating the hardness of ultrathin films as thin as the 25 nm films used in this study. The increase of the hardness versus indentation depth curves is due to the fact that the silicon substrate is harder than all three films. This is clear evidence that one has to be very cautious in obtaining hardness values for ultra thin films. Considerations of the indentation depth and the substrate effects are necessary. It was observed that the values of the hardness between 4 and 6 nm are relatively constant, which indicates the effect of the substrate was not significant in this range. Hence, it is acceptable to compare the hardnesses of the different films without significant influence from the substrates using the values obtained for indentation depths less than 6 nm. To illustrate the relation between the hardness and hydrogen content for the hydrogenated carbon films, a plot of these hardness values is shown in Fig. 4 versus hydrogen gas content. The tendency is that the hardness decreases with the hydrogen concentration. 3.2. Nano -wear tests Fig. 5 shows the wear depth versus wear cycles for the hydrogenated carbon films obtained from the nanowear tests with a loading force of 28 uN acting on the diamond tip. For the film with 2% hydrogen, the wear rate is very low, with a wear depth of only 6 nm at the 24th wear cycle. The film with 28% hydrogen has a

0

4

8

12

Wear Cycles Fig. 5. Wear films.

depth

versus

16

20

24

(cycles)

wear cycles for the hydrogenated

carbon

higher wear rate with a wear depth at the 24th wear cycle of 15.5 nm. Both curves are nearly linear with wear cycles because the film thickness is 25 nm. The wear depth of the film with 40% hydrogen increases relatively fast with wear cycles, with the wear depth reaching 29.5 nm at the 16th wear cycle. After 16 wear cycles, the wear rate drops dramatically to almost zero. The reason for this is that the film is worn off, and the tip scans the silicon substrate with a load too low to wear it. The drop in the wear rate indicates again that the silicon substrate is harder and more wear resistant than the film. Typical wear marks for the three hydrogenated carbon films after 16 wear cycles are shown in Fig. 6. For a clearer observation, the wear marks are displayed by their inverse images, showing depth as height. Clearly, the film with 40% hydrogen has the largest wear depth, with a value of 29.5 nm (Fig. 6 (a)). The wear depth for the film with 28% hydrogen is 11.0 nm (Fig. 6 (b)). The film with 2% hydrogen has the smallest wear depth, 6.0 nm. To illustrate the effect of hydrogen content on the wear depth, a plot of wear depth versus hydrogen concentration after 16 wear cycles for the films is shown in Fig. 7. It is observed that for the three values

79

Z. Jiang et al. / Thin Solid Films 258 (1995) 75-81

considered the wear depth concentration.

increases with hydrogen

3.3. Nano-friction tests The friction force versus loading force curves are shown in Fig. 8 for the three carbon films with a diamond tip sliding on their surfaces. There is a turning point on each curve, which divides the curve into two distinct regimes: a low friction regime and a high friction regime. The friction increases linearly with the loading force before the turning point, whereas the growth rate becomes much faster after the turning point. Further observation shows that the critical loading forces corresponding to the turning points are different for the three films. Fig. 9 shows the wear depth versus loading force curves for the films measured, corresponding to each test point in Fig. 8. It is seen that the critical points in Fig. 8 correspond to the load points for the beginning of wear on the film surfaces. The low friction region is a no-wear regime, whereas the high friction regime is accompanied by surface wear. The film with 40% hydrogen starts to wear at the lowest load ( 131.4 uN), and the wear depth reaches 39 nm at 192.1 uN. The

(bj

0

Cc) Fig. 6. Wear marks for the hydrogenated carbon films after 16 wear cycles. (a) Carbon film with 40% hydrogen; (b) carbon film with 28% hydrogen; (c) carbon film with 2% hydrogen.

3

30.

$

30-

;

50

100

150

Loading Force Fig. 8. Friction carbon films.

force

versus

--

40% Hydrogen 28 %IHydrogen 2% Hydrogen

9 4 n

loading

force

200

(pN) for

the hydrogenated

20m 10 -

lo-

B 0

* 0

10

20

30

40

Hydrogen Concentration (96) Fig. 7. Wear depth versus hydrogen concentration genated carbon films after 16 wear cycles.

for the hydro-

50 100 Loading Force Fig. 9. Wear depth versus loading films.

150 (CCN)

force for the hydrogenated

carbon

80

2. Jiang et al. 1 Thin Solid Films 258 (1995) 75-81

0

10

30

20

Hydrogen Concentration Fig. 10. Critical load versus genated carbon films.

hydrogen

concentration

40

for the hydro-

40% Hydrogen 28% Hydrogen 2% Hydrogen

0.08

20

30

Fig. 12. Friction coefficient hydrogenated carbon films.

versus

hydrogen

46

(5%)

concentration

for the

all nearly constant in the low friction regimes (no-wear regimes). After the turning points, the friction coefficients dramatically increase with loading force for all three films due to the wear effects. The friction coefficients in the low friction regimes versus hydrogen content are shown in Fig. 12.

4. Conclusions The hydrogen content in the hydrogenated carbon films has significant effects on their nanomechanical or nanotribological characteristics, such as nano-hardness, nano-wear depth, critical load for wear and nano-friction coefficient. We measured three hydrogenated carbon films with 2%, 28% and 40% hydrogen content respectively, and it was concluded that the nanoindentation hardness and critical load for wear decrease with the hydrogen content in the films, but the nano-wear depth and nano-friction coefficient increase.

Acknowledgments This research was supported by the Computer Mechanics Laboratory at the University of California at Berkeley and the KANEKO Research Laboratory at the NTT Interdisciplinary Research Laboratories.

0.12

-

10

Hydrogen Concentration

wear occurs at 161.7 uN for the film with 28% hydrogen, the wear depth is 30 nm at 192.1 uN. For the film with 2% hydrogen, the wear starts at the highest value (181.9 IN), the wear depth is only 10 nm at 192.1 uN. Hence, the critical loads are actually the maximum loading forces of the particular diamond tip that the film surfaces can support without wear. Again, to illustrate the effect of hydrogen content on the critical load, the relation of critical loads versus hydrogen concentration for the three films is shown in Fig. 10. The critical load decreases with the increase of hydrogen content. A very interesting point is that the tendency of the critical load curve is quite similar to that of hardness (Fig. 4). The reason for this is very clear: both hardness and critical load reflect the resistance of films against local failure. The advantage of the critical load measurement over hardness measurement is that we do not need to measure the indentation depth, which is the primary difficulty in the indentation hardness tests on ultrathin films. The friction coefficients versus loading force for the three films were calculated from the data in Fig. 8 by dividing the friction force by the loading force. The results are shown in Fig. 11. The friction coefficients are

3 ‘G g Q)

0

(96)

References

s .ag

[II N.H.

004 .

0.00

Fig. 11. Friction carbon films.

50

100 Loading Force

coefficient

versus loading

150

2ot

@IV)

force for the hydrogenated

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