Mo Films Research Articles

Development of Multi-Layer Fabrication Process for SFQ Large Scale Integrated Digital Circuits

We have developed a fabrication technology for the development of large-scale superconducting integrated circuits with Nb-based Josephson junctions. The standard fabrication process with 10 mask levels uses four metal layers including 3 Nb superconducting layers and a Mo resistor layer. The influence of deposition parameters on film stress, electrical properties, and surface roughness were studied systematically. High quality Nb, Al, Mo, and SiO2 films were successfully deposited for the subsequent fabrication of circuits. The circuit fabrication started with the fabrication of Mo resistors with a target sheet resistance Rsh of 2 Ome, followed by the deposition of Josephson-junction. The target critical current density Jc was set at 6 kA per cm2. The thicknesses and etch depths of the films were monitored during fabrication with on-wafer process-control-monitor (PCM) patterns for all the wafers.

Na incorporation into Cu2ZnSnS4 thin film absorbers from RF-sputtered NaF precursors

In this work, we report the incorporation of sodium (Na) alkaline element into copper-zinc-tin-sulphide (Cu-Zn-Sn-S) precursor films by adding radio frequency (RF)-sputtered sodium fluoride (NaF) intermediate layers bringing a new approach to the bifacial sodium-incorporated treatment (BIST) and its effect on the properties of Cu2ZnSnS4 (CZTS) absorber films. Due to the absence of alkaline diffusion barrier layers on soda lime glass (SLG) substrates, Na additions from both NaF intermediate layers and SLG substrate were evaluated together. With additional NaF intermediate layers among the elemental metal stacks, the films exhibited better crystalline properties and a more compact structure. However, the amount of the Cu2-xS secondary phases formed on the surfaces increased by the additional NaF layers. Raman spectra confirmed the formation of the kesterite phase and showed the peaks corresponding to Cu3SnS4 and Cu2-xS secondary phases. Na diffusion originating from NaF layers deposited on molybdenum (Mo) films occurred in both directions of the CZTS film and the SLG substrate. The photoluminescence (PL) bands located at around 1.40 eV were attributed to the band to band (BB) transitions. The film with additional NaF intermediate layers exhibited higher carrier concentrations and lower resistivity than the other film due to the presence of the higher amount of Cu2-xS secondary phases on its surface.

Centers of cold electron emission from molybdenum thin films

Thin-film semiconductor devices and functional metal-oxide-semiconductor structures have drawn attention as being applicable for on-chip electronics. Similar structures, however, were also reported to be promising as efficient sources of electrons. This paper presents the results of scanning electron microscopy analysis of molybdenum thin films capable of low-macroscopic-field electron emission. Supposedly, the images show the centres of cold-field emission at different stages of their life cycles from activation to destruction. It is assumed that initially continuous Mo films experience dewetting under heating and ion bombardment, which are caused by the emission current flow. The results contribute to understanding the mechanism of low-field emission from thin metal films, which seems to be different from those previously proposed for thin carbon coatings.

The preparation and superconducting properties of Mo2N nano films by high temperature nitriding method

We successfully obtained the Mo2N nano-films after the high-temperature nitriding of the Mo films prepared by RF magnetron sputtering. Currently, this is the first report that Mo2N nano-films were obtained by direct nitriding with Mo films. The prepared Mo2N films were characterized by XRD, SEM and EDS. Besides the influence of different power and deposition time on the crystal structure of the Mo2N nano-films were analyzed. Compared with other preparation methods, this method formed a Mo2N nano-film with a strict stoichiometric ratio of Mo:N = 2:1, no non-stoichiometric or N-rich Mo2N films have been formed during the nitridation process. The Mo2N nano-films exhibited a superconducting effect at 3.8 K.

Electrical and optical properties of electrochromic device with WO3:Mo film prepared by RF magnetron Co-Sputtering

In this work, we investigate the electrical and optical properties of an electrochromic device (ECD) that has molybdenum-doped tungsten trioxide (WO 3 :Mo) electrode. Also, it's surface morphology, coloration efficiency and light transmittance are evaluated. To fabricate the ECD, the WO 3 :Mo film as the working electrode and the Nickel oxide (NiO) film as the counter electrode are prepared by RF magnetron co-sputtering deposition respectively. WO 3 and Mo are set as targets and fluorine-doped tin oxide (FTO) glass, size 2 × 2 cm, is prepared as a substrate. Both targets are sputtered under the same sputtering conditions such as gas pressure, target distance and rotational speed of substrate at 15 mTorr, 5 cm and 5 rpm, respectively. In the co-sputtering process, all conditions were the same for the WO 3 target and Mo target, but only the RF power applied to each target was changed. The RF power applied to the WO 3 target was fixed at 150 W, while the RF power to the Mo target was increased from 50 W to 250 W in 100 W increments giving a difference in dopant concentrations. At the RF power to the Mo target of 50 W, 150 W, and 250 W, respectively, the coloration efficiency of the ECDs was 67.58 cm 2 C −1 , 40.60 cm 2 C −1 , 36.89 cm 2 C −1 . The optimum RF power to the Mo target for the fabrication of an WO 3 :Mo film for use as an electrode in ECD was 50 W at the fixed 150 W RF power to the WO 3 target, and the fabricated films showed an amorphous structure and the highest coloration efficiency. The transmittances of the prepared ECD were 64.63% and 17.17%, respectively, in the bleaching and coloring state. • RF Magnetron co-sputtering was used for fabricate the WO 3 :Mo thin film for made the electrochromic devices. • The concentration ratio was adjusted by varies the RF power applied to Mo target. • The optimal parameter was obtained from RF power was 150 W to WO 3 target and 50 W to Mo target. • The CE of the ECD with WO 3 :Mo electrode were increased to 67.58 cm 2 ·C -1 compare to undoped WO 3 electrode 41.45cm 2 ·C -1 .

Formation of Thin-Film Nanoparticle Materials for Single-Photon Avalanche Detectors

The results of the formation of an original material based on thin films of metal nanoclusters and the investigation of its chemical composition, surface morphology, and structure are presented. Monodisperse thin films comprised of individual nanoclusters of Ta, Mo, and Ni metals were manufactured by gas-phase magnetron sputtering. Images of the surface were obtained by scanning electron microscopy and it was shown that the Ta and Mo films had a well-developed porous structure and consisted of separate nanoclusters contacting each other only at the interface. The chemical composition of the resulting films immediately after deposition (in situ) and after exposure to the atmosphere (ex situ) was investigated by X-ray photoelectron spectroscopy. It was shown that the exposure to the atmosphere led to the oxidation of the obtained films, namely, to the formation of oxide shells in nanoclusters that formed the films. The possibility of creating thin-film photodetectors based on nanocluster films to detect the decay of isomeric nuclear transition in 229Th nuclei is discussed.

On the Generation of Intense Cs K X-Rays upon Electron Excitation of a Mo Target Heat Treated in Cs and O2 Vapors

The possibility of stable generation of intense characteristic Cs radiation upon electronic excitation of a Mo anode target heat treated in Cs and O2 vapors is demonstrated. The radiation source is a microfocus transmission type X-ray tube with a beryllium window and an S20 photocathode. A Cs-Mo-O layer is formed on the initial target in the form of a 1-µm-thick Mo film during heat treatment. The L and K spectral series are measured in an anode—cathode voltage range of 10—47 kV. A stable generation mode is observed at an electron flux power density up to 600 W/cm2. The energy of the Cs Kβ spectral line is in the region of photoabsorption peaks of the I and Xe atoms, which are widely used in the composition of contrast agents for medical diagnostics. This allows obtaining two- and three-dimensional images with the maximum contrast.

Role of disorder in magnetic and conducting properties of U–Mo and U–Mo–H thin films

Crystal structure, magnetic and electrical properties of thin U–Mo films and their hydrides prepared by reactive sputter deposition from metallic targets were studied. The films are metallic but with high values of electrical resistivity. Both bcc U0.79Mo0.21 thin film and the hydride (UH3)0.74Mo0.26 (β-UH3 structure type) have a very strong texture. Superconductivity is identified below Tc = 0.55 K for U0.79Mo0.21. The critical field of 1.0 T is exceeding that of α-U (0.3 T). The hydride film (UH3)0.74Mo0.26 is a ferromagnet with the Curie temperature TC = 165 K, higher concentration of Mo somewhat reduces the TC value. Deposition on a cooled substrate reduces the grain size of the hydride, approaching the limit of amorphous state. The U–Mo films and hydrides exhibit the net negative temperature coefficient of resistivity, TCR = dρ/dT, attributed to the randomness on atomic scale, yielding very strong scattering of electrons and weak localization.

Amorphous IGZO TFTs with Low Electrical Hysteresis By Using Two-Photomask Process

Amorphous oxide semiconductor (AOS)-based TFTs have been considered as potential channel materials for high-resolution TFT-LCDs and AMOLEDs [1-3]. However, the electrical characteristics of AOS TFTs frequently deteriorate with increasing operating cycle times because of the increased trap states present in the AOS semiconductor layer. In this work, we demonstrated the amorphous Indium-Gallium-Zinc-oxide (a-IGZO) TFT fabricated by using two-photomask process [4], which can effectively reduce the process complexity and manufacture cost.ExperimentThe TFT manufacture processes are as followed. A Mo film was deposited by DC sputtering on glass substrate as gate electrode. After patterning, the tri-layer structure of HfO2 /IGZO/ HfO2 film was deposited as gate insulator, channel and passivation layer respectively. The pattern of channel and passivation layer was defined by photolithography with back exposure. Finally, using DC sputtering deposited the MO films and patterned to form the source/drain electrode.Result and DiscussionIn this work, experimental results have shown that the 2-photomask fabricated a-IGZO TFT can exhibit superior performance with a subthreshold swing of 66 mV/dec., threshold voltage of -0.37V, field effect mobility of 10 cm2/V∙S, Ion/Ioff current ratio of ~107 and extremely low electrical hysteresis loop ∆V=10mV was obtained. The reliability of a-IGZO TFTs also was measured and investigated, it reveals the great reliability because the passivation layer can effectively isolate the reaction between the back-channel and atmosphere. It is highly promising to be applied for the emerging transparent display backplane technologies.Reference[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, nature, vol. 432, no. 7016, pp. 488-492, 2004.[2] J. Yao, N. Xu, S. Deng, J. Chen, J. She, H.P. D. Shieh, P.T. Liu, and Y.P. Huang, IEEE Transactions on Electron Devices, vol. 58, no. 4, pp. 1121-1126, 2011.[3] P.Y. Kuo, C.M. Chang, I.H. Liu, and P.T. Liu, Scientific reports, vol. 9, no. 1, pp. 1-7, 2019.[4] Y. Kuo, Journal of The Electrochemical Society, vol. 138, no. 2, pp. 637, 1991. Figure 1

(Invited) Reliability of Ultra Nano-sheet InWO Thin Film Transistor for Back-end of Line Applications

The amorphous indium-tungsten oxide (a-IWO) ultra nano-sheet (UNS) thin-film transistor (TFT) has been developed successfully and exhibited high device performance. It is also suitable for the back end of line (BEOL) technologies due to its superior electrical characteristic and low operation voltage[1, 2]. In this work, we study the reliability of TFT with nano-sheet IWO as channel layer for BEOL applications. The backchannel passivation with an organic layer was proposed to effectively isolate the humidity in the atmosphere. Experimental results have shown that the a-IWO nano-sheet TFT can exhibit the great potential and excellent reliability for BEOL IC applications. Experiment The bottom gate structure was used. A Mo films was deposited by DC sputter as gate electrode. Then, a layer of HfO2 film was deposited through the atomic layer deposition (ALD). Next, the 4nm-thick nano-sheet a-IWO thin film was deposition as the channel layer by RF sputter. The Mo source/drain contacts were deposited by DC sputter. Finally, the organic passivation layer covered on the device with spin coating method. The conditions of thermal annealing process were studied for the optimization of the interface quality between IWO nano-sheet backchannel and organic passivation layer. Result and discussion The amorphous oxide semiconductor (AOS) channel layer is easily affected by the atmosphere from the environments, like oxygen and moisture causing the electrical degradation [3, 4]. Therefore, the passivation effectively avoids the degradation of device features under positive bias thermal stress (PBTS) measurement, as shown in Fig. 1. The passivation layer can effectively improve the TFT device reliability and reduce the positive shift of threshold voltage. Reference [1] P. Y. Kuo, C. M. Chang, and P. T. Liu, pp. 21-22.[2] P. Y. Kuo, C. M. Chang, I. H. Liu, and P. T. Liu, Scientific reports, vol. 9, no. 1, pp. 1-7, 2019.[3] J. Yao, N. Xu, S. Deng, J. Chen, J. She, H. P. D. Shieh, P. T. Liu, and Y. P. Huang, IEEE Transactions on Electron Devices, vol. 58, no. 4, pp. 1121-1126, 2011.[4] P. T. Liu, Y. T. Chou, and L. F. Teng, Applied Physics Letters, vol. 95, no. 23, pp. 233504, 2009. Figure 1

ALD-Based Synthesis of Wafer-Scale Transition Metal Dichalcogenides Towards Nanoelectronics and Optoelectronics Applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS2 and WS2 have been widely studied due to their unique physical and electronic properties even at one atomic layer thickness. Since the first report on the MoS2 field-effect transistor (FET) device [1], great efforts have been made in the past decade to extend the intriguing features of TMDs to practical applications. However, such integration has been severely bottlenecked by the lack of effective approaches to achieve wafer-scale, uniform, crystalline and stoichiometric TMD films.Till now, it is still prevalent to fabricate TMD-based electronic devices using mechanical exfoliation methods. Although single-crystal flakes with least defects can be obtained, the size, thickness and location of the exfoliated TMD flakes are largely uncontrollable, making it unsuitable for large-scale device integration and massive production. So far, various synthetic approaches have been proposed to grow large-area TMD thin films. Chemical vapor deposition (CVD)-based method is one of the major routines to synthesize high-quality monolayer and few-layer TMD flakes [2-4]. For example, in typical CVD processes to grow MoS2, the films can be achieved either by the reduction reaction between the S and MoO3 source powders or by sulfurizing the Mo (or MoOx) films pre-deposited by using physical vapor deposition. However, the nucleation sites in the CVD process are uncontrollable which may lead to the vertical stacking of the MoS2 flakes in triangle shape deteriorating the crystalline properties of the film. On the other hand, the pre-deposited Mo or MoOx film by using e-beam evaporation or sputtering usually suffer from rough surface morphology and unsatisfactory film uniformity which further limit the homogeneity of the large-scale film and device integration.Atomic layer deposition (ALD) is a surface-controlled film fabrication and can provide a possible route towards the synthesis of TMD thin films since the ALD approach follows the layer-by-layer deposition mechanism [5,6]. As compared to the conventional CVD method, ALD can enable precise thickness control on atomic scale and excellent film uniformity as well as good stoichiometry and crystallinity with proper annealing steps. In this work, we employed the ALD-based synthesis method to grow wafer-scale MoS2 and WS2 thin films. Non-toxic MoCl5, WCl5 and hexamethyldisilathiane (HMDST) precursors have been used which are different and superior to those used in previous work. Film characterizations have confirmed the wafer-level uniformity, crystallinity and stoichiometry of the synthesized films. Homogeneous electrical behaviors have also been obtained from the fabricated FET device arrays. In addition, optoelectronic devices such as photodetectors and simple logic gates such as n-type inverter arrays have been demonstrated using the wafer-scale MoS2 FET arrays. Furthermore, we have used the substitutional doping in the ALD process to achieve controllable p-type doping of MoS2 films, which can provide solid basis for the complimentary metal-oxide-semiconductor (CMOS) integrated circuit applications. This work shows that the ALD-based synthesis of the 2D TMD thin films is an attractive approach and has provided a promising platform to further push forward the circuit and system level applications of the TMD materials.

Friction control by engineering the crystallographic orientation of the lubricating few-layer MoS2 films

Tribological properties of a surface are controlled by its topography and chemical structure as suggested by theoretical studies. However, the effect of crystallographic orientation on the tribological properties is still controversial. By tuning the heating rate during sulfurization of CVD Mo films, we prepared the few-layer MoS2 films with similar surface topography but different crystallographic c-axis orientation. This resulted in distinctly different tribological behavior both on the nanoscale and macroscale that can be attributed to different surface chemistry and surface electronic and phonon structures. In particular, horizontally aligned MoS2 sheets with c-axis oriented along the surface normal favor the weak van der Waals forces which resulted in a twice lower coefficient of friction comparing to the vertically aligned MoS2 sheets. These results demonstrate the possibility to optimize tribological properties by crystallographic orientation as required for the thin solid-state lubricating layers for extreme conditions.

Enhanced photo-response of CdTe Thin film via Mo doping prepared using electron beam evaporation technique

CdTe and 1–5 wt% molybdenum-doped CdTe materials were prepared by ball milling method. Then the ball-milled samples were deposited on a glass substrate at room temperature by an electron beam evaporation method. The structural, optical, morphological, electrical, and photo-response properties of CdTe and Mo-doped CdTe (CdTe:Mo) samples were investigated. The prepared samples exhibit a cubic structure of CdTe. X-ray diffraction pattern (XRD) showed that the deposited films belong to the cubic structure with a preferential growth along (111) plane. The peak shift in the lower side of 2θ was observed due to incorporation of Mo into the CdTe matrix. Surface morphology of the prepared CdTe and CdTe:Mo ball-milled samples showed the formation of various agglomerated structures. Scanning electron microscope images depicted the formation of high dense agglomerated grains on the film surface with varying sizes as a function of Mo concentration. The optical bandgap was increased from 1.38 to 1.54 eV with raising the Mo concentration in CdTe films. Enhanced optical absorption coefficients (~ 17 × 104 cm−1) were observed for 3 and 4 wt% Mo doped CdTe thin films. Hall measurement studies revealed that Mo doped CdTe (3 wt%) film possesses a low resistivity of 34 × 103 Ω cm. The CdTe and CdTe:Mo films exhibit p-type conduction. The high photo-response of 0.088 AW−1 is observed for 3 wt% Mo-doped CdTe thin film.

Investigation on the properties of Cu 2 ZnSnSe 4 and Cu 2 ZnSn(S,Se) 4 absorber films prepared by magnetron sputtering technique using Zn and ZnS targets in precursor stacks

Cu2ZnSnSe4 and Cu2ZnSn(S,Se)4 absorber films were grown on soda lime glass and Mo-coated soda lime glass substrates by deposition of the precursor films via RF magnetron sputtering method in the stacking orders of Cu/Sn/Zn/Mo/SLG and Cu/Sn/ZnS/Mo/SLG using metallic Zn or binary sulfide ZnS targets and subsequently carrying out of selenization process. It was aimed to find out the effect of Zn or ZnS target types and the small amount of S originated from ZnS target material used in the deposition of precursor films on the structural, morphological, optical, and electrical characteristics of the films to be selenized. XRD and Raman spectroscopy analysis showed that the kesterite CZTSe structure was predominantly formed in both cases where Zn and ZnS targets were used. According to the EDX analysis, S content in the prepared film using ZnS target was only around 1.79 at%. This indicated that a considerable amount of S in the film was driven out during the selenization process. Scanning electron microscopy analysis revealed that ZnS target material contributed to the achievement of the absorber films with larger grain size. It was also determined that the thickness of the interfacial MoSe2 film between the absorber and Mo films decreased by using ZnS target in the precursor film. This was attributed to ZnS layer with a high melting point acting as a barrier layer over Mo film and retarding the diffusion of Se into the Mo film during the selenization process. In addition, the band gap energy values of the Cu2ZnSnSe4 and Cu2ZnSn(S,Se)4 films were found to be 1.18 and 1.28 eV, respectively.

The Pitting Corrosion Behavior of the Austenitic Stainless Steel 308L-316L Welded Joint

The pitting corrosion resistance of the austenitic stainless steel 308L-316L welded joint was investigated by electrochemical tests. It is found that the weld zone was the most critical for pits to initiate in the welded joint due to relatively instable passive film with few Mo and inhomogeneous passive film induced by multiple (Mn, Al, and Si) oxides and continuous network of 13.94 vol.% δ ferrites. By statistical analysis, 53.8% pits initiated at (Mn, Al, and Si) oxides, 23.0% in austenite, and 23.2% at interface between ferrite and austenite. In addition, heat-affected zone was prone to have pitting corrosion compared with the base metal since residual strain was much higher in the region.

Stress and Microstructure Evolution in Mo Thin Films without or with Cover Layers during Thermal-Cycling.

The intrinsic stress behavior and microstructure evolution of Molybdenum thin films were investigated to evaluate their applicability as a metallization in high temperature microelectronic devices. For this purpose, 100 nm thick Mo films were sputter-deposited without or with an AlN or SiO2 cover layer on thermally oxidized Si substrates. The samples were subjected to thermal cycling up to 900 °C in ultrahigh vacuum; meanwhile, the in-situ stress behavior was monitored by a laser based Multi-beam Optical Sensor (MOS) system. After preannealing at 900 °C for 24 h, the uncovered films showed a high residual stress at room temperature and a plastic behavior at high temperatures, while the covered Mo films showed an almost entirely elastic deformation during the thermal cycling between room temperature and 900 °C with hardly any plastic deformation, and a constant stress value during isothermal annealing without a notable creep. Furthermore, after thermal cycling, the Mo films without as well as with a cover layer showed low electrical resistivity (≤10 μΩ·cm).

Effects of Process Parameters on the Microstructural Characteristics of DC Magnetron Sputtered Molybdenum Films on Graphite Substrate

Graphite, due to its extreme purity and high-temperature stability, is considered as the material of choice for nuclear fusion reactors. However, it undergoes rapid oxidation and erosion at extreme environmental conditions like those experienced by the plasma-facing components (PFCs) of a fusion reactor. On the other hand, molybdenum (Mo) does offer a combination of thermal, mechanical, and chemical properties that make it an aspirant candidate for PFCs and other applications demanding substantial physical, chemical, and thermal stability as well as significant thermal conductivity. Mo films on graphite is one of the attractive options to sustain against such harsh environmental conditions. Here, in this work, we develop Mo films over graphite substrate through sputter deposition and analyze the effects of process parameters like the sputter power and the substrate temperature on the morphology and film quality. It is shown that an increase in sputter power, as well as substrate temperature, induces significant changes in the topography of the films. For low-temperature depositions, the grain morphology does not change with increasing sputter power, however, for relatively higher substrate temperatures, significant changes in morphology are observed with increasing sputter power. Similarly, dependences of crystallite size, film roughness, and electrical conductivity are discussed with changing sputter power and substrate temperature. Our results suggest that the kinetic energy of the sputtered Mo particles plays a crucial role in determining the final film quality. This study manifests that optimum sputter parameters guarantee quality Mo films over graphite substrate.

Fabrication and Characterization of Molybdenum Thin-Film Resistor for Superconducting Quantum Devices

As part of the efforts to establish a reliable fabrication process of superconducting integrated circuits at Center for Excellence in Superconducting Electronics (CENSE) in China, we investigated the influences of various deposition conditions on the properties of molybdenum (Mo) thin films. Thin-film resistors were fabricated based on these films by using a lift-off process. It was found that Mo structures made from films peeled off easily from the substrate after lift-off. We have optimized the deposition conditions to grow Mo films with compressive or weakly tensile stress. Mo thin-film resistors with a sheet resistance of several ohm/square and surface roughness of less than 1 nm were successfully fabricated and were used in the fabrication of superconducting quantum interference devices and small-scale superconducting integrated circuits.

Enhancing the photocatalytic efficiency of ZnO thin films by the addition of Mo and rGO

In the present study, ZnO:Mo/rGO thin films were deposited on stainless steel mesh substrates using nebulizer spray method. The photocatalytic efficacy of the ZnO:Mo/rGO film were studied under solar light against two dyes viz. Methylene blue (MB) and Rhodamine B (RhB) and the results were compared with undoped ZnO films and Mo doped ZnO films. The band gap values are found to be 2.71 eV, 2.92 eV and 3.23 eV for ZnO:Mo/rGO, ZnO:Mo and ZnO films, respectively. The ZnO:Mo/rGO film as a photocatalyst is more responsive to visible light than the other two sets of samples. The PL, SEM, XRD and FTIR studies support the discussion on the mechanism behind this improvement.

Controlled wrinkling patterns and surface friction by thickness gradient

Gradient changing patterns often endow the surfaces with intriguing functionalities. Spontaneous wrinkling of film-substrate bilayer opens up a new possibility to create heterogeneous surfaces. In this work, we prepared the gradient changing wrinkle surfaces of Ag films and Mo films with identical thickness gradient on polydimethylsiloxane substrates by magnetron sputtering. It is revealed that Ag films show only labyrinth patterns, regardless of thickness gradient, while Mo films exhibit the pattern transition from labyrinth to herringbone along the thickness gradient direction. This can be attributed to the combined effect of the compressive stress, the thickness gradient and material property. The wavelength of Ag films and Mo films increases with increasing distance. Ag films show isotropic friction, which increases with increasing distance due to the increased surface roughness. In contrast, the friction properties of Mo films exhibit two-regime behavior, namely, isotropic friction and anisotropic friction, depending on the wrinkling patterns. The isotropic/anisotropic friction is closely related to the orientation-dependent topography. These results provide better understanding of the combined effect of thickness gradient and film constituent on the gradient changing wrinkle patterns and pave a new way for designing gradient surfaces with controlled friction behavior by harnessing wrinkling instability.