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Experimental analysis of galvanic corrosion of a thin metal film in a multilayer stack for MEMS application
Materials Science in Semiconductor Processing 16 (2013) 449–453
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Experimental analysis of galvanic corrosion of a thin metal film in a multilayer stack for MEMS application A. Ravi Sankar a,n, S. Das b a b
School of Electronics Engineering, VIT University, Chennai Campus, Chennai 600127, India School of Medical Science and Technology, Indian Institute of Technology, Kharagpur 721302, India
a r t i c l e i n f o
abstract
Available online 27 August 2012
Experimental analysis of galvanic corrosion of an aluminium (Al)–chromium (Cr)–gold (Au) multilayer stack is presented in this paper. The use of two or more stacks of different metal films is common for realisation of various microelectromechanical system (MEMS) devices. However, patterning of the multilayer metal films by lithographic and etching process is very critical due to galvanic corrosion. In a multilayer metal stack film, the knowledge of etch rate of the individual metal layers is very important for designing the process flow for the fabrication of micro-sensors. In the present study, galvanic corrosion characteristics of Al–Cr binary metal stack and Al–Cr–Au ternary metal stack in different etching solutions have been studied. The intermetallic contact area and the exposed metal area in the electrolyte solution were varied using an innovative process step involving silicon shadow mask technique and lithographic process. It is observed from the experimental results that for an intermetallic contact area to exposed metal area ratio of 2, etch rate of aluminium layer is increased by more than two times in aluminium etchant and 80% in Cr etchant as compared to the etch rate of the aluminium layer without intermetallics effect. The results obtained from this study have been applied for designing the fabrication flow and successful realisation of a MEMS piezoresistive accelerometer. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Galvanic corrosion Multilayer metal stack MEMS piezoresistive accelerometer Selective metal deposition Electroplating Shadow mask technique
1. Introduction When two or more different types of metals come into contact in the presence of an electrolyte, a galvanic couple is set up due to the difference of electrode potential of various metals, which leads to galvanic corrosion of the least noble metal [1]. For various technological applications, the use of two or more layers of different metal films is quite common [2,3]. One such example is a MEMS based silicon piezoresistive accelerometer [3], where three metal layers, i.e. aluminium (Al), chromium (Cr)
n Corresponding author. Tel.: þ91 44 3993 1274; fax: þ 91 44 3993 2555. E-mail address: [email protected] (A. Ravi Sankar).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.08.003
and gold (Au) are used. Aluminium is used for signal pick-up whereas a gold metal film is used to increase the proof mass weight and hence to enhance the performance of the device [3]. On the other hand, Au films require underneath Cr thin films for better adhesion. Processing of the three metal films for the accelerometer fabrication may lead to galvanic etching of the aluminium film which is the least noble metal in the present case. Galvanic metal etching is influenced by many parameters like electrode potential of different metals used, contact and exposed area of different metals in an electrolyte, properties of the electrolytes used, temperature of the electrochemical solution, etc. Even though electrode potential of different metals is known [1], it gives only qualitative information of the corrosion. Moreover, the etch rate of bulk metals is significantly different from that of thin films. The metal
450
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deposition technique and its associated process parameters coupled with the composition of the etchants produce significant etch rate variation in a thin metal film [4]. Since different lithographic and etching processes involving various electrolytic chemicals are required for realisation of the MEMS piezoresistive accelerometers, galvanic corrosion of the least noble metal cannot be avoided during various process steps. However, the etch rate of all the metals in the multilayer stack needs to be precisely measured for designing the fabrication process flow. Thus, in the present work a systematic study of galvanic corrosion of Al–Cr–Au metals stack in various metal etchants has been performed for quantitative estimation of the etch rates of different metal films. 2. Experimental procedures To analyse the galvanic effect, initially the etch rates of individual Al, Cr and Au metal films without galvanic formation were measured. In the present study, aluminium etchant (type-A), chromium etchant (1020) and gold etchant (TFA) commercially available from TranseneTM, USA, were used as electrolytes. The etchants were used as received and all the etching experiments were carried out at room temperature without a stirrer. Around 100 ml volume of etchant was used for each measurement. Al, Cr and Au/Cr metal films were deposited by a thermal evaporation process atop a thermally oxidised silicon wafer. The metal films were deposited at a base vacuum of 2 10 6 mbar and a substrate temperature of 60 1C. In the next step, a suitable photo mask was used to create various patterns with vertical sidewalls using ultraviolet (UV) photolithography process. Thickness of the initial metal films was precisely measured using a Dektak3 surface profilometer. Subsequently, these metal films were immersed in the above mentioned etchants for a fixed duration of time at room temperature (25 1C). Etch rates of the metal films were calculated by measuring the thickness difference of the metals before and after etching and then time averaging the total etch depth measured by Dektak3 surface profilometer. Results obtained from this study are given in Table 1. The galvanic corrosion effect was experimentally analysed using (1) a binary metal stack of Al and Cr and (2) a ternary metal stack of Al, Cr and Au layers. The schematics of the binary and ternary metal stacks are shown in Fig. 1(a) to (d), respectively. In the galvanic corrosion study, the exposed area eA of the metal film to be etched and the contact area cA of that film with the other metal film are important parameters. The exposed area is defined as the immersed region of the single layer metal Table 1 Measured etch rates of metals in different etching solutions. Material
Al Cr Au
Etch rate of materials in various etchants (nm/min) Al etchant
Cr etchant
Au etchant
210 – –
140 130 –
620 62 240
film in direct contact with the electrolytic solution whereas the contact area is the overlapping region of the metal stack layers as schematically shown in Fig. 1(a). Thus in the present study, samples were prepared with various exposed areas of the individual film along with a fixed overlapping region of different metal films in binary metal stack as shown in Fig. 1(b). A schematic of the top view of the Al–Cr–Au ternary metal stack is shown in Fig. 1(c) with its cross-sectional views across A–A0 and B–B0 in Fig. 1(d). In the ternary metal stack layer (Al–Cr–Au) experiment, cA is the intermetallic contact area of Au layer at the top of the Al–Cr stack and eA is the exposed area of the Al or Cr layer. In this structure, exposed areas of the Cr and Al film were kept equal. However, the exposed areas of Cr layer and Al layer were placed orthogonally to each other on the silicon surface as shown in Fig. 1(c). For both binary and ternary metals stacks, five different samples with the area ratio cA =eA of 0.25, 0.5, 1, 1.5 and 2 were used for the etch rate study. For accurate etch rate measurements, lithographically defined edges are necessary. However, once a metal is patterned, another metal cannot be deposited and subsequently patterned atop the already patterned metal, since it may damage underneath metal film edges due to the galvanic effect. Hence, in the present study a novel process step with the combination of lithography process and selective metal deposition using shadow mask technique [5,6] was used for sample preparation. Silicon wafers were used for shadow mask preparation by the anisotropic wet etching technique because of the smooth edges and wafer surface after anisotropic etching. In the present study, initially the first metal film was deposited and lithographically patterned. Subsequently the second metal film was selectively deposited using the silicon shadow mask technique to achieve predefined exposed area and contact area. However, in some cases lithography and etching were performed to properly define the exposed area of the metal film keeping the contact or overlapping area fully protected by a photoresist to avoid the galvanic corrosion of the underneath metal layer. The process flow for sample preparation of binary metal stack is given below with the schematics of each individual step as shown in Fig. 2(a) to (d). An aluminium film of thickness 1 mm was thermally deposited atop a Si/SiO2 layer (Fig. 2a) and lithography was performed using a suitable mask (Fig. 2b). Subsequently a 50 nm thick Cr film was thermally deposited atop the existing Al metal film using a shadow mask as shown in Fig. 2(c). Then lithography was performed to define a sharp edge for Cr film as shown in Fig. 2(d). During this process the underlying Al film was covered by a photoresist. By varying the dimensions of lithographic mask and shadow mask, the contact area and exposed area of different metal films were varied and Fig. 1(b) shows the binary stack of Al–Cr with different exposed areas ðeA Þ of the film to be etched with constant contact area ðcA Þ. Similarly, sample for ternary metals stack of Al–Cr–Au was prepared with a thermally deposited 200 nm thick Au layer atop the Cr layer. After preparing the samples, both the binary and ternary metals stacks were dipped in different etching solutions at 25 1C for a fixed time and subsequently
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Fig. 1. Binary and ternary metal stacks. (a) top view of the Al–Cr binary stack, (b) a cross-sectional view of the binary metals stack across A–A0 with a varying exposed area of Al layer, (c) top view of the Al–Cr–Au ternary metal stack and (d) cross-sectional views of the ternary metals stacks across A–A0 and B–B0 .
Fig. 2. Process flow of Al–Cr binary metal stack for galvanic corrosion measurements.
the etch rates profilometer.
were
measured
using
the
surface
3. Results and discussion Initially, the etch rates of the three materials (Al, Cr and Au) in the different etching solutions were experimentally measured and the results are shown in Table 1. It has been found that Al material has poor selectivity in Cr etchant and Au etchant. Similarly Cr material is also less selective in Au etchant but it has high selectivity in Al etchant. Conversely, the Au metal film is not affected by both Cr and Al etchants. These results have been used to
quantitatively study the galvanic corrosion of Al, Cr and Au in the subsequent sections. The mechanism of galvanic corrosion is a localised phenomenon by which a metal can be preferentially etched in an electrolytic solution. The occurrence of galvanic corrosion depends on the following conditions. (a) Metals must be far apart in galvanic series. In the galvanic or electrochemical series the metals are placed according to their potential measured with respect to a standard hydrogen electrode (SHE). The anodic or less noble metals show more negative potential than a more noble metal in the galvanic series. (b) The two different metals must be in electrical contact with each other and (c) metal films must be bridged by an electrolyte. Amongst the three metals in galvanic series, Al has the least electrode potential of 1.66 V whereas Cr and Au have 0.744 V and þ1.83 V [1] respectively with respect to a standard hydrogen electrode (SHE) and accordingly Al is the least noble material followed by Cr. Gold is a noble metal as compared to Al and Cr and has a positive electrode potential of 1.83 V. Thus, corrosion of Al metal will occur when a layer of Al makes contact with a more noble metal such as Cr or Au in an electrolyte. The etch rate of Al metal in binary metal stack of Al–Cr versus cA =eA is shown in Fig. 3. From the graph, it is evident that Al etch rate increases with increasing area ratio. For cA =eA ¼2, Al etch rate increases by 22% in Al etchant and 40% in Cr etchant as compared to the Al etch rate without intermetallic effect due to galvanic corrosion. Cr etchant has greater influence on etching of Al as compared to Al etchant. However, with reducing cA =eA ratio, galvanic etch rate of Al approaches the chemical etch rate value without the galvanic effect in both the etchants. A few experiments were carried out to estimate the galvanic effect on Cr film in Al and Cr etching solutions using an Al–Cr binary metal layer with different area ratios. The results show that there is less than 5% variation of Cr etch rate in the Cr etchant with
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Fig. 3. Galvanic etch rate of Al in the binary metal stack of Al–Cr.
intermetallics effect. Moreover, galvanic corrosion had no effect on the etch rate of Cr thin film in Al etchant. These results imply that the galvanic element formation is insignificant for the Cr film when it forms a binary metal stack with a less noble metal. The relative area of an anode with respect to a cathode in a metal stack will affect the amount of corrosion that occurs due to galvanic corrosion. A small anode i.e., less noble metal like Al connected to a large cathode i.e. more noble metal like Cr by an electrolyte will result in a high current density on the Al layer resulting in a higher rate of corrosion. Thus the rate of corrosion is influenced by the area difference. Conversely, if the area of the anode is larger than that of the cathode, the corrosion effect is diluted. Even though difference in electrode potential between Cr and Al in galvanic series available from literature [1] is relatively large, galvanic potential has insignificant effect on Al etch rate in both Al and Cr etchants for (cA =eA ) o1 as observed in the present study. This implies the significance of the contact area to exposed area ratio in the binary metal stack of Al and Cr. Fig. 4 shows that the etch rate of Al increases in the ternary metal stack of Al–Cr–Au with the increase in the area ratio. It is observed from the results that for cA =eA ¼2, etch rate of Al layer is increased by more than two times in Al etchant and 80% in Cr etchant as compared to the etch rate of Al layer without intermetallics effect. On the other hand for cA =eA ¼0.25, Al etch rate is increased by 62% in Al etchant and 52% in Cr etchant as compared to the chemical etch rate of Al without the intermetallics effect. The above results indicate that unlike in binary metal stack, galvanic effect is more significant and variation of the area ratio is comparatively less trivial on Al etch rate in ternary metal stack. This is because Al and Au are relatively far apart in the electrochemical series [1] as compared to the Al and Cr metal combination and thus generate more electrochemical potential in the electrolytic solution, resulting in increase of galvanic corrosion. Etch rate of Al in gold etchant could not be
Fig. 4. Galvanic etch rate of Al in the ternary metal stack of Al–Cr–Au.
Fig. 5. Galvanic etch rate of Cr in the ternary metal stack of Al–Cr–Au.
measured accurately to quantify the corrosion rate, because the etch rate of Al was increased by more than 10 times due to the galvanic effect. It is thus observed from the above experimental results that in the ternary stack of Al–Cr–Au, types of the electrolyte and metal film are more significant for galvanic corrosion than the exposed area of the metal film to be etched. Fig. 5 shows the etch rate of Cr in the ternary metal stack of Al–Cr–Au for different area ratios. The etch rate of Cr is increased by 56% in the Cr etchant and 29% in the Au etchant in the ternary metal stack for cA =eA ¼2 as compared to etch rate of Cr without intermetallic effect. For cA =eA ¼0.25, the etch rate of Cr is increased by 40% in the Cr etchant and 13% in the Au etchant as compared to the chemical etch rate of Cr without the galvanic effect. The results imply that the reduction of cA =eA ratio has comparatively less significant effect on the galvanic etch rate of Cr in ternary metal stack. It has also been observed that Al etchant did not have any effect on the etch rate of
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process technique bypasses the wet chemical etching process of metal layers and thus completely avoids the galvanic corrosion effect. The accelerometer device fabricated using the innovative process is shown in Fig. 6. From the figure, both Al interconnection lines and electroplated gold layer atop the proof mass can be seen. The detailed fabrication process sequence and test results are published elsewhere [3]. 4. Conclusions
Fig. 6. Photograph of a fabricated accelerometer device with an electroplated gold layer and Al metal lines.
Cr and Au. Moreover, in ternary metal stack the etch rate of Au is not affected by the etchants considered and by the variation of cA =eA ratio. The measurement results show that the galvanic effect influences the etch rate of Al in the binary and ternary multilayer metal stack. However, in the binary metal stack of Al–Cr, the cA =eA area ratio has a very significant role in altering the etch rate of Al. In the ternary metal stack of Al– Cr–Au the galvanic etch rate of Al is enhanced significantly as compared to the etch rate of Al without intermetallics effect. The phenomenon of galvanic enhanced etch rate of a thin film has been effectively used to realise nano-fluidic channels [2]. Comparing with the binary metal stack, it has been observed that in the ternary stack, the type of electrolyte and the choice of the metal film in galvanic series are more significant for galvanic corrosion than the exposed area of the metal film to be etched. In various application specific electronics and MEMS devices, the use of two or more different metal films is quite essential for proper functioning of the device. One such example is a MEMS based silicon piezoresistive accelerometer where Al metal is used as an interconnect for signal pick-up and a Cr–Au layer is used to increase the proof mass weight [3]. The performance characteristics of the accelerometer device are improved by increasing the proof mass weight [7,8]. Realisation of this device requires wet chemical etching of all the three metal films to achieve the proper geometrical layout. However, the present study shows that it is quite difficult to achieve the proper geometrical pattern of the metal film such as its mask dimension by the standard process sequence due to the galvanic effect. Moreover, if the metals in use are comparatively far apart in the electrochemical series, then galvanic corrosion is much more dominant, resulting in more distortion of the metal lines. To overcome this difficulty, the combined method of lithographic process and shadow mask technique proposed in the present study is used to fabricate a MEMS piezoresistive accelerometer with an electroplated gold layer. This innovative
Galvanic corrosion of Al thin film metal in the binary and ternary metals stacks is presented in this paper. Experimental results show that the galvanic effect influences the etch rate of Al in the binary and ternary multilayer metal stacks. However, in the binary metal stack of Al–Cr, the cA =eA area ratio has a very significant role in altering the etch rate of Al. In the ternary metal stack of Al–Cr–Au the galvanic etch rate of Al is enhanced significantly as compared to the etch rate of Al without intermetallics effect. Comparing with the binary metal stack, it has been observed that in the ternary stack, the type of electrolyte and the choice of the metal film in galvanic series are more significant for galvanic corrosion than the exposed area of the metal film to be etched. Considering the galvanic effect of Al metal film in other electrolytic solutions and resulting pattern distortion, a piezoresistive accelerometer with patterned gold film atop the proof mass has been fabricated with an innovative process step combining shadow masking technique and electrochemical deposition.
Acknowledgement The authors would like to express their gratitude to (Late) Prof. S. Kal for his valuable suggestions. The authors acknowledge the members of Microelectronics and MEMS Laboratory, IIT Kharagpur, for their help in the experiment. References [1] A. Peter, Physical Chemistry, 6th ed. W.H. Freeman and Company, New York, 1997. [2] H. Zeng, A. Wan, A.D. Feinerman, Nanotechnology 17 (2006) 3183–3188. [3] A. Ravi Sankar, S.K. Lahiri, S. Das, Journal of Micromechanics and Micro engineering 19 (2009). http://dx.doi.org/10.1088.09601317.19.2.025008. [4] K.R. Williams, K. Gupta, M. Wasilik, Journal of Microelectromechanical Systems 12 (2008) 761–778. [5] A. Tixier, Y. Mita, J.P. Gouy, H. Fujita, Journal of Micromechanics and Microengineering 10 (2000) 157–162. [6] J.M. Hong, J. Zou, Journal of Micromechanics and Microengineering (2008) doi:10.1088.0960-1317.18.5.055002. [7] N. Yazadi, K. Najafi, A.S. Salian, Journal of Microelectromechanical Systems 12 (2003) 479–486. [8] R.P. Van Kampen, R.F. Wolffenbutte, Sensors and Actuators A 64 (1998) 137–150.
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Experimental analysis of galvanic corrosion of a thin metal film in a multilayer stack for MEMS application A. Ravi Sankar a,n, S. Das b a b
School of Electronics Engineering, VIT University, Chennai Campus, Chennai 600127, India School of Medical Science and Technology, Indian Institute of Technology, Kharagpur 721302, India
a r t i c l e i n f o
abstract
Available online 27 August 2012
Experimental analysis of galvanic corrosion of an aluminium (Al)–chromium (Cr)–gold (Au) multilayer stack is presented in this paper. The use of two or more stacks of different metal films is common for realisation of various microelectromechanical system (MEMS) devices. However, patterning of the multilayer metal films by lithographic and etching process is very critical due to galvanic corrosion. In a multilayer metal stack film, the knowledge of etch rate of the individual metal layers is very important for designing the process flow for the fabrication of micro-sensors. In the present study, galvanic corrosion characteristics of Al–Cr binary metal stack and Al–Cr–Au ternary metal stack in different etching solutions have been studied. The intermetallic contact area and the exposed metal area in the electrolyte solution were varied using an innovative process step involving silicon shadow mask technique and lithographic process. It is observed from the experimental results that for an intermetallic contact area to exposed metal area ratio of 2, etch rate of aluminium layer is increased by more than two times in aluminium etchant and 80% in Cr etchant as compared to the etch rate of the aluminium layer without intermetallics effect. The results obtained from this study have been applied for designing the fabrication flow and successful realisation of a MEMS piezoresistive accelerometer. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Galvanic corrosion Multilayer metal stack MEMS piezoresistive accelerometer Selective metal deposition Electroplating Shadow mask technique
1. Introduction When two or more different types of metals come into contact in the presence of an electrolyte, a galvanic couple is set up due to the difference of electrode potential of various metals, which leads to galvanic corrosion of the least noble metal [1]. For various technological applications, the use of two or more layers of different metal films is quite common [2,3]. One such example is a MEMS based silicon piezoresistive accelerometer [3], where three metal layers, i.e. aluminium (Al), chromium (Cr)
n Corresponding author. Tel.: þ91 44 3993 1274; fax: þ 91 44 3993 2555. E-mail address: [email protected] (A. Ravi Sankar).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.08.003
and gold (Au) are used. Aluminium is used for signal pick-up whereas a gold metal film is used to increase the proof mass weight and hence to enhance the performance of the device [3]. On the other hand, Au films require underneath Cr thin films for better adhesion. Processing of the three metal films for the accelerometer fabrication may lead to galvanic etching of the aluminium film which is the least noble metal in the present case. Galvanic metal etching is influenced by many parameters like electrode potential of different metals used, contact and exposed area of different metals in an electrolyte, properties of the electrolytes used, temperature of the electrochemical solution, etc. Even though electrode potential of different metals is known [1], it gives only qualitative information of the corrosion. Moreover, the etch rate of bulk metals is significantly different from that of thin films. The metal
450
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deposition technique and its associated process parameters coupled with the composition of the etchants produce significant etch rate variation in a thin metal film [4]. Since different lithographic and etching processes involving various electrolytic chemicals are required for realisation of the MEMS piezoresistive accelerometers, galvanic corrosion of the least noble metal cannot be avoided during various process steps. However, the etch rate of all the metals in the multilayer stack needs to be precisely measured for designing the fabrication process flow. Thus, in the present work a systematic study of galvanic corrosion of Al–Cr–Au metals stack in various metal etchants has been performed for quantitative estimation of the etch rates of different metal films. 2. Experimental procedures To analyse the galvanic effect, initially the etch rates of individual Al, Cr and Au metal films without galvanic formation were measured. In the present study, aluminium etchant (type-A), chromium etchant (1020) and gold etchant (TFA) commercially available from TranseneTM, USA, were used as electrolytes. The etchants were used as received and all the etching experiments were carried out at room temperature without a stirrer. Around 100 ml volume of etchant was used for each measurement. Al, Cr and Au/Cr metal films were deposited by a thermal evaporation process atop a thermally oxidised silicon wafer. The metal films were deposited at a base vacuum of 2 10 6 mbar and a substrate temperature of 60 1C. In the next step, a suitable photo mask was used to create various patterns with vertical sidewalls using ultraviolet (UV) photolithography process. Thickness of the initial metal films was precisely measured using a Dektak3 surface profilometer. Subsequently, these metal films were immersed in the above mentioned etchants for a fixed duration of time at room temperature (25 1C). Etch rates of the metal films were calculated by measuring the thickness difference of the metals before and after etching and then time averaging the total etch depth measured by Dektak3 surface profilometer. Results obtained from this study are given in Table 1. The galvanic corrosion effect was experimentally analysed using (1) a binary metal stack of Al and Cr and (2) a ternary metal stack of Al, Cr and Au layers. The schematics of the binary and ternary metal stacks are shown in Fig. 1(a) to (d), respectively. In the galvanic corrosion study, the exposed area eA of the metal film to be etched and the contact area cA of that film with the other metal film are important parameters. The exposed area is defined as the immersed region of the single layer metal Table 1 Measured etch rates of metals in different etching solutions. Material
Al Cr Au
Etch rate of materials in various etchants (nm/min) Al etchant
Cr etchant
Au etchant
210 – –
140 130 –
620 62 240
film in direct contact with the electrolytic solution whereas the contact area is the overlapping region of the metal stack layers as schematically shown in Fig. 1(a). Thus in the present study, samples were prepared with various exposed areas of the individual film along with a fixed overlapping region of different metal films in binary metal stack as shown in Fig. 1(b). A schematic of the top view of the Al–Cr–Au ternary metal stack is shown in Fig. 1(c) with its cross-sectional views across A–A0 and B–B0 in Fig. 1(d). In the ternary metal stack layer (Al–Cr–Au) experiment, cA is the intermetallic contact area of Au layer at the top of the Al–Cr stack and eA is the exposed area of the Al or Cr layer. In this structure, exposed areas of the Cr and Al film were kept equal. However, the exposed areas of Cr layer and Al layer were placed orthogonally to each other on the silicon surface as shown in Fig. 1(c). For both binary and ternary metals stacks, five different samples with the area ratio cA =eA of 0.25, 0.5, 1, 1.5 and 2 were used for the etch rate study. For accurate etch rate measurements, lithographically defined edges are necessary. However, once a metal is patterned, another metal cannot be deposited and subsequently patterned atop the already patterned metal, since it may damage underneath metal film edges due to the galvanic effect. Hence, in the present study a novel process step with the combination of lithography process and selective metal deposition using shadow mask technique [5,6] was used for sample preparation. Silicon wafers were used for shadow mask preparation by the anisotropic wet etching technique because of the smooth edges and wafer surface after anisotropic etching. In the present study, initially the first metal film was deposited and lithographically patterned. Subsequently the second metal film was selectively deposited using the silicon shadow mask technique to achieve predefined exposed area and contact area. However, in some cases lithography and etching were performed to properly define the exposed area of the metal film keeping the contact or overlapping area fully protected by a photoresist to avoid the galvanic corrosion of the underneath metal layer. The process flow for sample preparation of binary metal stack is given below with the schematics of each individual step as shown in Fig. 2(a) to (d). An aluminium film of thickness 1 mm was thermally deposited atop a Si/SiO2 layer (Fig. 2a) and lithography was performed using a suitable mask (Fig. 2b). Subsequently a 50 nm thick Cr film was thermally deposited atop the existing Al metal film using a shadow mask as shown in Fig. 2(c). Then lithography was performed to define a sharp edge for Cr film as shown in Fig. 2(d). During this process the underlying Al film was covered by a photoresist. By varying the dimensions of lithographic mask and shadow mask, the contact area and exposed area of different metal films were varied and Fig. 1(b) shows the binary stack of Al–Cr with different exposed areas ðeA Þ of the film to be etched with constant contact area ðcA Þ. Similarly, sample for ternary metals stack of Al–Cr–Au was prepared with a thermally deposited 200 nm thick Au layer atop the Cr layer. After preparing the samples, both the binary and ternary metals stacks were dipped in different etching solutions at 25 1C for a fixed time and subsequently
A. Ravi Sankar, S. Das / Materials Science in Semiconductor Processing 16 (2013) 449–453
451
Fig. 1. Binary and ternary metal stacks. (a) top view of the Al–Cr binary stack, (b) a cross-sectional view of the binary metals stack across A–A0 with a varying exposed area of Al layer, (c) top view of the Al–Cr–Au ternary metal stack and (d) cross-sectional views of the ternary metals stacks across A–A0 and B–B0 .
Fig. 2. Process flow of Al–Cr binary metal stack for galvanic corrosion measurements.
the etch rates profilometer.
were
measured
using
the
surface
3. Results and discussion Initially, the etch rates of the three materials (Al, Cr and Au) in the different etching solutions were experimentally measured and the results are shown in Table 1. It has been found that Al material has poor selectivity in Cr etchant and Au etchant. Similarly Cr material is also less selective in Au etchant but it has high selectivity in Al etchant. Conversely, the Au metal film is not affected by both Cr and Al etchants. These results have been used to
quantitatively study the galvanic corrosion of Al, Cr and Au in the subsequent sections. The mechanism of galvanic corrosion is a localised phenomenon by which a metal can be preferentially etched in an electrolytic solution. The occurrence of galvanic corrosion depends on the following conditions. (a) Metals must be far apart in galvanic series. In the galvanic or electrochemical series the metals are placed according to their potential measured with respect to a standard hydrogen electrode (SHE). The anodic or less noble metals show more negative potential than a more noble metal in the galvanic series. (b) The two different metals must be in electrical contact with each other and (c) metal films must be bridged by an electrolyte. Amongst the three metals in galvanic series, Al has the least electrode potential of 1.66 V whereas Cr and Au have 0.744 V and þ1.83 V [1] respectively with respect to a standard hydrogen electrode (SHE) and accordingly Al is the least noble material followed by Cr. Gold is a noble metal as compared to Al and Cr and has a positive electrode potential of 1.83 V. Thus, corrosion of Al metal will occur when a layer of Al makes contact with a more noble metal such as Cr or Au in an electrolyte. The etch rate of Al metal in binary metal stack of Al–Cr versus cA =eA is shown in Fig. 3. From the graph, it is evident that Al etch rate increases with increasing area ratio. For cA =eA ¼2, Al etch rate increases by 22% in Al etchant and 40% in Cr etchant as compared to the Al etch rate without intermetallic effect due to galvanic corrosion. Cr etchant has greater influence on etching of Al as compared to Al etchant. However, with reducing cA =eA ratio, galvanic etch rate of Al approaches the chemical etch rate value without the galvanic effect in both the etchants. A few experiments were carried out to estimate the galvanic effect on Cr film in Al and Cr etching solutions using an Al–Cr binary metal layer with different area ratios. The results show that there is less than 5% variation of Cr etch rate in the Cr etchant with
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Fig. 3. Galvanic etch rate of Al in the binary metal stack of Al–Cr.
intermetallics effect. Moreover, galvanic corrosion had no effect on the etch rate of Cr thin film in Al etchant. These results imply that the galvanic element formation is insignificant for the Cr film when it forms a binary metal stack with a less noble metal. The relative area of an anode with respect to a cathode in a metal stack will affect the amount of corrosion that occurs due to galvanic corrosion. A small anode i.e., less noble metal like Al connected to a large cathode i.e. more noble metal like Cr by an electrolyte will result in a high current density on the Al layer resulting in a higher rate of corrosion. Thus the rate of corrosion is influenced by the area difference. Conversely, if the area of the anode is larger than that of the cathode, the corrosion effect is diluted. Even though difference in electrode potential between Cr and Al in galvanic series available from literature [1] is relatively large, galvanic potential has insignificant effect on Al etch rate in both Al and Cr etchants for (cA =eA ) o1 as observed in the present study. This implies the significance of the contact area to exposed area ratio in the binary metal stack of Al and Cr. Fig. 4 shows that the etch rate of Al increases in the ternary metal stack of Al–Cr–Au with the increase in the area ratio. It is observed from the results that for cA =eA ¼2, etch rate of Al layer is increased by more than two times in Al etchant and 80% in Cr etchant as compared to the etch rate of Al layer without intermetallics effect. On the other hand for cA =eA ¼0.25, Al etch rate is increased by 62% in Al etchant and 52% in Cr etchant as compared to the chemical etch rate of Al without the intermetallics effect. The above results indicate that unlike in binary metal stack, galvanic effect is more significant and variation of the area ratio is comparatively less trivial on Al etch rate in ternary metal stack. This is because Al and Au are relatively far apart in the electrochemical series [1] as compared to the Al and Cr metal combination and thus generate more electrochemical potential in the electrolytic solution, resulting in increase of galvanic corrosion. Etch rate of Al in gold etchant could not be
Fig. 4. Galvanic etch rate of Al in the ternary metal stack of Al–Cr–Au.
Fig. 5. Galvanic etch rate of Cr in the ternary metal stack of Al–Cr–Au.
measured accurately to quantify the corrosion rate, because the etch rate of Al was increased by more than 10 times due to the galvanic effect. It is thus observed from the above experimental results that in the ternary stack of Al–Cr–Au, types of the electrolyte and metal film are more significant for galvanic corrosion than the exposed area of the metal film to be etched. Fig. 5 shows the etch rate of Cr in the ternary metal stack of Al–Cr–Au for different area ratios. The etch rate of Cr is increased by 56% in the Cr etchant and 29% in the Au etchant in the ternary metal stack for cA =eA ¼2 as compared to etch rate of Cr without intermetallic effect. For cA =eA ¼0.25, the etch rate of Cr is increased by 40% in the Cr etchant and 13% in the Au etchant as compared to the chemical etch rate of Cr without the galvanic effect. The results imply that the reduction of cA =eA ratio has comparatively less significant effect on the galvanic etch rate of Cr in ternary metal stack. It has also been observed that Al etchant did not have any effect on the etch rate of
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process technique bypasses the wet chemical etching process of metal layers and thus completely avoids the galvanic corrosion effect. The accelerometer device fabricated using the innovative process is shown in Fig. 6. From the figure, both Al interconnection lines and electroplated gold layer atop the proof mass can be seen. The detailed fabrication process sequence and test results are published elsewhere [3]. 4. Conclusions
Fig. 6. Photograph of a fabricated accelerometer device with an electroplated gold layer and Al metal lines.
Cr and Au. Moreover, in ternary metal stack the etch rate of Au is not affected by the etchants considered and by the variation of cA =eA ratio. The measurement results show that the galvanic effect influences the etch rate of Al in the binary and ternary multilayer metal stack. However, in the binary metal stack of Al–Cr, the cA =eA area ratio has a very significant role in altering the etch rate of Al. In the ternary metal stack of Al– Cr–Au the galvanic etch rate of Al is enhanced significantly as compared to the etch rate of Al without intermetallics effect. The phenomenon of galvanic enhanced etch rate of a thin film has been effectively used to realise nano-fluidic channels [2]. Comparing with the binary metal stack, it has been observed that in the ternary stack, the type of electrolyte and the choice of the metal film in galvanic series are more significant for galvanic corrosion than the exposed area of the metal film to be etched. In various application specific electronics and MEMS devices, the use of two or more different metal films is quite essential for proper functioning of the device. One such example is a MEMS based silicon piezoresistive accelerometer where Al metal is used as an interconnect for signal pick-up and a Cr–Au layer is used to increase the proof mass weight [3]. The performance characteristics of the accelerometer device are improved by increasing the proof mass weight [7,8]. Realisation of this device requires wet chemical etching of all the three metal films to achieve the proper geometrical layout. However, the present study shows that it is quite difficult to achieve the proper geometrical pattern of the metal film such as its mask dimension by the standard process sequence due to the galvanic effect. Moreover, if the metals in use are comparatively far apart in the electrochemical series, then galvanic corrosion is much more dominant, resulting in more distortion of the metal lines. To overcome this difficulty, the combined method of lithographic process and shadow mask technique proposed in the present study is used to fabricate a MEMS piezoresistive accelerometer with an electroplated gold layer. This innovative
Galvanic corrosion of Al thin film metal in the binary and ternary metals stacks is presented in this paper. Experimental results show that the galvanic effect influences the etch rate of Al in the binary and ternary multilayer metal stacks. However, in the binary metal stack of Al–Cr, the cA =eA area ratio has a very significant role in altering the etch rate of Al. In the ternary metal stack of Al–Cr–Au the galvanic etch rate of Al is enhanced significantly as compared to the etch rate of Al without intermetallics effect. Comparing with the binary metal stack, it has been observed that in the ternary stack, the type of electrolyte and the choice of the metal film in galvanic series are more significant for galvanic corrosion than the exposed area of the metal film to be etched. Considering the galvanic effect of Al metal film in other electrolytic solutions and resulting pattern distortion, a piezoresistive accelerometer with patterned gold film atop the proof mass has been fabricated with an innovative process step combining shadow masking technique and electrochemical deposition.
Acknowledgement The authors would like to express their gratitude to (Late) Prof. S. Kal for his valuable suggestions. The authors acknowledge the members of Microelectronics and MEMS Laboratory, IIT Kharagpur, for their help in the experiment. References [1] A. Peter, Physical Chemistry, 6th ed. W.H. Freeman and Company, New York, 1997. [2] H. Zeng, A. Wan, A.D. Feinerman, Nanotechnology 17 (2006) 3183–3188. [3] A. Ravi Sankar, S.K. Lahiri, S. Das, Journal of Micromechanics and Micro engineering 19 (2009). http://dx.doi.org/10.1088.09601317.19.2.025008. [4] K.R. Williams, K. Gupta, M. Wasilik, Journal of Microelectromechanical Systems 12 (2008) 761–778. [5] A. Tixier, Y. Mita, J.P. Gouy, H. Fujita, Journal of Micromechanics and Microengineering 10 (2000) 157–162. [6] J.M. Hong, J. Zou, Journal of Micromechanics and Microengineering (2008) doi:10.1088.0960-1317.18.5.055002. [7] N. Yazadi, K. Najafi, A.S. Salian, Journal of Microelectromechanical Systems 12 (2003) 479–486. [8] R.P. Van Kampen, R.F. Wolffenbutte, Sensors and Actuators A 64 (1998) 137–150.