hal , version 1-7 May 2008

Transcription

1 Author manuscrit, ulished in "N/P" 9-11 Aril 2008 Process nano scale mechanical roerties measurement of thin metal films using a novel addle cantilever test structure Chi-Jia Tong & Ming-Tzer Lin Institute of Precision Engineering, National Chung Hsing University, Taichung, 402, Taiwan hal , version 1-7 May 2008 Astract-A new technique was develoed for studying the mechanical ehavior of nano-scale thin metal films on sustrate is resented. The test structure was designed on a novel addle cantilever eam secimens with dimensions as few hundred nanometers to less than 10 nanometers. This eam is in triangle shae in order to rovide uniform lane strain distriution. Standard clean room rocessing was used to reare the addle samle. The exeriment can e oerated y using the electrostatic deflection on the addle uniform distriuted stress cantilever eam and then measure the deosited thin metal film materials on to of it. A caacitance technique was used to measurement on the other side of the deflected late to measure its deflection with resect to the force. The measured strain was converted through the caacitance measurement for the deflection of the cantilever. System erformance on the residual stress measurement of thin films are calculated with three different forces on the addle cantilever eam, including the force due to the film, comliance force and electrostatic force. I. INTRODUCTION Microelectromechanical systems (MEMS) technologies are develoing raidly with increasing study of the design, farication and commercialization of microscale systems and devices. The roadma for MEMS lans for develoing the comlexity and acking density of devices into the future allowing ever smaller in the range of nanometer scale and more densely acked structures to e faricated. Continued growth of Microsystems technologies requires still further miniaturization, with a corresonding need to understand how length scales affect mechanical ehavior of all the comonents. Accurate knowledge on the mechanical ehaviors of thin film materials used for MEMS is imortant for successful design and develoment of MEMS. Although many revious studies had erformed on the characterization of mechanical ehavior on thin films, the results otained from different measurement techniques were vary widely for nominally identical samles due to the difficulty with the techniques. Examle such as the nanoindentation exeriment on thin film is stronger correlated its sustrate. Therefore, the ISO (International Standard Organization) had defined the roer measurement range of using Nano Indentation test that the contact deth has to e <200nm & h<t/10, where t is film thickness. As a result, it limited the aility to do extensive study of thin film. Here a new technique was develoed for studying the mechanical ehavior of Nano-scale thin metal films on sustrate. The test structure was designed on a novel addle cantilever eam secimens with dimensions as few hundred nanometers to less than 10 nanometers. This eam is in triangle shae in order to rovide uniform lane strain distriution. The exeriment is designed to e oerated y using the electrostatic deflection on the addle uniform distriuted stress cantilever eam and then measure the deosited thin metal film materials on to of it. Thus allow us to exlore mechanical function mechanisms in thin films and film of the thickness on nanoscale regime. II. DESIGN & DEVELOPMENT PROCEDURES In traditional microeam ending, cantilevered eams of the film of interest are faricated y micromachining. The end of the eam is deflected using an indentation aaratus which senses vertical dislacement. Elastic roerties of the thin film material can e determined from the measured load-dislacement characteristics [1-3]. If a thin film is deosited on a thicker free-standing cantilever, (e.g. silicon) lastic roerties of the thin film can e roed as well [3]. However, the rimary drawacks in common eam ending, due to the non-uniform distriution of stresses, only a small ortion of the samle (at the root of the eam) is exosed to the maximum load. One would thus exect to see higher values for yield strength from eam ending exeriments comared to techniques which samle larger volumes. In addition, using indentation aaratus roviding vertical dislacement for the eam will generate localized effects in the vicinity of the loading oint due to its destructive force and comlicated analysis rocesses. The addle cantilever eam aroaches here was to design a constant stress cantilever eam that eliminate the non-uniform distriution of stresses along the cantilever. In a cantilever eam with a single localized load at the free end, the ending moment varies linearly from zero at the oint of load alication to a maximum at the uilt in end. Thus for a arallel sided eam, as in the receding sketch ig. 1, the axial stress on the eam surface is roortional to the ending moment; and, is exressed y:

2 hal , version 1-7 May 2008 Where: σ (x) = ending stress on the eam surface at a distance X from the oint of load alication, si (N/m 2 ) M(x) = ending moment at distance X, in-ls. (mn) c = t/2 = distance from neutral axis to eam surface, in(m) I = t 3 /12 = Moment of inertial of eam cross-section, in 4 (m 4 ) Z = t 2 /6 = Section modulus of eam, in 3 (m 3 ) = eam width, in (m) t = eam thickness, in (m) Thus, through the design of a cantilever section which modulus can e made roortional to X y making the width roortional to X, and holding the thickness constant, thus stress is constant from one end of the eam to the other. This constant-stress addle cantilever eam is shown in igure 1 and was rimary discussed in various studies in ulk structure [4]. igure 1: A constant-stress cantilever eam [4] In addition, the electrostatic deflection force on the designated late to drag addle cantilever eam can e used instead of using indentation aaratus to rovide vertical dislacement for the eam. Thus can reduce localized effects in the vicinity of the loading oint. A. Samle design and farication The single test chi is designed (igure 2) to fit in an aaratus where one side of the addle late is ulled y an electrostatic field. The measured strain was converted through the caacitance measurement for the deflection of the cantilever. The samle is shown as the structure of a 20mm 20mm chi with addle as shown aove in lane view and cross section. The thin silicon eam (40 um thick) has a triangular shae in lane view to rovide uniform strain in a metallic film on its surface when the eam is ent with force on the addle. igure 2 shows the front side and ackside view of the samle Aril 2008 The samle farication rocedure is using standard clean room rocessing for MEMS structures. We utilize these techniques and develo one of the simlest samle farication rocesses to conduct an exeriment that can maintain consistent samle rearation and excellent yield rocedures. The samle is faricated on a standard 4 inches diameter doule olished silicon wafer. Each wafer consists of 7 test chis after standard clean room farication. arication can e carried out either with crystallograhic etching in KOH or dee Si lasma etching. The detail of the farication rocess sequence is outlined: A 220 um thick douly olished Si wafer with 75 mm in diameter is coated y LPCVD SiN x on oth sides. The two SiN x surface are atterned with masks as shown aove. The wafer is then etched from oth sides in hot KOH. After 110um of the silicon has een etched from each side, the addle is free from the surrounding frame exect at the left-hand end. Etching is continued until the eam section has desired (40um) thickness after which the wafer is rinsed and dried. The addle has the full thickness of the wafer. Cleavage streets 75 um dee (defined y the width of the atterned line on one mask) are etched from one side. The individual chis may e cleaved from the wafer at this oint as the test chi carrier. Then either individually lanket metallized thin film, or the whole wafer may e lanket metallized can e deosited with desired thickness and film materials for the following tests of interested. The structure can e metallized from oth sides. No rocessing is required after the metal has een deosited. If desired, the SiN x may e removed y reactive ion etching (RIE) efore metal deosition. Later, a metallic conducting surface is required on oth sides of addle for stale deflection and caacitance measurements to e made. igure 3 shows the KOH rocess schematic. frontside ackside igure 2: Schematics view of samle igure 3 KOH etching rocess for an examle of thin metal samle Taered sidewalls are naturally roduced with KOH etching. The alternative is to use dee RIE. It roduces the vertical sidewalls and the rocesses are clean and easy. Processing with dee Si etching is also a two-mask rocess. Etching from the front can define the addle geometry through

3 the full thickness of the wafer. Etching from the ack creates the desired eam thickness. It may e desirale to the rocess with slightly non-vertical sidewalls to facilitate roducing continuously conducting surfaces on oth sides of a chi when it is metallized on tested thin film. igure 4 shows the dee RIE rocess schematic Aril 2008 igure 7: A finished addle samle hal , version 1-7 May 2008 igure 4: RIE etching rocess for an examle of thin metal samle The finish addle samle is shown in igure 5 and 6. The figure 5 is the whole samle view; the white zone is after KOH etching and the igure 6 shows just addle late. The igure 7 is the wafer just finish KOH etching. igure 5: A finished addle samle igure 6: A finished addle samle B. Testing System The aaratus is custom design and the system set-u is design to measure eam deflection y caacitance. It consists of the guard-ringed caacitor electrode, the metal sacer, the caliration samle and the deflection electrode. igure 8 shows the schematic of the measuring system. The samle chi is mounted together with a guard-ringed caacitor electrode as shown elow. A sacing of 25 to 125um to the window frame chi surface around the addle structure is defined y a metallic sacer. 100KHz C signal or DC for resonance Printed circuit oard DC or DCAC for resonance igure 8: System schematic During the exeriment, we aly an electrostatic force to the secimen and measure the caacitance change. The deflection of the addle eam can e measured from the caacitance value which is descried y A C = ε 0 d Metal sacer Where A is the caacitor late area, d is the sacer thickness and ε 0 is dielectric constant. Thus we can have linear correlation etween deflection sace and the accurate caacitance value. A second electrode is mounted elow the addle late. This electrode is used for electrostatic deflection of addle. The whole chi is at DC ground ut driven at 100 khz with amlitude of a few volts. That rovides a dislacement current to central electrode of the caacitor late which is roortional to the caacitance, and hence inversely roortional to the ga. Deending on the sacing selected, the caacitance is etween 2 and 4. The measurement of the caacitance can e made to a recision of aroximately 0.1 f so that addle sacing changes of 50 nm are readily determined. The addle can e ulled u with a DC voltage on the guard-ringed electrode or ulled down with a DC voltage on the lower electrode. The

4 hal , version 1-7 May 2008 caacitance measurement can e made with a time resolution of ± 10 msec. C. Measurements of secimen ending and strain or the electronic setu of the caacity measurement, a sine-wave generator at 100 khz is alied to the film simultaneously measuring caacity of the addle caacitor while a second generator drives at the same frequency for a test caacitor which has a known caacity. The two units are couled (one master, one slave) and have a hase shift of igure 9 shows the circuits. The out of hase currents from the two generator-caacity airs are summed at inut of change sensitive reamlifier. The amlified sum is measured with lock-in amlifier with the reference signal from one of the frequency generators. When the current flowing through the two caacitors is aroximately equal, the lock-in will show a value close to zero. In this case the ratio of the caacities is inversely-roortional to the ratio of the amlitudes of the driving: V 1 V 2 V C ~ = ~ V C 2 Z 1 Z 2 If now the caacitance of the addle caacitor is changed y a change in y, the out of hase currents are unalanced and the lock-in amlifier will measure the difference. igure 9: Electronic setu for the caacity measurement The measurement is controlled y PC through National Instrument LaVIEW rogram. The control electronics include a controller, amlifier and waveform generator. Monitored signals are conditioned and then fed into an A/D oard which is located in a PC. Data acquisition is erformed with LaVIEW software. During samle testing, it is laced inside the chamer, and the samles are monitored with an otical microscoe. After the samle is eing locked inside the system, then wait until the system is reaching the thermal equilirium and the caacitor read out is clear, the test can e erform. D. System Caliration The caliration of the testing system is done to see if a linearity of caacitor value resect to the sace etween the addle face and to electrode anel can e otained. After the system is uilt, we can set u sacer of different thickness etween 25 and 125um to hold the metal film on the samle at a fixed distance from metallic film on the Pyrex late. Together with the metal film on the samle this late forms a caacitor. A guard ring connected to ground surrounds the caacitor, shields it from stray signals and minimized fringe caacitance. The outermost region of the attern metal film rovides electrical connection to the samle. With different sacer we can calculate resected caacitance value at the same time we can also measuring the caacitance with different sacer and check the 9-11 Aril 2008 linear correlation etween the caacitance value and the sacer thickness. igure 10 shows an examle of calculated caacitance value versus 1/d which can e used as the caliration reference line of the system. igure 10 Calculated Caacitance versus 1/sacer thickness III. CALCULATION RESULTS & DISCUSSIONS The mathematical studies of system erformance were done to verify the erformance of the systems. or all the measurement, the overall erformance on the caacitance, electrostatic electrode and the addle cantilever as shown in igure 11 is essential. igure 11 Schematic arrangements of caacitance, electrostatic electrode and the addle cantilever Whenever the cantilever is ent, the caacitance of the addle cantilever structure is ( ) l C y = ε dx 0l d y sloe 0 X c where ε 0 is the dielectric constant of vacuum and l is the width of the addle. y is the distance from the end osition of the eam when it is flat to its osition when the eam is ent the y a force (initial stress or electrostatic force). If the film has no initial stress y =0 and any electrostatic force will made it negative, If the film have initial tensile stress y is ositive with no electrostatic force and may e or negative with electrostatic deflection. igure 12 Schematic view of electrostatic force deflection. In this case y(x) = d c - y - sloe x and the sloe of the addle lane is 2y /l. This yields

5 hal , version 1-7 May 2008 ε 0l l d c y = 2 y ln d y 2 c ( y ) C y l l y and y have the geometric relation y l y =1 l Thus we can use an examle to calculate the samle deflection versus the caacitance. An examle of the caacitance vs y for a ε 0 of 8.85E-12 force/m, a l of 3 mm, a l of 5 mm, a d c of 100um. is shown in igure 13. igure 13: Caacitance versus y osition for d c = 100um; l = 5mm; l = 3mm. The electrostatic force on the addle is given y 2 ε 0l l = V dx e ( d y sloe X ) e The electrostatic force is always downward so that e ε 0l l = V 4 y d e y 2 y l d y e l Where d e is the distance from the addle ottom lane to the electrode and V is the alied voltage. The electrostatic force divided y V 2 is shown in igure 14 as a function of y. igure 14: Electrostatic force versus y osition for l =5mm; l =3mm; d e =100um or these dimensions the maximum value of y is 61.53um ecause at that oint the end of the addle touches the caacitance late. The minimum value is um ecause the addle then touches the deflection electrode. The total force on the addle is T c e = P P P P where the force on the addle center due to stress in the film 9-11 Aril 2008 is P = de dy = E ε V dε dy and the comliance force of the eam c 1 = y comliance, y 6 l ( l l ) comliance = = 3 P EKt If we neglect electrostatic force on the addle the total force is T = P P C P We can write Eq. in the other word C 0 1 = E V ( ) P P ε ε K 1 y comliance Where ε 0 is the initial strain of the film deends on initial stress and ε is a function of y. So we can written Eq. like elow t y l l l 3 C 0 t EKt = E V ( ) l ( l l ) l ( l l ) y P P ε 6 Where t is 40um; l is 3mm; l is 5mm; K is 0.3; E is iaxial young s modulus. The film and eam force together vs y is shown in igure 15. igure 15: ilm and eam force together versus y osition for initial stress in the film is 100, 200 and 300MPa As we can oserve in this lot when the force is equal to zero imly the system is in its equiliration osition with no electrostatic force. IV. CONCLUSION A novel addle cantilever eam secimens with dimensions as few hundred nanometers to less than 10 nanometers is designed and faricated. The exeriment can e oerated y using the electrostatic deflection on the addle uniform distriuted stress cantilever eam and then measure the deosited thin metal film materials on to of it. Caacitance techniques were used to measurement on the other side of the deflected late to measure its deflection with resect to the force. The measured strain was converted through the caacitance measurement for the deflection of the cantilever. System erformance on the residual stress measurement of thin films are calculated with three different forces on the addle cantilever eam, including the force due to the film, comliance force and electrostatic force. The calculation hels to redict system erformances including caacity versus ending high, driving voltage versus electrostatic force, maximum deflection and the free end of cantilever eam osition in different residual stress. They also rovided roof of the testing aroach as well as otential use for the design and develoment of MEMS

6 materials. ACKNOWLEDGMENT The authors are grateful to Prof. Walter Brown of Lehigh University for valuale discussions. This work was suorted y Taiwan National Science Council; grant numer NSC E Aril 2008 REERENCES [1] Weihs, T. P., S. Hong, J. C. Bravman et al., Mechanical deflection of cantilever microeams: a new technique for testing the mechanical roerties of thin films, Journal of Materials Research 3 (5), (1988). [2] Nix, W.D., Metallurgy. Trans. 20A 2217 (1989). [3] Schweitz, J.A., Mechanical characterization of thin films y micromechanical Techniques MRS Bulletin 17 (7), (1992). [4] Dr. Kingsury in the Integrated Mechanical Testing Laoratory, Arizona State University, Laoratory notes, unulished. [5] S. Hyun, W. L. Brown, and R. P. Vinci Thickness and temerature deendence of stress relaxation in nanoscale aluminum films Alied Physics Letters Vol. 83, N NOV 2003 hal , version 1-7 May 2008