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Tribological investigations on micromachined silicon sliders
Tribology International 36 (2003) 279–283 www.elsevier.com/locate/triboint
Tribological investigations on micromachined silicon sliders Hans H. Gatzen ∗, Michael Beck Institute for Microtechnology, Callinstrasse 30A, 30167 Hanover, Germany
Abstract The continuous decrease in flying height and subsequent machining challenges spurred an interest in alternative slider materials, particularly single phase ones. Furthermore, due to a desire of the recording head industry to more closely follow semiconductor fabrication technologies, silicon has been taken into consideration as an interesting alternative to alumina-titanium carbide (Altic), used presently. In case silicon is to be introduced, extensive tribological investigations are required. This paper attempts to shed some light on some basic tribological properties of silicon sliders used on DLC coated metal disks by doing a pin-on-disk test at a low rotational velocity. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Microfriction; Tribological properties; Pin-on-disk tests of silicon on DLC
1. Introduction Since the introduction of thin film sliders in 1979, the slider material of choice has been aluminatitanium carbide. However, the continuous decrease in flying height and subsequent machining challenges spurred an interest in alternative slider materials, particularly single phase ones. Furthermore, due to a desire of the recording head industry to more closely follow semiconductor fabrication technologies, silicon has been taken into consideration as an interesting alternative slider material. In case silicon is to be introduced, extensive tribological investigations are required. Our investigations focussed on basic tribological investigations on micromachined silicon sliders used on DLC coated metal disks. Outside the data storage industry, these results are also of interest for micro electro-mechanical systems (MEMS), in particular microactuators. To keep the investigations as general as possible, and not be affected excessively by ABS geometry issues, the following approach was taken: two types of dummy sliders with rather rough ABS were tested on a commercial disk. By choosing a nominal average roughness Ra in the order of 15–20 nm, ten times the roughness of a typical production slider, the effects of adhesion were
∗
Corresponding author. Fax: +49-511-7622867. E-mail address: [email protected] (H.H. Gatzen).
eliminated. To take differences in contact geometry (i.e. ABS contour) into account in a most general fashion, a pico size slider (1.2 mm length×1.0 mm width) was built in two versions. A type I slider was left unpatterned and had a flat contact area the size of the slider footprint (hence the need for a sufficient surface roughness to avoid “joeblocking” due to adhesion). A type II slider was patterned and featured round protruding contact areas, each with a 0.45 mm diameter, and the same roughness as slider type I. This patterned slider was intended to simulate a three point contact.
2. Experimental 2.1. Set-up A test head consisted of a dummy slider mounted on a flexure (Fig. 1). As on disk drives, the preload was created by a combination of the geometry of the flexure’s
Fig. 1.
0301-679X/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0301-679X(02)00198-6
View of a test head consisting of a dummy slider and a flexure.
280
H.H. Gatzen, M. Beck / Tribology International 36 (2003) 279–283
leaf spring and by choosing the appropriate distance between the mounting height and the disk surface (“z”height). The z-height is a designed feature of a flexure, in a nominal mounting position the gimbal angle has to be such that the slider is positioned parallel to the disk. For the tests, the gram load of the test heads was adjusted consecutively between 10 and 60 mN. As a counterpart for the sliders, commercial type rigid disks, taken from state of the art rigid disk drives, were used. The disk base material was aluminum coated with the films customary on rigid disk media. The two top layers are DLC film coated with a lubricant. While no specification was available for this disk this should be irrelevant for executing the test, since all tests were done under the same conditions. 2.2. Probe preparation For fabricating production type rigid disk sliders, a wafer level process creates the thin film read/write elements. After completing wafer fabrication the wafer is separated into row bars. Each row bar represents a row of future sliders. One kerf sidewall (the one close to the read/write elements’ pole face) represents the future air bearing surface, while the opposite is the slider mounting surface. The read/write elements, originally located at the wafer’s surface, end up on the slider’s trailing edge. After lapping or nanogrinding the row bars to create the ABS plane the row bars, turned by 90° with the mounting surface face down, are rejoined together in an “artificial” wafer for ABS patterning. Through a sequence of batch processes consisting of photolithography and physical ion beam etching (IBE) or a physical-chemical reactive ion etching (RIE), the desired air bearing surface profile is created. Finally, the row bar “wafer” tends to receive other treatment like DLC coating or addition of studs. Afterwards, the row bars are separated and diced into sliders. For assembling the sliders into recording heads, the sliders are mounted on a flexure and the read/write elements are contacted using a microflex cable. For creating silicon slider dummies, the respective process steps of slider fabrication as outlined before were used. In particular, the test surfaces were created by a combination of photolithographic masking and ion beam etching. Since the wear behavior of silicon was to be tested, no DLC coating was applied. The raw material for fabricating the slider dummies was a polished 525 µm thick silicon wafer with a {111} crystal orientation at its surface. Through a dicing process the wafer was cut into 1 mm×1.2 mm (pico size) slider dummies. The contact surface was machined flat by means of nanogrinding and the desired average surface roughness Ra of app. 15–20 nm was achieved (type I). Additionally, for the type II slider, three round spots on the slider surface, with a combined contact area of
0.6 mm2, were covered with photoresist. In a two hour ion beam etching (IBE) process, the areas not covered by photoresist were recessed. By doing so three plateau areas. 1–2 µm above ground were created. Fig. 2 depicts the geometries of the contact surfaces. The variation between the photolithographically defined surface areas is less than 5%. The nanoroughness measurements taken with an Atomic Force Microscope are shown in Fig. 3. 2.3. Test stand As a test stand, a pin on disk tester was used [1]. Fig. 4 shows a schematic view. The tester features an air bearing rotary table with an extraordinary high mass which allows to rotate it with rather low velocities of one revolution every 330 s (0.18 min⫺1). An electronic commuting motor is driving the rotary table through a friction drive. This combination results in minimal table vibrations. Despite its low velocity this tester achieves an absolutely even rotation of the disk. A test disk was mounted on the rotary table. The tester is equipped with a stage to hold the test head. To allow an appropriate adjustment of the test head z-height, the stage is vertically adjustable. For measuring the frictional force of the test head, the pin on disk tester is equipped with a commercial optical frictional force measurement. It consists of a spring system allowing a deflection in direction of the head’s frictional load and a fiberoptic system for measuring the degree of deflection. The light intensity varies proportional to the drag force dependent on the degree of overlapping between a light dot and a sensor area (Fig. 5). At the beginning of each test, each test head was mounted on the tester. For verifying the nominal force of a test head for a test run, the probe table was replaced with a gram load cell. Next, the head was mounted in the tester and was lowered on the gram load cell through the test system’s z-height adjustment until the nominal value was reached. In this position, the gram load was determined. In case the gram load was off, the gram load was readjusted by appropriately bending the flexure’s leaf spring. For running the friction tests, the gram load measuring system was replaced with a probe table holding the disk. Before each test, the slider was cleaned in an aqueous ultrasonic bath followed by a dip in isopropanol and drying with nitrogen. The tests were executed for gram loads between 10 and 60 mN. As mentioned before, the time per revolution was 330 s, resulting in a relative velocity at the head disk interface of 0.76 mm per s.
3. Tests and results Fig. 6 depicts a graph of a typical frictional force over time during the measurement. It is obvious that there are
H.H. Gatzen, M. Beck / Tribology International 36 (2003) 279–283
Fig. 2.
281
Surface topography of the two types of picosliders (Ieft hand: flat, type I, right hand: three protrusions, type II).
Fig. 4.
Schematic representation of the pin-on-disk tester.
major variations over the test period. Such variations were already described by Bogy et al. [2]. A variation of frictional values within one revolution of up to 100% was found. For a further comparison of the test results, an average friction value was used. Fig. 7 depicts the relationship between the nominal force and the average frictional value for both types of sliders. For each series of tests three sets of measurements were taken. Fig. 8 presents the relationship between the apparent contact area and the surface pressure. As mentioned before, the tests were performed with loads ranging from 10–60 mN. For the unpatterned slider (type I), the average friction value µI is app. 0.24, while the patterned slider with the three contact dots (type II) show an average friction value µII of app. 0.34. Besides looking at the frictional values as a function of the gram load, their dependence on the surface pressure (based on the apparent contact area) is of interest. Fig. 9 shows the results. As before, the unpatterned slider had the lowest frictional value.
4. Discussion Fig. 3.
Nanoroughness of the two types of sliders and of the disk.
It is interesting to compare these silicon friction data with Altic test results. Xu and Bhushan [3] report coef-
282
H.H. Gatzen, M. Beck / Tribology International 36 (2003) 279–283
Fig. 5.
Pin-on-disk tester.
Fig. 6. Typical variation of the coefficient of friction µ measured over one revolution (330 s per revolution, slider type I, 28.5 mN gramload).
Fig. 8. Average surface pressure as a function of gram load for a slider type I (unpatterned) and type II (patterned).
Fig. 7. Average friction over one revolution as a function of gram load for a slider type I (unpatterned) and type II (patterned).
Fig. 9. Average friction over one revolution as a function of surface pressure for a slider type I (unpatterned) and type II (patterned).
ficients of friction µ for Altic of 0.22 up to 0.4. depending on the time the disk is in use. Furthermore, it may be observed that a decrease in apparent contact area results in an increase of the coefficient of friction for silicon. The experimental data show that there is a relationship between the frictional value and the apparent contact area of a slider. An increase of the apparent contact area results in a decrease of friction. As a result
it may be expected that the shape of the ABS will affect the frictional value of the slider. Scherge et al. [4] made similar investigations but came to different conclusions. In their case an increase of the contact area also resulted in an increase of friction. However, for their experiment they were using polished silicon surfaces, while in our case the sliders do feature a definitive roughness and also, the disk is lubricated. Scherge concluded that the
H.H. Gatzen, M. Beck / Tribology International 36 (2003) 279–283
increase was caused by an increase of adhesion forces due to the coplanarity of the two interface surfaces. Due to the fact that the tests reported on in this paper were done with rather rough sliders most likely explains the different results.
5. Conclusion and outlook With this series of tests, some rather basic results regarding the frictional behavior were achieved. Obviously, there is quite a spectrum of work to be done should a silicon slider be introduced. Since it is rather unlikely that silicon sliders would be used without DLC, investigations comparing the frictional behavior of DLC
283
coated Altic sliders with comparable silicon sliders seem to be most pressing. References [1] Beck M, Wortmann A, Lu¨ thje H, Gatzen HH. Novel test equipment to measure the properties of microtribological interfaces. 13th International Conference on Wear of Materials, Vancouver, 2001. [2] Bogy DB, Yun X, Knapp BJ. Enhancement of Head-disk interface durability by use of diamond-like carbon overcoats on thes slider’s rails. IEEE Transactions on Magnetics, 1994; p. 369–74. [3] Xu J, Bhushan B. Friction and durability of ceramic slider materials in contact with lubricated thin-film rigid disks. Proc Instn Mech Eng 1997;211(J):303–16. [4] Scherge M, Schaefer JA. Microtribological investigation of stick/slip phenomena using a novel oscillatory friction and adhesion tester. Tribology Letters 1998;4:37–42.
Tribological investigations on micromachined silicon sliders Hans H. Gatzen ∗, Michael Beck Institute for Microtechnology, Callinstrasse 30A, 30167 Hanover, Germany
Abstract The continuous decrease in flying height and subsequent machining challenges spurred an interest in alternative slider materials, particularly single phase ones. Furthermore, due to a desire of the recording head industry to more closely follow semiconductor fabrication technologies, silicon has been taken into consideration as an interesting alternative to alumina-titanium carbide (Altic), used presently. In case silicon is to be introduced, extensive tribological investigations are required. This paper attempts to shed some light on some basic tribological properties of silicon sliders used on DLC coated metal disks by doing a pin-on-disk test at a low rotational velocity. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Microfriction; Tribological properties; Pin-on-disk tests of silicon on DLC
1. Introduction Since the introduction of thin film sliders in 1979, the slider material of choice has been aluminatitanium carbide. However, the continuous decrease in flying height and subsequent machining challenges spurred an interest in alternative slider materials, particularly single phase ones. Furthermore, due to a desire of the recording head industry to more closely follow semiconductor fabrication technologies, silicon has been taken into consideration as an interesting alternative slider material. In case silicon is to be introduced, extensive tribological investigations are required. Our investigations focussed on basic tribological investigations on micromachined silicon sliders used on DLC coated metal disks. Outside the data storage industry, these results are also of interest for micro electro-mechanical systems (MEMS), in particular microactuators. To keep the investigations as general as possible, and not be affected excessively by ABS geometry issues, the following approach was taken: two types of dummy sliders with rather rough ABS were tested on a commercial disk. By choosing a nominal average roughness Ra in the order of 15–20 nm, ten times the roughness of a typical production slider, the effects of adhesion were
∗
Corresponding author. Fax: +49-511-7622867. E-mail address: [email protected] (H.H. Gatzen).
eliminated. To take differences in contact geometry (i.e. ABS contour) into account in a most general fashion, a pico size slider (1.2 mm length×1.0 mm width) was built in two versions. A type I slider was left unpatterned and had a flat contact area the size of the slider footprint (hence the need for a sufficient surface roughness to avoid “joeblocking” due to adhesion). A type II slider was patterned and featured round protruding contact areas, each with a 0.45 mm diameter, and the same roughness as slider type I. This patterned slider was intended to simulate a three point contact.
2. Experimental 2.1. Set-up A test head consisted of a dummy slider mounted on a flexure (Fig. 1). As on disk drives, the preload was created by a combination of the geometry of the flexure’s
Fig. 1.
0301-679X/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0301-679X(02)00198-6
View of a test head consisting of a dummy slider and a flexure.
280
H.H. Gatzen, M. Beck / Tribology International 36 (2003) 279–283
leaf spring and by choosing the appropriate distance between the mounting height and the disk surface (“z”height). The z-height is a designed feature of a flexure, in a nominal mounting position the gimbal angle has to be such that the slider is positioned parallel to the disk. For the tests, the gram load of the test heads was adjusted consecutively between 10 and 60 mN. As a counterpart for the sliders, commercial type rigid disks, taken from state of the art rigid disk drives, were used. The disk base material was aluminum coated with the films customary on rigid disk media. The two top layers are DLC film coated with a lubricant. While no specification was available for this disk this should be irrelevant for executing the test, since all tests were done under the same conditions. 2.2. Probe preparation For fabricating production type rigid disk sliders, a wafer level process creates the thin film read/write elements. After completing wafer fabrication the wafer is separated into row bars. Each row bar represents a row of future sliders. One kerf sidewall (the one close to the read/write elements’ pole face) represents the future air bearing surface, while the opposite is the slider mounting surface. The read/write elements, originally located at the wafer’s surface, end up on the slider’s trailing edge. After lapping or nanogrinding the row bars to create the ABS plane the row bars, turned by 90° with the mounting surface face down, are rejoined together in an “artificial” wafer for ABS patterning. Through a sequence of batch processes consisting of photolithography and physical ion beam etching (IBE) or a physical-chemical reactive ion etching (RIE), the desired air bearing surface profile is created. Finally, the row bar “wafer” tends to receive other treatment like DLC coating or addition of studs. Afterwards, the row bars are separated and diced into sliders. For assembling the sliders into recording heads, the sliders are mounted on a flexure and the read/write elements are contacted using a microflex cable. For creating silicon slider dummies, the respective process steps of slider fabrication as outlined before were used. In particular, the test surfaces were created by a combination of photolithographic masking and ion beam etching. Since the wear behavior of silicon was to be tested, no DLC coating was applied. The raw material for fabricating the slider dummies was a polished 525 µm thick silicon wafer with a {111} crystal orientation at its surface. Through a dicing process the wafer was cut into 1 mm×1.2 mm (pico size) slider dummies. The contact surface was machined flat by means of nanogrinding and the desired average surface roughness Ra of app. 15–20 nm was achieved (type I). Additionally, for the type II slider, three round spots on the slider surface, with a combined contact area of
0.6 mm2, were covered with photoresist. In a two hour ion beam etching (IBE) process, the areas not covered by photoresist were recessed. By doing so three plateau areas. 1–2 µm above ground were created. Fig. 2 depicts the geometries of the contact surfaces. The variation between the photolithographically defined surface areas is less than 5%. The nanoroughness measurements taken with an Atomic Force Microscope are shown in Fig. 3. 2.3. Test stand As a test stand, a pin on disk tester was used [1]. Fig. 4 shows a schematic view. The tester features an air bearing rotary table with an extraordinary high mass which allows to rotate it with rather low velocities of one revolution every 330 s (0.18 min⫺1). An electronic commuting motor is driving the rotary table through a friction drive. This combination results in minimal table vibrations. Despite its low velocity this tester achieves an absolutely even rotation of the disk. A test disk was mounted on the rotary table. The tester is equipped with a stage to hold the test head. To allow an appropriate adjustment of the test head z-height, the stage is vertically adjustable. For measuring the frictional force of the test head, the pin on disk tester is equipped with a commercial optical frictional force measurement. It consists of a spring system allowing a deflection in direction of the head’s frictional load and a fiberoptic system for measuring the degree of deflection. The light intensity varies proportional to the drag force dependent on the degree of overlapping between a light dot and a sensor area (Fig. 5). At the beginning of each test, each test head was mounted on the tester. For verifying the nominal force of a test head for a test run, the probe table was replaced with a gram load cell. Next, the head was mounted in the tester and was lowered on the gram load cell through the test system’s z-height adjustment until the nominal value was reached. In this position, the gram load was determined. In case the gram load was off, the gram load was readjusted by appropriately bending the flexure’s leaf spring. For running the friction tests, the gram load measuring system was replaced with a probe table holding the disk. Before each test, the slider was cleaned in an aqueous ultrasonic bath followed by a dip in isopropanol and drying with nitrogen. The tests were executed for gram loads between 10 and 60 mN. As mentioned before, the time per revolution was 330 s, resulting in a relative velocity at the head disk interface of 0.76 mm per s.
3. Tests and results Fig. 6 depicts a graph of a typical frictional force over time during the measurement. It is obvious that there are
H.H. Gatzen, M. Beck / Tribology International 36 (2003) 279–283
Fig. 2.
281
Surface topography of the two types of picosliders (Ieft hand: flat, type I, right hand: three protrusions, type II).
Fig. 4.
Schematic representation of the pin-on-disk tester.
major variations over the test period. Such variations were already described by Bogy et al. [2]. A variation of frictional values within one revolution of up to 100% was found. For a further comparison of the test results, an average friction value was used. Fig. 7 depicts the relationship between the nominal force and the average frictional value for both types of sliders. For each series of tests three sets of measurements were taken. Fig. 8 presents the relationship between the apparent contact area and the surface pressure. As mentioned before, the tests were performed with loads ranging from 10–60 mN. For the unpatterned slider (type I), the average friction value µI is app. 0.24, while the patterned slider with the three contact dots (type II) show an average friction value µII of app. 0.34. Besides looking at the frictional values as a function of the gram load, their dependence on the surface pressure (based on the apparent contact area) is of interest. Fig. 9 shows the results. As before, the unpatterned slider had the lowest frictional value.
4. Discussion Fig. 3.
Nanoroughness of the two types of sliders and of the disk.
It is interesting to compare these silicon friction data with Altic test results. Xu and Bhushan [3] report coef-
282
H.H. Gatzen, M. Beck / Tribology International 36 (2003) 279–283
Fig. 5.
Pin-on-disk tester.
Fig. 6. Typical variation of the coefficient of friction µ measured over one revolution (330 s per revolution, slider type I, 28.5 mN gramload).
Fig. 8. Average surface pressure as a function of gram load for a slider type I (unpatterned) and type II (patterned).
Fig. 7. Average friction over one revolution as a function of gram load for a slider type I (unpatterned) and type II (patterned).
Fig. 9. Average friction over one revolution as a function of surface pressure for a slider type I (unpatterned) and type II (patterned).
ficients of friction µ for Altic of 0.22 up to 0.4. depending on the time the disk is in use. Furthermore, it may be observed that a decrease in apparent contact area results in an increase of the coefficient of friction for silicon. The experimental data show that there is a relationship between the frictional value and the apparent contact area of a slider. An increase of the apparent contact area results in a decrease of friction. As a result
it may be expected that the shape of the ABS will affect the frictional value of the slider. Scherge et al. [4] made similar investigations but came to different conclusions. In their case an increase of the contact area also resulted in an increase of friction. However, for their experiment they were using polished silicon surfaces, while in our case the sliders do feature a definitive roughness and also, the disk is lubricated. Scherge concluded that the
H.H. Gatzen, M. Beck / Tribology International 36 (2003) 279–283
increase was caused by an increase of adhesion forces due to the coplanarity of the two interface surfaces. Due to the fact that the tests reported on in this paper were done with rather rough sliders most likely explains the different results.
5. Conclusion and outlook With this series of tests, some rather basic results regarding the frictional behavior were achieved. Obviously, there is quite a spectrum of work to be done should a silicon slider be introduced. Since it is rather unlikely that silicon sliders would be used without DLC, investigations comparing the frictional behavior of DLC
283
coated Altic sliders with comparable silicon sliders seem to be most pressing. References [1] Beck M, Wortmann A, Lu¨ thje H, Gatzen HH. Novel test equipment to measure the properties of microtribological interfaces. 13th International Conference on Wear of Materials, Vancouver, 2001. [2] Bogy DB, Yun X, Knapp BJ. Enhancement of Head-disk interface durability by use of diamond-like carbon overcoats on thes slider’s rails. IEEE Transactions on Magnetics, 1994; p. 369–74. [3] Xu J, Bhushan B. Friction and durability of ceramic slider materials in contact with lubricated thin-film rigid disks. Proc Instn Mech Eng 1997;211(J):303–16. [4] Scherge M, Schaefer JA. Microtribological investigation of stick/slip phenomena using a novel oscillatory friction and adhesion tester. Tribology Letters 1998;4:37–42.