Technology of Electroless Copper Plating for MEMS

Technology of Electroless Copper Plating for MEMSYi Li, Xiang Han, Yilong Hao, Guizhen Yan, Wengang WuInstitute of Microelectronics, Peking University, Beijing 100871, ChinaAbstractSome newly-developed techniques of electroless copper plating for MEMS are reported in this paper. In order to make theelectrolessly-plated copper film adhere to the silicon surface perfectly, we propose using oxygen plasma bombardmentcombined with ion implantation and KOH etching to do physical treatment on silicon surface. The copper film’s thickness andsheet resistance are found 1500 angstroms and ڤ/‏‎0.14Ω (this datum is given by a four-point probe instrument), respectively,and the copper crystals of approximate 300 nanometers in diameter are fine and symmetrical. It is found that the copper can beselectively plated on silicon surface but not on the glass surface, and the silicon structures are encapsulated completely by thecopper film. So far the average thickness of the electrolessly-plated copper film has reached 3 microns since the newtechniques, such as oxygen-plasma-bombardment silicon surface treatment, are applied in the electroless copper plating.Key words: MEMS, electroless plating, oxygen plasma bombardment1 INTRODUCTIONThe technology of electroless plating different from theelectroplating , which is actually a kind of chemical platingwithout the introduction of electricity, is widely used inmany fields such as copper interconnects in print circuitboards. At the same time, this technology has been appliedin microelectronics and microelectromechical system. [1]First, the copper film can be plated on the surface of thepolysilicon. Second, this process can be realized on siliconwhich is the most widely used material in themicroelectronics. We have employed this process infabricating the micro-accelerators and micro-gyroscopes forits selectivity and conformality. So the device’s parasiticresistor can be reduced, consequently the quality factor andperformance can be improved. Because the parasitic resistorof passive devices is the main restricting factor, a thickercopper film is necessary in RF.The basic principle of the electroless copper plating is thatthe oxidation and reduction reactions are performed on theactivated solid surface in the solution. The reducer reducesthe copper ion into copper film. Oxidation reaction:Cu 2+ + 2e - Cu (1)Reduction reaction:2HCHO + 4OH — 2HCOO - + 2 H 2 O + 2e - (2)Based on the theory of chemical thermodynamics, thereaction can take place only when the potential of thereducer is lower than that of the copper ion. In thealkalescence solution, formaldehyde can reduce the copperion. In order to keep the PH value of the solution in theproper range, it is necessary to add the potassium hydroxideinto the solution. In order to prevent the precipitation in thesolution, we should reduce the concentration of thedissociated copper ion and add the complexing agent intothe solution. The amount of the complexing agent should betaken into careful account. There are a lot of subsidiaryreactions in the solution. In order to prevent these reactions,it is necessary to add additives into the solution. In order toreduce the stress of the plated copper film and improve thequality of the copper film, it is necessary to add surfactantinto the solution. [2]The base material which can be plated with copper filmshould be activated by catalyst. Palladium chloride is themost widely used catalyst. It’s basic process is the formationof catalytic center - palladium particulate, crystal element orcrystal layer.We have employed different treating methods and discussedthe improvement of silicon surface’ character and theinfluence on the plated layer due to these treating ways. Wefurther investigate the process condition of plating thickcopper film. This process indicates that the quality of thecopper film has close relationship with the silicon surface’scondition. Silicon as a semiconductor material is differentfrom the metal copper in the electrical and mechanicalcharacters. It is necessary to treat the silicon surface beforethe plating. Besides the chemical activation to the siliconsurface using the palladium chloride, we should improve thesilicon surface’ physical aspect condition. On the other hand,the stress between the silicon and copper layer will influencethe quality of the thick copper film.2 THE PROCESS AND CONDITIONThe recipe of electroless plating copper consists of thefollowing components: activation solution: palladiumchloride (0.2g/L) and hydrofluoric acid (150ml/L);Oxidation agent: cupric salt of cupric sulfate (5g/L);reduction agent: formaldehyde; brightener:2,2’-dipyridyl(25ml/L); complexing agent: EDTA(15g/L);surfactant:RE-610(2.4mg/L). We also use the potassiumhydroxide to adjust the PH value. The plating temperature is60 0 C.[3]

The process steps are treating before the plating, activationand plating. We use the (100),heavily n-doped 4” wafer. Thetreating procedures include the improvement of thecleanness and enhancement of the coarseness of the siliconsurface. For the first aspect, we can use the nanostrip( amixture of sulfuric acid and hydrogen peroxide). But it iseasy to damage the metal electrode on the glass for thedevices are fabricated using the standard bulk siliconprocess .For the second aspect, besides the ion implantation(35KeV, 5E15, P + ), we also employ the oxygen plasmabombardment, so the quality of the plated layer can beimproved greatly. Activation is a process that stripping of thenative oxide layer and formation the catalytic center-palladium particulate on the silicon surface. Generally, thelonger the activation time is, the more catalytic centersformed on the silicon surface. But considering thehydrofluoric acid’ eroding to the glass, the activation time iscontrolled in a minute. The devices are put into the strainermade by teflon in the plating solution.If we first use potassium hydroxide to erode the surface andthen employ ion implantation, the surface’ condition can beimproved. The silicon surface shown in the Fig. 2(a) istreated after these steps: eroding the surface by potassiumhydroxide, wiping off potassium ion, ion implantation,activation and plating. The stress between the plating layerand silicon surface makes the plating layer bubble. Thecopper layer is not uniform. When we increase the platingtime, the stress makes the bubble break and then can bereleased. This phenomenon is reflected in the Fig. 2(b). Theeroding to surface by potassium hydroxide reduces thecoarse degree of the surface, but at the same time producesthe adverse influence to the binding strength between platinglayer and base material.The basic experiment equipments are: microwave utensilcontaining the active solution (2 litres), glass beakercontaining the plating solution (3 litres), strainer, severalutensils containing DI water, thermometer, graduated flaskand scale.3 EXPERIMENT AND DISCUSSION3.1 Binding Strength between Plating Layer and BaseMaterialIn order to increase the binding strength between platinglayer and base material, we employ the ion implantation tomake the silicon surface coarse. There is meshwork on thesilicon surface shown in the Fig. 1. The silicon surface in themeshwork is implanted while the surface around themeshwork is not implanted. The plating layer formed in theion implantation area bubbles, while the plated layer formedaround the ion implantation does not bubble after the platingbecause it is difficult for the plated layer formed in the ionimplantation area to release the stress.(a)(b)Figure 2(a). The plated layer formed after eroding thesurface by potassium hydroxide, wiping off potassium ion,ion implantation, activation and plating (a)10 minutes and(b)15 minutesFigure 1. The plated layer formed after ion implantation,activation and platingThis stress prevents the copper layer adhering to the siliconsurface tightly. So it is difficult to achieve the thick copperlayer. We employ the oxygen plasma bombardment to treatthe silicon surface when plating the thick copper layer.

The silicon surface shown in the Fig. 3 is treated after thesesteps: eroding the surface by potassium hydroxide, wipingoff potassium ion, ion implantation, removing the nativeoxide layer by hydrofluoric acid, oxygen plasmabombardment, activation and plating. When the silicon isdipped into the active solution, there are black flocculesfloating from the silicon surface. The copper film may beamorphous at first and then becomes large column crystalwhen the copper layer becomes thick. We investigate theprofile of copper layer in the Fig. 3(b). The copper crystal ismeticulous and the crystal particulates are symmetrical insize. The diameter of column crystal particulate is 300nm.The copper layer is smooth and bright. The thickness ofcopper layer is about 1500 angstroms and the squareresistance is 0.14Ω/□. When the plating time is increased,the stress between plated layer and base material makes thecopper layer bubble.(a)(b)Figure 3(a). The plated layer formed after eroding thesurface by potassium hydroxide, wiping off potassium ion,ion implantation, removing the native oxide layer byhydrofluoric acid, oxygen plasma bombardment, activationand plating. (b) The profile of copper layer shown in the (a)The copper layer adheres to the surface well and its qualitycan be improved greatly though eroding the surface bypotassium hydroxide, ion implantation and oxygen plasmabombardment.3.2 Investigation to Activation MechanismWhen the silicon is dipped into the active solution, there areblack floccules floating from the silicon surface. Becausethere only exist hydrofluoric acid (150ml/L) and palladiumchloride (0.2g/L) in the solution and metal palladium isblack, the black floccules should be metal palladium. Weonce put the silicon into tin (Π) chloride dehydrate solutionand then into palladium chloride solution, but the quality ofcopper layer is bad. This experiment indicates that theelectron provided by the tin (Π) chloride dehydrate canreduce the palladium ion, but it is not benefit to the qualityof copper layer. So we can make a deduction that for thesilicon with the crystal orientation (100), there exists suchstructure (=SiH 2 ) after hydrofluoric acid strips of the siliconsurface’s native oxide layer. Not every silicon atom has thisstructure with the double hydrogen ternimation. Because ofthe metal ion adulterated and the ineluctable defect of crystallattice formed in the process of silicon fabrication, somesilicon ions on the surface have dangling bonds with thesingle electron formed in the process of covalent bondbreaking. Silicon surface has the character of minus charges.The catalytic reaction is:=SiH 2 +Pd 2+ =SiPd +2H + . (3)It is the silicon surface that provides palladium ion withelectrons.[ 4,5,6]In order to prove this deduction, we fill hydrofluoric acidand palladium chloride into two different containersrespectively. The silicon is dipped into the hydrofluoric acidand the native silicon oxide layer on the silicon surface canbe removed then it is activated in the palladium chloridesolution. The black floccules also are found floating fromthe silicon surface.The silicon surface shown in the Fig. 4 is treated after thesesteps: eroding the surface by potassium hydroxide, ionimplantation, stripping of the native oxide layer byhydrofluoric acid, activation and plating. The quality ofcopper layer is well in this experiment. Compared to theresult indicated in the Fig. 3, the crystal is not even enough.Because the removing of the native silicon oxide layer andthe formation of palladium center take place closely, thequality of the copper layer is better than that shown in theFig. 3.

Figure 4. The plated layer formed after eroding the surfaceby potassium hydroxide, ion implantation, stripping of thenative oxide layer by hydrofluoric acid, activation andplating.3.3 Selectivity to the Base MaterialWe find that the copper layer cannot be plated on the surfaceof the glass, silicon nitride and silicon dioxide. It indicatesthat the selectivity to the base material in this experiment.The silicon surface is oxidized first and then a silicondioxide of 800 angstrom is formed. Silicon dioxide can beetched and then a meshwork is formed. So silicon is in themeshwork and silicon dioxide is around the meshwork. Wefirst bombard this silicon plate using oxygen plasma andthen put it into the active solution. It is found that the copperfilm cannot be plated on the surface of silicon dioxide. Weinvestigate the amplificatory results in the Fig 5(b), and findthat the boundary is very clear.(b)Figure 5 (a). The selectivity of this plating and (b)amplificatory results3.4 Exploring to the Plating Thick Copper LayerThe silicon surface shown in the Fig. 6 is treated after thesesteps: oxygen plasma bombardment, activation and plating.The thickness of the copper layer is about 5 microns. Thecopper layer bubbles and the coralloid crystal is formedbecause copper layer are very thick. So we conclude thatsilicon surface becomes coarser after oxygen plasmabombardment. The thick copper layer can be achieved easily,but its quality is not very good and the crystal is not regular.Figure 6. The plated layer formed after oxygen plasmabombardment, activation and plating3.5 Analysis on the Copper Layer plated on the Surfaceof Device’ Structure(a)The copper layer on the surface of micro-gyroscope isshown in the Fig. 7(a). The device is fabricated using thestandard bulk silicon process. The thickness of the structureis 60 microns. The distance between the comb and the glassis 4 microns. This kind of plating has good conformality andthe copper film is also plated on the sidewalls. We furtherinvestigate the copper film in Fig. 7(c) and find that thecopper layer bubbles somewhere. This phenomenonindicates that quality of copper layer has close relationship

with the structure.Figure 7(a) The copper layer on the surface of devicestructure. (b) Conformality. (c) Copper layer bubbles.4 CONCLUSIONS(a)The active solution consists of hydrofluoric acid andpalladium chloride in this experiment. The function ofhydrofluoric acid is not only stripping of native oxide layeron the surface but making it coarse and with dangling bond.It is just this structure that reduces the palladium ion. So thepalladium atom deposits on the surface and then furtherforms the crystal layer. When the silicon is dipped into theactive solution, there are black floccules floating from thesilicon surface. The surface of silicon has a layer of blacksubstance after activation. This substance should be metalpalladium. This phenomenon indicates that palladium ion isnot reduced in the plating solution, but in the active solution.We find out the condition that realizes the high-qualitythin copper layer shown in the Fig. 3. It is treated after thesesteps: eroding the surface by potassium hydroxide, wipingoff potassium ion, ion implantation, stripping of the nativeoxide layer by hydrofluoric acid, oxygen plasmabombardment, activation and plating. When the plating timeis increased, the stress between the silicon and plating layermake the copper layer bubble and even break.Only through the oxygen plasma bombardment thethick copper layer can be realized. But there are coralloidcrystals on the copper layer.This kind of plating has selectivity. We have foundthat the copper layer can not be plated on the glass, siliconnitride and silicon dioxide. This kind of plating will haveadvantage if it is used in the standard bulk silicon process.This kind of plating has the great conformality whichis another advantage when compared to the electroplating.ACKNOWLEDGEMENTS(b)(c)This work is supported in part by the National NaturalScience Foundation of China under Grant No. 90307007.REFERENCE[1] H.Jiang, Y.wang, J.L.A.Yeh, and N. C. Tien, “Fabricationof high-performance on-chip suspended spiral inductors bymicromachining and electroless copper plating,” IEEE2000MTT-S Digest[2] Xiao Xia Jiang, The fundamentals and practice ofelectroless plating, National Defence Industry Press,2000[3] J.L.Andrew Yeh, Hongrui Jiang , Hercules P.Neves,“ Copper-Encapsulated Silicon Micromachined Structures,”JMMS ,9, 2000[4] TONG Hao and Wang Chun Ming, “Application Studyof open circuit potential-time technology and atomic forcemicroscopy on Si(100)/solution interface in Ag electrolessdeposition,” Acta Chimica Sinica, 60, 2002[5] G.S.Higashi, Y.J.Chabal, G.W.Trucks, andKrishnanRaghavachari, “Ideal hydrogen termination of theSi(111) surface,” AT&T Bell Laboratories, Murray Hill,New Jersey 07974

[6] G.S.Higashi, R.S. Becker, Y.J. Chabal, and A.J.Becker,“Comparison of Si(111) surface prepared using aqueoussolutions of NH 4 F versus HF,” AT&T Bell Laboratories,Murray Hill, New Jersey 07974BiographiesYi Li ,Graduate student in the Peking UniversityEmail: liyi@ime.pku.edu.cnXiang Han, undergraduate in the Peking UniversityYilong Hao,Professor in the Peking UniversityGuizhen Yan ,Professor in the Peking UniversityWengang Wu, Professor in the Peking UniversityEmail: wuwg@ime.pku.edu.cn