Selected-area growth of nickel micropillars on aluminum thin films by electroless plating for applications in microbolometers

We have presented an electroless nickel plating process for applying in surface micromachining of microbolometers upon an IC substrate. The optimization of Ni-P containing bath compositi[r]

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Original Article

Selected-area growth of nickel micropillars on aluminum thin ﬁlms by electroless plating for applications in microbolometers

Do Ngoc Hieua,b, Dang Nguyen Ha Myb,c, Vu Thi Thu c, Nguyen Quoc Hungb,

Do Ngoc Chungd, Nguyen-Tran Thuatb,*

aFaculty of Engineering Physics and Nanotechnology, VNU University of Engineering and Technology, Vietnam National University, 144 Xuan Thuy, Hanoi,

Viet Nam

bNano and Energy Center, VNU University of Science, 334 Nguyen Trai Street, Hanoi, Viet Nam

cDepartment of Advanced Materials Science and Nanotechnology, University of Science and Technology of Hanoi, 18 Hoang Quoc Viet, Hanoi, Viet Nam dCenter for High Technology Development, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Viet Nam

a r t i c l e i n f o Article history:

Received April 2017 Received in revised form 12 May 2017

Accepted 12 May 2017 Available online 18 May 2017

Keywords:

Nickel electroless plating Micropillars

MEMS Microbolometer Selected-area growth

a b s t r a c t

An optimization process of electroless plating of nickel was carried out with NiCl2as the nickel ion source, NaH2PO2as the reduction agent, CH3COONa and Na3C6H5O7as complexing agents Electroless plated nickel layers on sputtered aluminum corning glass substrates with a resistivity of about 75.9mUcm and a nickel concentration higher than 93% were obtained This optimum process was successfully applied in growing nickel micropillars at selected areas with a well-controlled height The microstructure of the masking layers was fabricated by means of optical photolithography for subsequent growth of nickel micropillars on selected areas Micropillars size was deﬁned by the opening size and the height was controlled by adjusting the plating time at a growth rate of 0.41mm/min This result shows that electroless nickel plating could be a good candidate for growing micropillars for applications in microbolometers © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

The integration between micro electro-mechanical systems (MEMS) and complementary metal oxide semiconductor (CMOS) circuits allows a huge range of smart applications starting from simple actuator functionalities to sophisticated sensing andﬂuidic system-on-a-chip devices[1] Microbolometers are typical MEMS-based devices whose performance had been enhanced by the monolithic integration of MEMS upon integrated circuit (IC) sub-strates using MEMS-last processing via layer deposition and surface micromachining Since then MEMS-based microbolometers have been used for the detection of long wavelength infrared (LWIR)

[2e5]and terahertz electromagnetic radiation[6e8]

Electroless or autocatalytic plating of metals is a well-developed coating technique which involves the presence of a chemical agent in a solution to reduce metallic ions into its bulk or thin metalﬁlm state[9] The autocatalytic plating is deﬁned as the deposition of a metal layer by a controlled chemical reduction which is catalyzed by the metal or alloy being deposited This plating process has been used to yield metal deposits, such as Ni, Pd, Cu, Au, and Ag as well as some alloys containing these metals with impurities such as P, B or N[10] In semiconductor industry, electroless plating and electro-plating have been used in making interconnection between metal layers[11]

On the other hand, electroless plating and electroplating of nickel is one of the important fabrication steps in MEMS process, which is described by the term LIGA (lithography, galvanoforming and moulding)[12e14] Among other metals, nickel is one of the com-mon elements to be electroless plated on a metal surface, such as aluminum, copper or alloys Electroless nickel (EN) plating has been widely used in MEMS fabrication process thank to its simplicity

[15e17] EN plating was reported in Ref.[4]for making pillars of microbolometer arrays with a good sensitivity and detectivity but the sacriﬁcial planarization layer has not been intentionally

* Corresponding author Nano and Energy Center, VNU University of Science, Room 503, 5thﬂoor, T2 building 334 Nguyen Trai street, Thanh Xuan, Hanoi, Viet Nam Fax:ỵ84 435 406 137

E-mail address:thuatnt@vnu.edu.vn(N.-T Thuat)

Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2017.05.004

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removed for enhancing the thermal isolation of microbolometers The growing of these pillars in EN plating process must be occurred only in selected aluminum areas, thus requiring a masking layer to protect un-wanted deposition of nickel on other areas[18] The drawback is that this masking layer could be damaged by a large range of pH level of the electroless plating bath and the pretreat-ment, activation and zincating solutions[15,16,19,20]

In this paper, we present an experimental study on electroless nickel plating for applications in microbolometers The study starts from an optimization of the electroless plating bath for depositing nickel on aluminum surface and ends up at the controlled growth of nickel micropillars on selected areas with the masking layer patterned by a conventional optical lithography process

2 Experimental procedures

Nickel electroless plating processes were carried out as following: (i) aluminum samples preparation, (ii) zincating and rinsing, (iii) EN plating in a Ni-P containing a bath Bulk aluminum samples with a purity of 99.99% were used as a reference and the optimization of bath composition Thin aluminumfilms were used after obtaining the optimum bath composition These Al thinfilms, with thicknesses about 200 nm, were deposited on clean corning glass substrates by sputtering a two-inch aluminum target, 99.99% of purity, at 60 W for 60 min, on a 4-mangetron-gun sputtering system from SYSKEY Before the electroless plating step, aluminum samples were pretreated by a zincating procedure, with the composition of each solution described onTable For bulk Al samples the zincating time was 15 s for each dipping step, while it was s for thin Alfilm samples After the zincating step, all samples were rinsed in deionized water and the electroless plating step was taken place immediately afterward

Compositions of nickel electroless plating baths are showed on

Table The optimization of the bath composition was carried out

only on bulk aluminum samples with the starting “plain bath”

composition The optimum composition is denoted as“bath with

additives”, after using CH3COONa and Na3C6H5O7as additives The

plating temperature wasﬁxed at 90C, and the pH level of the bath

was kept around to

For selected-area growth of nickel by electroless plating, only

aluminum thinﬁlms samples were used The masking layer, with

various opening size ranging from 6mm to 8mm, was patterned by using a conventional photolithography contact procedure The photoresist AZ5214E, from Microchemicals, was used as both the transferring layer and the masking layer Since the zincating and plating duration was quite short, the hard-baked photoresist layer was not damaged during all experiments After the pattering pro-cess by photolithography, nickel micropillars were grown on un-masked areas by the optimized electroless plating process showed above

The morphology of all samples was characterized on a con-ventional optical microscope and by scanning electron microscopy (SEM), on a SEM Hitachi S-4800 system The chemical composition of nickel electroless plated was characterized by energy-dispersive X-ray spectroscopy (EDS) on the same SEM system The sheet resistance of electroless plated nickel was measured on a four-point RM3000 Jandel prober The thickness of nickel micropillars was characterized on an Alpha-Step DEKTAK150 proﬁlometer

3 Results and discussion 3.1 Electroless plating of nickel

The optimization of electroless nickel plating was performed on bulk aluminum samples with the starting“plain bath” composition showed onTable The morphology, the chemical composition of samples' surface and sheet resistance of the sample were used an optimizing parameter In order to obtain this“plain bath” compo-sition, many trial experiments have been done by changing the bath temperature, the order of putting the substrate and the reduction agent (NaH2PO2) into the solution, and different

combi-nation of nickel source (NiCl2 and NiSO4) On the one hand, the

concentration of Ni in the“plain bath”, about 7.5 g/L, was quite similar to the concentration of electroless plating of nickel in in-dustry At this range of nickel concentration, the deposition rate of electroless plating is not much affected by theﬂuctuation of nickel concentration in the bath[21,22] On the other hand, the pH value of the“plain bath” composition was smaller than the value yielding low value of phosphorus concentration of plated nickelﬁlm[21] This led to the introduction of additives into the bath for the sub-sequent optimization For the availability of chemicals in local, CH3COONa and Na3C6H5O7were chosen As found in the literature,

acetates and citrates are complexing agents They play three roles: (i) maintain stable pH level, (ii) prevent precipitation of nickel salts such as phosphites, (iii) reduce the concentration of free nickel ion

[23e25] The optimizing “bath with additives” composition is showed inTable For this optimized bath, the insertedﬁgures of

Fig 1show (a) a bulk aluminum surface before and (b) after the electroless nickel plating process It can be seen that the aluminum

Table

100 ml zincating solutions composition

1st beaker 2nd beaker 3rd beaker

Volume (ml) 20 50 20

Composition NaOH 0.1 g HNO3 10% NaOH 0.1 g ZnO 0.05 g

Table

100 ml baths composition

Bath description NiCl2$6H2O (g) NaH2PO2$H2O (g) CH3COOH 80% (ml) CH3COONa (g) Na3C6H5O7(g) pH level

Plain bath 3 0 2e3

Bath with additives 6e7

Fig EDS spectrum of electroless nickel plating on bulk aluminum sample Inserted ﬁgures are bulk aluminum sample (a) before and (b) after electroless nickel plating with additives

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surface changes substantially; it is no longer shiny and is covered by an electroless plated nickel layer The graph ofFig 1exhibits the EDS spectrum taken from this electroless plated nickel deposition on bulk aluminum surface It is worth noting that all EDS spectra in

this study are used for the purpose of chemical elements analysis only OnFig 1Ni and P peaks are found suggesting the abundance of nickel and the presence of phosphorus as an impurity Surpris-ingly, there is no aluminum peak observed even though this

Fig Optical images of thin aluminum sample: (a) as deposited surface, (b) after zincating treatment, (c) after of electroless nickel plating and (d) after 10 of electroless nickel plating

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spectrum was taken on a bulk aluminum sample It shows that the aluminum surface is coated completely by a P-containing Ni thick ﬁlm and the nickel concentration was estimated roughly to be higher than 90% The residual phosphorus in the electroless plated nickel was unavoidable since the reducing agent was NaH2PO2

The optimum “bath with additives” composition was then

applied to plate auto-catalytically nickel on 200 nm of thickness thin aluminumﬁlms.Fig 2shows the surface of an aluminum thin ﬁlm sample (a) as deposited by sputtering, (b) after the zincating treatment, (c) after and (d) after 10 of electroless plating

We can see that the aluminum surface is darken after the zincating treatment and the electroless plated nickel surface after is more homogenous than after 10

In order to shed some light on the morphology of the electroless plated nickel surface, Fig 3shows SEM micrographs of samples after each treatment step OnFig 3a, a nanostructured aluminum surface is observed after the zincating treatment This nano-structure surface favors the growth of nickel in the subsequent electroless plating step.Fig 3b, and respectivelyFig 3c, shows the morphology, and the cross-section respectively, of electroless plated nickel after min.Fig 3d, and respectivelyFig 3e, are for the sample after 10 of nickel plating time From these micrographs, we can see that the morphology of nickel layers are likely an as-sembly of micro-sized spheres These spheres became larger as the plating time increased from to 10 From the cross-section micrographs, we can see that the underneath layer exhibits a submicron-sized structure By measuring the thickness, which is 2.88mm for and 5.48mm for 10 sample, it is deduced that the thickness increases rather linearly with the plating time For the sample after 10 of plating, a resistivity of about 75.9mUcm was obtained from the sheet resistance measurement This is one order of magnitude higher than the resistivity of the bulk nickel material, which is about 6.93mUcm In addition,Fig 4shows the EDS spectra of these two samples There are only peaks characterizing nickel and phosphorus, giving an estimated value of nickel concentration higher than 90% for both samples By comparing with the literature, the value of resistivity and the concentration of phosphorus ob-tained in this study are in good agreement[23]

Fig EDS spectra of electroless nickelﬁlm on aluminum thin ﬁlm after and 10 of plating

Fig Simpliﬁed monolithic MEMS and IC integration using MEMS-last processing via layer deposition and surface micromachining: (a) starting CMOS wafer with aluminum pads, (b) masking for selected areas on aluminum pads, (c) electroless nickel plating on selected areas, (d) supporting layer deposition, (e) LWIR sensing layer deposition, (f) contact deposition, (g) encapsulation layer deposition and (h) masking layer removal

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4 Selected-area growth of nickel micropillars

In order to show how electroless plating of nickel plays a central role in micromachining microbolometers for LWIR detection,Fig

shows our simpliﬁed fabrication process of a monolithic MEMS and IC integration Starting from a CMOS wafer with aluminum pads for the connection with underneath circuits, onFig 5a, a masking layer was coated, on Fig 5b The structure of the masking layer was patterned by a conventional photolithography by using AZ5214e photoresist This layer plays not only the role of deﬁning the openings for subsequent electroless nickel plating, described by

Fig 5c, but also a sacrificial layer, which will be stripped off at the final step, described by Fig 5h All other layers for supporting, sensing, contacting and encapsulating are deposited upon the sacrificial layer and nickel micropillars, described byFig 5deg The height of the masking layer will define therefore the height of suspending microbolometers for absorbing LWIR radiation For the

central absorption wavelength of 10 mm, the thickness of the

sacriﬁcial layer should be around 2.5mm in order to have an effect of destructive reﬂection of incoming radiation

It is true that the selected-area growth of micropillars by elec-troless nickel plating is a crucial step of our microbolometers fabrication processflow Since the openings height of the masking layer is about 2.5 mm, and thicknesses of other layers (such as supporting, sensing, contacting and encapsulating) are about 100 nm, withoutfilling the openings it is quite difficult to micro-machine the suspending structure of microbolometers Nickel micropillars not onlyfill the openings for preparing a planar surface,

but also electrically conducting and mechanically supporting col-umns for the suspending structure The electroless plating tech-nique gives a simpleﬁlling method and may not need a subsequent surface planarization step if the growth were well controlled

Figs 6a and b shows optical images of different opening size of about 11mm, and mm, respectively, after the photolithography patterning step.Figs 6c and d show the corresponding micro-sized electroless plated nickel micropillars after removing the masking photoresist layer;Figs 6e and f are the corresponding zoomed SEM micrographs It can be seen easily that nickel micropillars were grown at intentionally selected areas on aluminum thinﬁlms The lateral sizes of nickel micropillars are similar to that of openings There will be little optimization for obtaining good shapes of micropillars OnFig 6c and d, the superposed features are images taken on our LWIR sensing multilayer, patterned by photolithog-raphy, for illustrating how microbolometers will be fabricated upon nickel micropillars.Fig 7shows in addition the EDS spectrum taken on a selected nickel micropillar with a lateral size of 11mm It is similar to the spectra taken on electroless plated nickel on bulk and aluminum surface showed onFigs and 4, respectively We can observe also that there are only Ni and P peaks, giving an estimated Ni concentration of 93% The value of P concentration in the plated Ni in our samples, about 7%, is ranged as middle impurity concen-tration In this paper, we focused on electrical properties of plated Ni layers, the structure of Ni-P was not fully characterized It was reported that Ni and P formed compounds such as Ni5P2, Ni3P in the

plated Ni layer[26] In order to reduce the impurity to the low level

of 2e5%, more complexing agents are needed such as the

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combination of malic acid and succinic acid[27] In Ref.[27], it was reported that the main source of P incorporation into the plated Ni was hypophosphorous acid H3PO2 and hydrogen radicals in the

bath Thus reducing the concentration of H3PO2and maintaining

high pH level are the crucial elements for lowering the phosphorus concentration In our devices, the P concentration in Ni micropillars may lead to the diffusion of P into the CMOS substrates in future devices The fabrication of nickel micropillars are considered as a post-CMOS surface micromachining step Fortunately, transistors are protected by insulator layers deposited during the intercon-nection back end of line steps So phosphorus residual in micro-pillars would not have high impact on the device performance Nevertheless, more complexing agents such as malic acid and succinic acid will be used for the future optimization of the bath composition

For controlling the height of nickel micropillars, a systematic series of electroless plating experiments with different plating time

were carried out Fig presents the dependence between the

micropillars height and the plating time For our optimum“bath with additives” composition, the deduced deposition rate is about 0.41mm/min As a consequence, in order to grow micropillars of 2.5mm of height, of plating time is enough The deposition rate could be decreased by diluting the nickel source, NiCl2, and the

reducing agent, NaH2PO2, but longer time in plating bath would

give some unwanted damage on the surface of device For future optimization of growing micropillars by electroless plating, sur-factants for enhancing the wettability and for smoothing the plated Ni surface, such as sodium dodecyl sulphate or cetyltrimethyl

ammonium bromide [28], will be considered with an optimized

concentration in order to avoid the organic contamination in the plating bath

5 Conclusion

We have presented an electroless nickel plating process for applying in surface micromachining of microbolometers upon an IC substrate The optimization of Ni-P containing bath composition was carried out on bulk aluminum samples The optimum plating bath was successfully applied on sputtered aluminum thinfilms by reducing the zincating time from 15 s to s A resistivity of 75.9 mUcm was obtained for electroless plated nickel film on a 200 nm of thickness aluminum thinfilm for 10 of plating time Selected-area growth of nickel micropillars was performed by using the optimum electroless plating process The lateral size of these micropillars was in a good agreement with the opening size, about 6mm and 11mm The height of micropillars was well controlled by adjusting the plating time For our optimum bath composition, a growth rate of 0.41mm/min was obtained, thus giving a way to control the height of micropillars for assuring the planarized sur-face before other subsequent sursur-face micromachining steps This study thus opens a potential perspective in fabricating micro-bolometers for LWIR imaging

Acknowledgments

The authors greatly acknowledge the ﬁnancial support of the

National Foundation for Science and Technology Development e

NAFOSTED under Project No 103.02-2015.79“Fabrication of micro sized coolers based on thermoelectric effect” The authors would like to thank the Vietnam National University Hanoi for research

equipment from the project named“Strengthening research and

training capacity in ﬁelds of Nanoscience and Technology, and

Application in Medical, Pharmaceutical, Food, Biology, Environ-mental protection and Climate Change adaptation in the direction of sustainable development”

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