Wet etching is a critical fabrication step for the mass production of micro and nanoelectronic devices. However, when an extremely corrosive acid such as hydrofluoric (HF) acid are used during etching, an undesirable damage might occur if the device includes a material that is not compatible with the acid. Polyimide thin films can serve as sacrificial/structural layers to fabricate freestanding or flexible devices. The importance of polyimide in microelectronics is due to its relatively low stress and compatibility with standard micromachining processes. In this work, a fast delamination process of a 4-μm-thin film of polyimide from a silicon substrate has been demonstrated. The films’ detachment has been performed using a wet-based etchant of HF acid. Specifically, the effect of HF concentration on the delamination time required to detach the polyimide film from the substrate has been investigated. This study is intended to provide the information on the compatibility of using polyimide films with HF, which can help in the design and fabrication of polyimide-based devices

Al-mashaal, Asaad K. Edaan

Cheung, Rebecca

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

https://doi.org/10.14311/AP.2021.61.0684Acta Polytechnica 61(6):684–688, 2021 © 2021 The Author(s). Licensed under a CC-BY 4.0 licencePublished by the Czech Technical University in PragueDELAMINATION OF POLYIMIDE IN HYDROFLUORIC ACIDAsaad K. Edaan Al-mashaala,∗, Rebecca Cheungba University of Basrah, Polymer Research Centre, Material Science Department, Basrah, 61004, Iraqb The University of Edinburgh, Scottish Microelectronics Centre, School of Engineering, Edinburgh EH9 3FF, UK∗ corresponding author: asaad.edaan@uobasrah.edu.iqAbstract. Wet etching is a critical fabrication step for the mass production of micro and nanoelectronicdevices. However, when an extremely corrosive acid such as hydrofluoric (HF) acid are used duringetching, an undesirable damage might occur if the device includes a material that is not compatiblewith the acid. Polyimide thin films can serve as sacrificial/structural layers to fabricate freestanding orflexible devices. The importance of polyimide in microelectronics is due to its relatively low stress andcompatibility with standard micromachining processes. In this work, a fast delamination process ofa 4-µm-thin film of polyimide from a silicon substrate has been demonstrated. The films’ detachmenthas been performed using a wet-based etchant of HF acid. Specifically, the effect of HF concentration onthe delamination time required to detach the polyimide film from the substrate has been investigated.This study is intended to provide the information on the compatibility of using polyimide films withHF, which can help in the design and fabrication of polyimide-based devices.Keywords: Polyimide, delamination, wet etching, hydrofluoric acid.1. IntroductionThe fabrication of microelectronic devices and theirsubsets involves the use of a range of acids, or-ganic/inorganic polymers, metals, and semiconduc-tor materials. The fabrication process includes sev-eral steps, such as deposition, photolithography, andetching. For example, it is common in the fabrica-tion of microelectromechanical systems (MEMS) thata sacrificial layer of a material of interest is depositedon a substrate before it is subsequently removed bya specific type of etchants. However, there might bea considerable reliability concern when the materi-als/processes used in the fabrication are not compat-ible with each other [1]. The incompatibility couldresult from fabrication processes that require the useof chemicals and materials with different properties [2].In this work, we focus on determining whether theapplication of a layer of polyimide is fully compatiblewith the etching process when a wet-based etchant ofhydrofluoric (HF) acid is used during the fabricationprocess.Polymers play a crucial role as essential functionaland structural materials in modern micromachiningtechnologies [3]. One of the polymeric materials thathave found their way to several applications, especiallyin microelectronics and MEMS devices, is polyimide.The importance of polyimide films is due to theiroutstanding thermal, mechanical, and chemical prop-erties. For example, the low modulus, high mechani-cal strength and high thermal stability have enabledthe polyimide films to be used as sacrificial layers insuspended MEMS-based structures [4, 5]. Another im-portant application of polyimide-based layers is theiruse as active structural components in MEMS sensorsand resonators [6–8]. In addition to their main func-tion as dielectric, alignment, passivation and insulatorlayers [9], polyimides have other useful applicationssuch as a flexible substrate for different types of softdevices [10–12].Hydrofluoric acid (HF) is one of the most widelyused etchants in manufacturing industries. Despitethe fact that etching in HF is a wet process, whichmay have a number of drawbacks, it is still consideredas a highly preferred etching process because of thehigh etch rate and compatibility with batch fabrica-tion processes [13, 14]. However, there seems to bea delamination issue with the use of HF solution whena layer of polyimide is attached to a silicon substrate.The delamination of the polyimide film occurs whenthe reacting ions of the HF solution are diffused intothe interfacial area between the polyimide film andthe substrate. Although a number of studies havebeen reported on both wet [15, 16] and dry [17–19]etching of polyimides, the delamination of polyimidefilms, especially in HF-based etchant solutions, hasnot been investigated yet. The purpose of the currentpaper is to demonstrate the detachment of a thin filmof polyimide from a rigid silicon substrate. Partic-ularly, we have investigated how the concentrationof the HF etchant affects the delamination time andetching rate of polyimide films. The main motivationfor conducting this work is to explore the possibil-ity of developing a fast release process that can beutilised to detach a layer of polyimide from a sili-con substrate without damaging the released layeror the substrate underneath. This is important as itcan help in realising polyimide-based electronic de-vices. It is worth pointing out that the delaminationprocess was conducted in a cleanroom environment(i. e., room temperature and atmospheric pressure).684vol. 61 no. 6/2021 Delamination of polyimide in hydrofluoric acid(a). (b).(c). (d).Figure 1. Fabrication and delamination processes for polyimide films. (A) Silicon substrate cleaned and primedwith a promoter of VM-652. (B) Polyimide spin coated on silicon substrate and fully cured. (C) Substrate withpolyimide on top immersed in HF-based acid. (D) Polyimide film delaminated from the substrate.The chemical identity/composition of polyimide (PI2611-HDMicroSystems) used in the present study isN-Methyl-2-pyrrolidone (85–95 %) and polyimide (10–20 %).2. ExperimentalA schematic diagram of the deposition and delamina-tion processes of polyimide films is shown in Figure 1.Firstly, a substrate of a p-type silicon wafer has beencleaned in acetone and isopropyl alcohol (IPA) sol-vents and treated with oxygen plasma (Barrel Asher)before being coated with a promoter of VM-652 (HD-MicroSystems) (Figure 1a). The purpose of adding thepromoter is to provide adhesion between the polymericfilm to be applied and the silicon substrate. Then,a thin layer of polyimide (PI 2611-HDMicroSystems)has been spin-coated at 4000 rpm for 45 seconds (Fig-ure 1b). An initial bake at 110 °C for 90 seconds hasbeen performed. In order to cure the polyimide filmcompletely, the sample has been held in an oven at200 °C for 60 minutes. The thickness of the appliedpolyimide film has been measured to be ∼ 4 µm. Afterbeing immersed in the HF-based solution (Figure 1c),the polyimide film has been detached from the sub-strate (Figure 1d). It should be noted that due tothe corrosive nature of HF, it must be handled withextreme caution.3. Results and discussionThe delamination process of polyimide films has beenperformed under the use of HF as the main etchantmaterial. The three etchants include HF (49 %),7:1 buffered hydrofluoric acid (BHF) and HF (49 %):deionised water (DI) with a ratio of 1:1. These propor-tions have been chosen as they are the most commonin HF-based etching. The delamination procedureinvolves the immersion of the silicon/film stack in theprepared etchants until the entire polyimide film isdelaminated. After submerging the silicon wafer thatbears the polyimide film in the prepared solution, thefilm delamination has been inspected visually usinga conventional camera. In such an inspection process,we have monitored how the film starts to delaminateat the edge and ultimately how the entire film is de-tached completely from the substrate. Both the edgeand the full delamination time of polyimide films hasbeen calculated and presented in Figure 2. Particu-larly, the approximate time required to delaminate thepolyimide film from the substrate from the momentthe entire silicon/film system was immersed in theetchant solution has been determined.In general, the reason that the delamination startfrom the film’s edge region is possibly associated withthe shear stresses that are concentrated near the edgesand film-substrate interfaces [20]. Figure 2a showsa prepared sample being submerged in the HF solution.As HF molecules penetrate the interfacial area at the685Asaad K. Edaan Al-mashaal, Rebecca Cheung Acta Polytechnica(a). (b).(c). (d).Figure 2. Delamination process of polyimide films in HF-based etchants. (A) Silicon substrate with the film ontop is dipped in HF. (B) The film being delaminated around the edge of the substrate with the inset shows thefilm peeled off (C) A full detachment of the film from the substrate. (D) A delaminated polyimide film floating ondeionised water subsequent to 4 minutes of immersion in HF.rim of the wafer, the polyimide film appears to loseits adhesion to the silicon substrate at these far edgeregions (Figure 2b). We believe that the promotercould play a role in the delamination process sinceit can be attacked by HF. It has been found thatthe size of delaminated regions is much smaller whenan adhesion promoter is applied [21]. The inset inFigure 2b shows how the edge-detached polyimide filmhas been peeled off from the silicon substrate. Oncethe etchant molecules penetrate deeply into the middlearea and reach the entire interfacial region, the filmcan be easily delaminated as shown in Figure 2c. Thefinal delaminated film can be seen floating on the DIwater surface (Figure 2d).Figure 3 shows the delamination time estimated forthe polyimide film with respect to the three differenttypes of HF-based etchants. For the first etchant solu-tion, 50 ml of HF (49 %) has been prepared. Theedge delamination has been found to occur after∼ 90 seconds of immersion. The time required toattain a full delamination has been found to be ap-proximately 4 minutes. In the case of the secondetchant, a mixture of 50 ml of 49 % HF and 50 ml ofDI water has been prepared. It appears that the edgeand full delamination of polyimide films immersed inHF: DI solution occurs after approximately 4 and 7minutes, respectively (see Figure 3). Compared to thefirst etchant, the polyimide delamination time in HF:DI etchant has been observed to be about two timeslonger.To prepare the third etchant of buffered hydrofluoricacid (BHF) solution, 7 parts of a buffering agent ofammonium fluoride (NH4F) have been mixed with1 part of HF (49 %). As shown in Figure 3, the timerequired for the polyimide film to detach from thesubstrate has been calculated to be ∼ 15 minutes inthe edge region. To achieve a complete detachment,however, the film must be immersed in the solution formore than 20 minutes. It is clear that the delaminationof polyimide films is relatively slower in the BHF thanthat in the concentrated HF and HF: DI.To inspect the etch rate of polyimide, the films’thickness has been measured using a reflectometer(Nanometrics Nanospec) before and after the immer-sion in the etchants. It has been found that thepolyimide in the HF is around 15–25 nm/min. Com-pared to the total thickness of the as-applied film (e. g.,4 µm), the proportion of the layer that has been etchedaway in HF during the delamination does not exceed5 %. No considerable difference has been measured in686vol. 61 no. 6/2021 Delamination of polyimide in hydrofluoric acid04812162024HF HF:DI BHFdelamination time (min)etchant  full delamination  edge delaminationFigure 3. Delamination time of polyimide films as a function of different types of HF-based etchants.the films’ thickness after submerging the films in theHD: DI and BHF etchants. Such etching rates are forfully cured polyimide films, and they are in agreementwith other reported findings [15]. It is worth pointingout that the etching rate of deposited or spin-coated(i. e., not cured) polyimide films is higher than thatof the fully cured films.The mechanism behind the delamination of thinpolymeric films from a solid substrate is related toseveral possible effects. For example, it could resultfrom the interfacial deformation process of the ap-plied film and the molecular reaction of the etchantsolution (liquid) and the silicon substrate (solid) [22].An alternative possible mechanism is that the HFattacks the interface, which causes the edge to peelup and delaminate. The residual induced stresseswill cause the film to lift and continuously expose thewhole interface to the attack of the HF. Under sucha cascading effect, a full delamination of the film canbe achieved. The HF has the ability to penetratethrough the polyimide layer and attack the interface.Since our delamination process is HF dependent, theHF molecules are absorbed into the polyimide filmand cause it to swell. As the HF is being absorbed,it is going to migrate to the polyimide/Si interfaceand consequently attack it. In general, we believethat both mechanisms could play a role. Compared toother reported techniques [23–25], the delamination ofpolyimide films in HF-based solutions that has beenimplemented in this work can be considered fast, sim-ple and effective. Our simple method can be furtherimproved to pave the way for a large-scale productionof polyimide thin films for a wide range of flexibledevices.4. ConclusionWe have demonstrated the delamination of thin poly-imide films in a wet-based etchant. Polyimide films ofa thickness of around 4 µm attached (spin coated) toa silicon substrate have been immersed in HF-basedetchant solutions to investigate how the etchants canaffect the detachment of the polymeric films from thesubstrate. It has been found that the purer (not fullydiluted) the etchant solution, the greater the delamina-tion of the film. A full delamination of the polyimidefilms occurs after ∼ 4, 7 and 15 minutes of immersionin HF (49 %), HF: DI and 7:1 BHF, respectively. Therelatively long delamination time can be compensatedby taking into account the least harmful etchant tothe deposited film and the substrate. The results ob-tained in this work provide useful information on thecompatibility between polyimide films and wet etchingprocesses when HF-based etchants are involved.References[1] J. Iannacci. Reliability of MEMS: A perspective onfailure mechanisms, improvement solutions and bestpractices at development level. Displays 37:62–71, 2015.https://doi.org/10.1016/j.displa.2014.08.003.[2] E. H. Cook, D. J. D. Carter. MEMS processcompatibility of multiwall carbon nanotubes. Journal ofVacuum Science & Technology B 29(6):06FE04, 2011.https://doi.org/10.1116/1.3662082.[3] B. J. Kim, E. Meng. Review of polymer MEMSmicromachining. Journal of Micromechanics andMicroengineering 15(1):013001, 2015.https://doi.org/10.1088/0960-1317/26/1/013001.[4] A. K. Al-mashaal, G. Wood, E. Mastropaolo,R. Cheung. 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Delamination of polyimide in hydrofluoric acid

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Delamination of polyimide in hydrofluoric acid

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