A technique was developed that removes Ni/Au contact metal films from their substrate to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN nanowire devices.
Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO2 interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO2 surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO2 substrate.
Single-NW devices are made by dispersing a NW suspension onto an insulating substrate and forming contact pads on the substrate via conventional photolithography and metal deposition, which results in randomly formed two-terminal devices. A thick SiO2 film on a Si wafer is typically used as an insulating substrate1,2. For metals deposited on a SiO2 surface, a common problem resulting from heat treatment is the occurrence of void formation at the metal/SiO2 interface. In addition to cracking and delamination of the metal film, this void formation can negatively affect device performance from an increase in resistance caused by a reduction of the contact area. Ni/Au contacts oxidized in N2/O2 atmospheres are the predominant contact scheme applied to p-GaN3-7. During heat treatment in a N2/O2, the Ni diffuses to the surface to form NiO and the Au diffuses down to the substrate surface.
In this work, excessive void formation at the contact/NW and contact/SiO2 interfaces was shown to occur during annealing of Ni/Au contacts to NWs on SiO28. The surface morphology of the annealed Ni/Au film, however, does not indicate the existence of voids or the degree to which void formation has occurred. To address this problem, we developed a technique for the removal of Ni/Au contacts and GaN NWs from SiO2/Si substrates in order to analyze the interface of the contact with the substrate and NWs. This technique can be used for the removal of any contact structure that has poor adhesion to the substrate. The Ni/Au films with GaN NWs embedded in them are removed from the SiO2 substrate with carbon tape. The carbon tape is adhered to a standard pin mount for characterization by use of scanning electron microscopy (SEM) along with several other tools. The detailed procedure for the fabrication of single GaN NW devices and analysis of their contact interface morphology are described.
The GaN NWs used in these experiments were grown by catalyst-free molecular beam epitaxy (MBE) on Si(111) substrates9. The general procedure for preparing the NW suspension from the substrate with the as-grown NWs is illustrated in Figure 1.
1. Nanowire Suspension Preparation
2. Substrate Preparation
The substrates used are heavily-doped (ρ ~0.001-0.005 Ω-cm) 3-inch Si wafers with 200 nm of thermally-grown SiO2 on both sides.
3. Nanowire Dispersal
4. Photolithography of Contact Pattern
Use standard photolithographic techniques to create the contact pattern in a clean room with ambient conditions of ~20 °C and ~40% relative humidity. Mask aligner intensity (step 4.6), exposure time (step 4.8) and develop time (step 4.9) will be equipment-dependent and should be adjusted to produce maximum pattern definition with a lift-off-resist (LOR) undercut of about 0.5 μm.
5. Sample Pretreatment Prior to Metal Deposition
Prior to loading the samples into the electron-beam evaporator for metal deposition, give the patterned wafer a UV ozone treatment and a HCl:H2O bath.
6. Electron-beam Evaporation of Contact Metals
7. Contact Metal Lift-off
8. Contact Anneal
Test devices before the contact anneal in order to compare these with annealed devices. Perform the contact anneal of the Ni/Au films using a rapid thermal annealer (RTA) with ultra-high-purity N2/O2 (3:1) as the process gas.
9. Ni/Au Film Removal
Since removal of the Ni/Au film is a destructive process, devices are typically imaged and tested before this step. The procedure for the Ni/Au film removal is illustrated in Figure 3.
An example of SEM analysis on annealed Ni/Au films removed from the SiO2 substrate using carbon tape is shown in Figure 4. The surface of a Ni/Au contact prior to removal is shown in Figure 4A. The underside of the same area of that particular Ni/Au film after removal is shown in Figure 4B. Comparison of the surface and underside morphology can help determine if there is a relationship between the two. For example, when the two images are compared, it can be seen that the dark spots in (a) coincide with the dark features in (b). At higher magnifications, critical features of the Ni/Au underside morphology can be discerned. Along with the use of energy dispersive spectroscopy (EDS), in order to determine the composition of the different features of the underside morphology, the general structure of the Ni/Au film on SiO2 after annealing can be ascertained. A removed Ni/Au film that was properly prepared is shown at a lower magnification in Figure 4C. The void formation is uniform across the film and no cracking or breaking of the film has occurred. Figure 4D is an example of a removed Ni/Au film that was poorly prepared. This sample had received no cleaning pretreatment prior to the metal deposition, and the residual contamination produced nonuniform void distribution and large macrovoids that resemble blisters. Upon removal of the film, the tape had come off some from the mount and wrinkled, causing the film to break apart.
One important application of this technique is analyzing the contact/NW interface morphology. Figure 5 shows SEM images of the underside of annealed Ni/Au films that had been deposited onto NWs dispersed onto SiO2/Si substrates. The NWs, which are embedded in the Ni/Au films, also come off with films upon removal with the carbon tape. At larger magnifications, like the image shown in Figure 5A, the distribution of voids relative to the NWs can be observed. At higher magnifications, such as the images in Figures 5B and C, the contact/NW microstructure can be more thoroughly studied. It is not uncommon for NWs to become dislodged from the Ni/Au film upon peeling it off of the substrate, as shown in Figures 5A and C. This allows for the examination of the contact/NW interface that would otherwise be obscured if the NW had remained in place.
A more quantitative analysis can be performed through the use of imaging software. The example shown in Figure 6 is based on the correlation of residual contamination from processing with void formation at the interface of the Ni/Au with the SiO210. The presence of this residual contamination can cause a significant increase in the number of voids observed at the contact/substrate interface. By quantifying the degree of void formation at the contact/substrate interface, the effectiveness of different cleaning methods can be evaluated. These experiments focused on the effectiveness of the various cleaning methods prior to deposition of the Ni/Au onto the SiO2 for the removal of the residual contamination. The area of the voided regions was determined using imaging software. Using SEM images, multiple 100 μm2 areas were analyzed for each sample and the average void area (as a percentage of the total area) for each of the different preparation and cleaning methods was determined. The data is plotted in Figure 6G with the standard deviation of the data set represented by error bars.
Figure 1. General procedure for NW suspension preparation and dispersal. Click here to view larger image.
Figure 2. Procedure for dispersal of NW suspension. Click here to view larger image.
Figure 3. Procedure for removal of annealed Ni/Au film from SiO2/Si substrate. (a) Step 9.6, Sample is gently laid onto the mount with carbon tape. (b) Step 9.7, Force is applied to the back of sample. (c) Step 9.8, Removal of substrate from carbon tape using a razor blade. (d) Step 9.8, Annealed Ni/Au film remains adhered to the tape. Click here to view larger image.
Figure 4. (a) SEM image of annealed Ni/Au contact. (b) SEM image of same Ni/Au film shown in (a) after being removed with carbon tape to reveal its underside. (c) Removed Ni/Au film that was properly prepared. (d) Removed Ni/Au film that was poorly prepared. (Images (a) and (b) taken from reference8). Click here to view larger image.
Figure 5. SEM images of the underside of annealed Ni/Au films that were deposited onto NWs that had been dispersed onto SiO2/Si substrates and then removed with carbon tape. (a) Area showing a NW within the annealed Ni/Au film alongside where a NW had been before dislodging. (b) Close-up view of a NW within the annealed Ni/Au film. (c) Close-up of where a NW had been before dislodging. (Images (b) and (c) taken from reference8). Click here to view larger image.
Figure 6. SEM images of the underside of annealed Ni/Au films deposited on SiO2 surfaces that had received varying surface treatments. (a)-(c) Samples where the SiO2 surface received no photolithographic processing prior to predeposition cleaning. (d)-(f) Samples where the SiO2 surface received photolithographic processing prior to predeposition cleaning. (g) Values of the areas of the voided regions plotted for each sample shown in (a)-(f). (Images and plot taken from reference10). Click here to view larger image.
Figure 7. (a) SEM image of an individual NW device. (b) Complete contact pattern. (c) Close-up view of contact pattern. Click here to view larger image.
The technique presented allows for analysis of the contact/substrate and contact/NW microstructure of single NW devices. The main advantages of this technique are its low cost and simplicity. It allows for qualitative and quantitative analysis of the contact interface on a large scale with the substrate as well as on a submicrometer scale with individual NWs. The use of carbon tape for the film removal and SEM pin stubs for sample mounting make it possible for analysis using characterization techniques that require clean low-pressure environments. In addition to using SEM for imaging the interface morphology, numerous other characterization techniques can be used, including EDS, x-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES) and atomic force microscopy (AFM).
One possible modification to the NW dispersal procedure would be to etch a pattern into the substrate prior to dispersal that shows the specific location of where each contact pattern will be in order for more exact placement of the NW suspension. This will require extra processing steps that increase the complexity of the process as well as the chance for residual contamination. Another issue for the NW dispersal process is the amount of NW suspension that is to be dispersed. The specific amount is dependent on the size of the contact pattern. For the 1 cm2 sized contact pattern that was used in our experiments, a 30 μl sized drop was sufficient to cover the contact pattern area. It is recommended that not more than two applications of NW suspension be applied in a given area in order to avoid excess build-up from the solvent. If a more dense NW population is desired on the substrate, a smaller amount of solvent should be added to the NW suspension during preparation.
The parameters of the photolithography process are dependent on the specific photoresists and developer used, and therefore the contact pattern photolithography may require modification in order to achieve optimal pattern definition. The exact exposure and developing conditions will also vary as a function of the ambient conditions in the clean room. For application of the photoresist, it is recommended that the sample size not be smaller than a quarter wafer and that the contact patterns not be placed along the edge of the sample due to the edge effects on the photoresist. The contact pattern used in these experiments was for producing 2-terminal NW devices, an example of which is shown in Figure 7A. The contact pattern (1 cm2) consisted of 4 arrays of 48 sets of 2 pads (250 μm x 500 μm) separated by a specific gap size, as shown in Figures 7B and C. Each array has a different gap size; array I is 3 μm, II is 4 μm, III is 5 μm, and IV is 6 μm. As shown in the Representative Results, Ni/Au film removal can also be accomplished without photoresist patterning of the Ni/Au film.
The simplicity of the Ni/Au film removal process makes it fairly straightforward but some steps may take some practice. In particular, steps 7 and 8 should be rehearsed using dummy samples in order to determine the correct amount of force to apply for the film to adhere to the tape without breaking the sample. In order to determine the void area using imaging software, the SEM images of the removed films must be of large uniform areas that are not excessively broken up like that shown in Figure 4C.
The authors have nothing to disclose.
The Authors would like to acknowledge the individuals in the Quantum Electronics and Photonics Division of the National Institute of Standards and Technology in Boulder, CO for their assistance.
REAGENTS and MATERIALS | |||
Lift-off resist | MicroChem | LOR 5A | Varies according to application |
Photoresist | Shipley | 1813 | Varies according to application |
Developer | Rohm and Haas Electronic Materials | MF CD-26 | Varies according to application |
Photoresist stripper | MicroChem | Nano Remover PG | Varies according to application |
Ni source | International Advanced Materials | 99.999% purity | |
Au source | International Advanced Materials | 99.999% purity | |
SiO2/Si wafers | Silicon Valley Microelectronics | 3-inch <100> N/As 0.001-0.005 Ohm-cm, 200 nm thermal oxide | |
Carbon tape | SPI Supplies | 5072, 8 mm wide | |
Solvents are standard semiconductor or research grade. Vendor is not important for the experimental outcome. | |||
Reactive ion etch gases and thermal annealing gases are high purity grade. Vendor is not important for the experimental outcome. | |||
EQUIPMENT | |||
Ultrasonic cleaner | Cole-Palmer | EW-08849-00 | Low power |
Micropipette | Rainin | PR-200 | Metered, disposal tips |
Reactive ion etcher | SemiGroup | RIE 1000 TP | Other vendors also used with different process parameters |
Mask aligner | Karl Suss | MJB3 | Other vendors also used with different process parameters |
UV ozone cleaner | Jelight | Model 42 | Other vendors also used with different process parameters |
E-beam evaporator | CVC | SC-6000 | Other vendors also used with different process parameters |
* Manufacturers and product names are given solely for completeness. These specific citations neither constitute an endorsement of the product by NIST nor imply that similar products from other companies would be less suitable. |