Laser slicing: a thin film lift-off method for GaN-on-GaN technology

A femtosecond laser focused inside bulk GaN was used to slice a thin GaN film with an epitaxial device structure from a bulk GaN substrate. The demonstrated laser slicing lift-off process did not require any special release layers in the epitaxial structure. GaN film with a thickness of 5 $\mu$m and an InGaN LED epitaxial device structure was lifted off a GaN substrate and transferred onto a copper substrate. The electroluminescence of the LED chip after the laser slicing lift-off was demonstrated.

Bulk GaN substrate is a crucial element in the production of reliable, high-current density GaN-based devices. However, the widespread use of GaN substrates is currently limited due to their high cost. Separation of the epitaxial device structure from the bulk GaN substrate allows the reuse of the expensive substrate multiple times and thus reduces the effective cost. Also, the separation of the epitaxial structure is necessary to create heterogeneously integrated devices [1][2][3], as well as to improve heat dissipation [2,4] and light extraction [5].
The lift-off methods based on the use of an intermediate layer require a complicated epitaxy process. The ion implantation method does not allow separating epitaxial device structures due to a large number of point defects formed by the implantation process [38,39,41]. The freecarrier-absorption LLO method is limited to separating undoped films from a heavily doped substrate [40].
We have proposed a laser slicing lift-off (LSLO) method for separating an epitaxial structure from a bulk GaN substrate, one that does not require any special release layers and can be used both for lifting off pure GaN * voronenkov@mail.ioffe.ru † y.shreter@mail.ioffe.ru films and for lifting off GaN films with device structure [42]. The method is based on the effect of GaN decomposition induced by an ultra-short laser pulse focused inside a bulk GaN material. The use of femtosecond lasers is preferable as it allows to reduce the heat-affected zone and the corresponding damage in the surrounding material [43]. This effect was earlier used to create hollow 3D-channels inside bulk GaN [44] and to dice transparent wafers with laser stealth dicing technology [45]. The principle of the LSLO method is as follows. To liftoff a thin GaN film with an InGaN multiple quantum well (MQW) device structure from a bulk GaN substrate, the pulses of a near-infrared femtosecond laser are focused inside the GaN layer, several microns under the surface ( fig.1). The laser photon energy is less than the InGaN bandgap, so both the InGaN device structure and the GaN substrate are transparent at the wavelength of the laser. The laser pulse energy is adjusted so that the nonlinear breakdown threshold is exceeded only in the focus area and the GaN material in this area is decomposed. Scanning of the focus position is performed in the X-Yplane until a continuous layer of decomposed material is created inside the wafer.
In this work, a proof-of-concept LED chip fabrication using the LSLO method is described. First, a 5-µm thick GaN film with an InGaN LED structure was grown homoepitaxially on a bulk GaN layer. After that, the film with the LED structure was lifted-off from the substrate and transferred to a copper carrier ( fig. 2). The electroluminescence of the LED chip after the lift-off was demonstrated. A detailed study of the LED chip for possible damage caused by the LSLO process will be published separately.
A GaN-on-sapphire HVPE template was employed as a bulk-like substrate for the InGaN LED structure epitaxy. The template was grown on a 2-inch c-plane (0001) sapphire substrate using a two-stage growth process [46]. The thickness of the bulk-like HVPE GaN layer was 50 µm. The surface of the HVPE GaN template was epipolished by a chemical-mechanical polishing process. After that, the InGaN MQW LED structure was deposited using a metal-organic chemical vapor deposition system. The LED structure consisted of a 5-µm thick n + doped GaN:Si (n=4·10 18 cm −3 ) layer; five pairs of InGaN quan- The chip isolation and the LSLO process were performed with an ytterbium-doped, diode-pumped solidstate laser with a wavelength of 1030 nm, a pulse duration of 350 fs, and a pulse energy of 400 µJ. A three-axis motorized translation stage was used to perform scanning in the X-Y -plane and to adjust the focus position along the Z axis.
First, a 500µm×500µm chip was isolated through the laser scribing process -grooves with a depth of 6 µm and a width of 5 µm were cut along the chip perimeter. Such grooves are necessary to release the gas produced by the GaN decomposition during the LSLO process. A 20× dry objective was used to focus the laser beam on the surface of the GaN layer. The pulse energy was 5 µJ, pulse repetition rate was 1 kHz and the scanning velocity was 1000 µm/s.
To perform the LSLO an oil immersion objective (100×, NA = 1.25) with an n=1.518 immersion oil was used. Before performing the LSLO, the breakdown threshold was determined: the laser was focused at a depth of 6 µm below the surface and the laser pulse energy was gradually increased until a breakdown in the focus area was observed at a pulse energy of 0.2 µJ. After that, the LSLO was performed using a pulse energy of 0.24 µJ. The laser beam, directed from the front side of the sample with a beam focus located 6 µm under the surface was used to create a thin decomposed layer 6 µm below the surface of the wafer. The scanning pattern exhibited a set of parallel rows. The scanning velocity along the row was 1000 µm/s and the pulse repetition rate was 2.5 kHz resulting in a pulse-to-pulse distance along the row of 0.4 µm; the distance between the rows was 2 µm.
After the LSLO process, the LED chip was bonded to a temporary carrier wafer using a cyanoacrylate adhesive. After the adhesive hardened, the chip was debonded from the substrate. Then, the cyanoacrylate adhesive was dissolved, and the chip was bonded to a copper substrate using an electrically conductive silver epoxy as a bonding material.
To observe the electroluminescence of the chip, an electrical current probe was connected directly to the surface of the chip, without applying any current-spreading layers. The light emitting image of the LED chip on a copper substrate is shown in fig. 3(a). The operating current was 1 mA. Blue-light emission was observed under the probe, confirming that the MQW LED structure was not destroyed during the LSLO process. A detailed study of the I-V characteristics and the electroluminescence spectrum, as well as a detailed comparison of the LED performance before and after the LSLO process will be published separately.
The SEM image of the top side (Ga-face) of the LED chip after the LSLO process is shown in fig. 3(b). No signs of damage caused by the LSLO were observed on the surface. The SEM image of the bottom side (nitrogen-face) of the LED chip after LSLO and debonding is shown in fig. 3(c). The remains of the decomposed layer were observed. The surface roughness of the bottom side of the chip was estimated using a tilted-view SEM image (not shown) to be about 1 µm, which corresponds to the focus depth of the laser beam. The thickness of the separated film was measured using a side-view SEM image (not shown) and was 5 µm, which corresponds to the position of the focus below the surface.
Following is a brief discussion of the technological limits and possible improvements of the method. The maximum thickness of the lifted-off layer is limited by the defocusing of the laser beam caused by spherical aberration and optical inhomogeneity of the material. Recent advances in the growth of pure GaN [47] make it possible to obtain an optically homogeneous material, and the adaptive optics techniques can compensate for spherical aberration when focusing to a depth of hundreds of microns [48]. The minimum thickness of the decomposed layer is limited by the depth of focus of the laser beam, which can be reduced by the use of high-numericalaperture objectives, such as solid immersion lenses [49]. Processing throughput is mainly limited by the scanning speed, therefore the use of parallel multi-beam machining methods [50,51] is highly desirable. Wafers with already deposited metallization layers can also be processed. For this, the focused laser beam is directed from the underside of the wafer. Other materials that decompose irreversibly under the action of laser breakdown can also be processed. The greatest practical interest of such materials is diamond, which is graphitized by laser breakdown [52].
In conclusion, a 5-µm thick GaN film with the InGaN LED MQW structure was lifted off from the bulk GaN layer by focusing near-infrared femtosecond laser pulses 6 µm under the surface of the GaN wafer. The film was transferred to a copper carrier and the electroluminescence of the LED structure was demonstrated.