Photoresist with Ultrasonic Atomization Allows for High-Aspect-Ratio Photolithography under Atmospheric Conditions

Transcription

1 Photoresist with Ultrasonic Atomization Allows for High-Aspect-Ratio Photolithography under Atmospheric Conditions 1

2 CONTRIBUTING AUTHORS Robb Engle, Vice President of Engineering, Sono-Tek Corporation Shane Ketcham, Applications Engineer, Sono-Tek Corporation Mike Delia, Engineering Intern, Sono-Tek Corporation, Senior at Worcester Polytechnic Inst. 2

3 HISTORY OF ULTRASONIC NOZZLES Dr. Harvey L. Berger, Ph.D. Inventor of the ultrasonic nozzle Founded Sono-Tek Corporation in 1975 The first ultrasonic nozzles were developed for use in oil burners and subsequently for the development of liquid fuel burners contracted by the U.S. military for use in portable power generation equipment. 3

4 ULTRASONIC ATOMIZATION When liquid is added to a resonating nozzle, waves are formed on the atomizing surface. λ L =((8*π*θ)/(ρ*f 2 )) 1/3 π = pi θ = Surface Tension ρ =Density F = Frequency of Nozzle (Rayleigh, Theory of Sound,1898) Increasing the power causes the wave peaks to get so high that droplets fall off the tips of the wave. These droplets have a mathematically definable size: D N, 0.5 =.34*λ L (Rayleigh, Theory of Sound,1898) Notice also that less of the atomizing surface is used (for a given flow rate). 4

5 PHOTORESIST SPRAY APPLICATIONS Increases in smaller micro-electro-mechanical systems (MEMS) on cell phones, tablets, autos, robots and even the potential Internet of Things requires the ability to uniformly coat high-aspect ratio devices with photoresist. High-aspect ratio devices have depth to width ratios that are 1:1 or greater at dimensions of 100 microns and smaller (i.e. features that are 400 microns deep and 100 microns wide). Examples include MEMS accelerometers on modern cell phones and emerging medical technologies. Figure 1: Complex MEMS Figure 2: Complex MEMS Sample (Electronic Specifier, 2013) (Shanghai Senodia Semi, n.d.) 5

6 PHOTORESIST PROCESS The photoresist etching process of MEMS wafers involves: Deposition of a photoresist onto a wafer Exposure of the resist Developing the resist Chemically etching the wafer, and Stripping the remaining resist away. The photoresist serves the function of a barrier for the chemical etching process by allowing its partial removal depending on the area exposed to UV light. The exposed area can be for removal or to stay in place depending on if the resist is positive or negative. Once developed, a chemical etching process can be performed on the substrate surface and the entire process repeated until the desired features have been etched to the desired depth. Thickness of the photoresist in this process is an important part as it affects the resolution and quality when developed. 6

7 TRADITIONAL PHOTORESIST SPIN COATING Spin coating became the preferred method of applying photoresist during the 1980 s because of its key advantages at the time: Deposition occurred in a very short amount of time (typically less than a minute) A uniform layer of photoresist (+/- 10% uniformity) was deposited A thickness of around 10 microns was desirable to industry standards. Spin coating challenges when aspect ratios are higher than 1:1 (depth to width): Feature size on the silicon has continued to shrink to keep up with Moore s law. As a result, deep wells suffer one of two process failures: 1. Resist flies over the well, leaving a void, or 2. The well is filled to an undesirable level but leaves thin coatings on vertical walls. 7

8 ULTRASONIC PHOTORESIST SPRAY COATING It has been demonstrated that the advantages of ultrasonic atomization can effectively be used to spray coat difficult features under atmospheric conditions with high yields and stable performance. It is possible to process more challenging features without the need for next-generationlithography or expensive ALD because: Droplets created by ultrasonic atomization are of a fundamentally uniform size. The process engineer has the ability to control droplet wetness through the nozzle distance variable. Ultrasonic spray allows independent control of both gas flow that shapes the plume and flow rate of photoresist. 8

9 DROPLET SIZE Successful spray coating of photoresist is based upon the following key principles: Drop size is governed by the frequency at which the ultrasonic nozzle vibrates, and by the surface tension and density of the liquid being atomized. The higher the frequency, the smaller the median drop size. The spray must be consistent over time and therefore the spraying device must not clog It is imperative that the spray droplet size be as uniform as possible Where feature size gets smaller and smaller (below 50 microns in width), smaller droplet sizes become crucial. 9

10 DROPLET UNIFORMITY (DROP DISTRIBUTION) Ultrasonic atomizers provide two key advantages to the process objectives. The size of the droplets are fundamentally uniform in accordance with the formula and data below and the nozzles are self-cleaning and therefore will not clog during the processing. Tight drop distribution plays an important role in the ability to spray very thin layers uniformly. Typical ultrasonic droplet size distribution versus typical non-ultrasonic nozzle (Sono-Tek Corp) 10

11 PROCESS CONTROLS ENABLED Height versus final droplet size and independent control of plume shaping pressure contributes to dryness, wettability and flow on the surface. Because ultrasonic atomization creates the droplets without the use of pressure or gas, the droplets have near zero kinetic energy. The spray plume is then shaped with a low velocity gas. Having control of this gas as a process engineer provides a method to control the dryness versus wetness of the coating created. Height plays a key role. The droplets will quickly start to evaporate in flight while traveling to the substrate. In controlled environments, this leads to repeatable shrinking of the droplet size and allows fine tuning of the rheology on the substrate surface. 11

12 ULTRASONIC SPRAY OF PHOTORESIST Ultrasonic spray systems are suitable for a number of different substrates and spray applications for photoresist. Common Substrates Silicon Wafers Glass Metal Ceramic Common Applications MEMS Microelectronics Flat panel displays Lenses Filters Microfluidic devices 12

13 ULTRASONIC SPRAY OF PHOTORESIST Ultrasonic spray systems are used to coat deep well* trenches as well as flat substrates. * Deep well topographies are defined as aspect ratios >3:1 where width is <100 microns. 1. Varying Topographies 2. Flat Substrates 13

14 ULTRASONIC SPRAY OF PHOTORESIST: Varying Topographies Traditional spraying and spin coating may produce non-conformal coatings. Ultrasonic spray deposits uniform coatings on all surfaces with low velocity, atomized spray. The ultrasonic nozzle traverses across the substrate instead of using a spin method. 14

15 ULTRASONIC SPRAY OF PHOTORESIST: Varying Topographies ULTRASONIC NOZZLE TIP High frequency 120khz nozzle is typically used due to its ability to produce very small droplets 13 MICRON DROPLETS (ALCOHOL) OFF THE TIP OF THE NOZZLE BECOME SMALLER WITH EVAPORATION AND LOW VELOCITY AIR ASSISTED SPRAY 15

16 ULTRASONIC SPRAY OF PHOTORESIST: Varying Topographies ULTRASONIC NOZZLE TIP High frequency 120khz nozzle is typically used due to its ability to produce very small droplets 13 MICRON DROPLETS (ALCOHOL) OFF THE TIP OF THE NOZZLE BECOME SMALLER WITH EVAPORATION AND LOW VELOCITY AIR ASSISTED SPRAY UNIFORM THIN FILM ON SIDE WALLS AND BOTTOM OF TRENCHES 16

17 ULTRASONIC SPRAY OF PHOTORESIST: Varying Topographies The vortexing ultrasonic nozzle is used for 3D topography with trenches and V-grooves. Typical coating thickness 2-8 microns 17

18 ULTRASONIC SPRAY OF PHOTORESIST: Varying Topographies The focusing ultrasonic nozzle is used for the most challenging high aspect ratio topographies and TSVs*. Typical coating thickness 2-8 microns * TSVs (Through-Silicon Vias) are defined as vertical electrical connections passing through a silicon wafer, enabling 3D chip manufacturing. Chip package size and power consumption is reduced while bandwidth is increased. Uniformity on aspect Vias as high as 5:1 can be achieved 18

19 ULTRASONIC SPRAY OF PHOTORESIST: Flat Substrates Traditional coating methods may be limited by size, create significant waste, or have poor repeatability. Ultrasonic spray deposits uniform coatings on all surfaces with low velocity, atomized spray. The ultrasonic nozzle traverses across the substrate instead of using a spin method. Ultrasonic spray uses less material to achieve same coating thickness as spin coating or other deposition methods. 19

20 ULTRASONIC SPRAY OF PHOTORESIST: Flat Substrates Two coating techniques for flat substrates Technique is dependent on application requirements and resist characteristics LAYER BY LAYER SINGLE FLOOD COAT 20

21 PHOTORESIST DEPOSITION TECHNIQUE Cross Hatch or Multi-pass method Each method results in ±2% uniformity at all coating thicknesses Typically used for substrates with topography to ensure droplets come in at all angles Typically used for planar substrates 21

22 SUMMARY Current Advantages of Ultrasonic Spray for Photoresist Coatings Uniform small droplets with low velocity allows them to adhere better to side walls of trenches. Independent control of air shaping, air flow and photoresist flow for increased process control and repeatability. Ultrasonic nozzles don t clog. Sudden expansion in MEMS capabilities may be seen in future expansion of ALD process, however this process is prohibitively expensive. Ultrasonic atomization allows today for the most complex MEMS designs and stable manufacturing repeatability. 22

23 Questions? 23