使用InGaN微LED的亚微光光刻技术

Abstract—The fabrication of gallium-nitride based (GaN) light  emitting diode (LED) arrays by a direct writing technique, itself  using micron-sized LEDs (micro-LEDs), is reported. CMOSdriven ultraviolet GaN-based micro-LED arrays were used to  pattern photoresist layers with feature sizes as small as 500nm.  Chequerboard-type square LED array devices were then  fabricated using such photoresist patterns based on either single  pixel or multi-pixel direct writing, and implemented as part of a  completely mask-less process flow. These exemplar arrays  comprised either 450nm-emitting 199x199µm2 pixels on a 200µm  pitch or 520nm-emitting 21x18µm2 pixels on a 23µm pitch. Fillfactors of 99% and 71.5% were achieved with optical output  power densities per pixel of 5W/cm2 and 20W/cm2 at 90mA and  6mA dc injected currents, respectively.

We report here on the use of 8x8 micro-LED arrays with  pixel sizes as small as 14µm-diameter in an imaging optical  setup (similar to the setup in ref.but with a 10:1  demagnification) to pattern PR with features down to 500nmwidth in order to process GaN-based LED heterostructures. First, a single pixel was used to pattern a  sacrificial PR mask for a “chequerboard-type” array with  individually addressable LEDs emitting at 450nm.The  chequerboard device was designed with a targeted 1µm gap  between adjacent pixels, resulting in a 99% filling factor  arrays in a suitable configuration for subsequent flip-chip  bonding to custom complementary metal-oxide-semiconductor  (CMOS). Typical measured optical output powers here  were 2mW (5W/cm2 ) per pixel at a driving current of 90mA.  Parallel direct writing is then implemented using 2 micropixels each of 24µm diameter to create a 32x32 chequerboard  design emitting at 520nm. This had 21 x 18µm2 pixels on a  23µm pitch, each delivering 77µW (20W/cm2 ) at 6mA  injection current. These arrays were fabricated in a suitable  configuration for applications including lab-on-chip sensors,  micro-displays, time-resolved spectroscopy and optoelectronic  tweezing, and provide demanding demonstrations of the  capabilities of micro-LED direct writing in the rapid and  accurate fabrication of active optoelectronic devices.

The parallel writing capability of our setup was then  demonstrated with 2 adjacent 24µm-diameter pixels (namely  pixels 7 and 8 from Fig. 2(a)) driven at the same forward bias  voltage, 7.5V (25.5mA typical dc injected current). The pixels  were both placed within the field of view of the collection  microscope objective so that their emission is collected  altogether. In this configuration and driven individually, each  pixel delivers a different optical power at focus of 0.163µW  (3.6W/cm2 ) and 0.17µW (3.75W/cm2 ), respectively, due to  variation in pixel-to-pixel performances as shown in Fig. 2(a).  However, due to the characteristics of the CMOS design, the  current flowing through the chip when several LEDs are driven  simultaneously induces a voltage drop at the LED electrodes  due to the ground bounce effect.Consequently, when driven simultaneously, the optical power projected is not the sum of  the individual optical powers, instead a total optical power at  focus of 0.131µW (2.9W/cm2 ) is estimated per driven pixel.  The chequerboard-design was again chosen to illustrate the  lithographic capability in this case for high density micro-LED  arrays emitting at 520nm. An array of 32 x 32 elements with a  23-µm pitch was thus coded for a 2 pixel writing experiment.  In the vertical direction (direction according to inset of Fig. 4.)  each trench is exposed using a single pixel while, in the  horizontal direction, both pixels are used consecutively. The  writing speed was fixed to 100µm/s in vertical giving a typical  exposure dose of 69mJ/cm2 , and 200µm/s in the horizontal  direction to take into account the overlapping exposure path  from the two pixels. After PR patterning, the same etching  process was applied as described previously resulting in an  array of individually-addressable rectangular pixels. In this  case the resulting fill-factor is reduced to 71.5% with  21x18µm2 pixels as shown in the optical micrograph inset of  Fig. 4. Probes were used here to electrically and optically  characterize the device. Fig. 4 summarizes the typical I-V and  L-I curves for a pixel of the array. The pixel delivers an optical  power of 77µW at an injection current of 6mA, giving an  optical power density of 20W/cm2 with current density of  1.6kA/cm2 .

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A custom-made 370-nm emitting micro-LED array bonded  on CMOS backplane was used as a maskless photolithography  tool to fabricate complex and tailored optoelectronic devices  through the use of an optical demagnifying setup and computer  control. This direct writing technique was first used to pattern  PR layers giving features as narrow as 500nm at 140µm/s  writing velocities. In combination with wet and dry etching  and inkjet printing, custom designed 450nm emitting, 99% fill factor, 8x8 broad-LED array and 520nm-emitting 32x32  micro-LED array were fabricated by single and two-pixels  parallel writing respectively. Typical optical output powers emitted by a single element were measured to be up to 5W/cm2 and 20mW/cm2 , respectively, at injected current of 90mA and  6mA. These results demonstrate the advantages of micro-LED  maskless lithography, including re-configurable written  patterns with sub-micron feature sizes and multi-beam parallel  writing and implementation as part of a mask-less process  flow, to address the easy fabrication of complex optoelectronic  and photonic devices such as, but not limited to, GaN-based  LED arrays.