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Coating Thickness Uniformaty를 향상시키기위한 조건

고품질 스핀코팅막을 얻기위한 테크닉

Photoresist for defining patterns in microcircuit fabrication on wafer.

Dielectric/insulating layers for microcircuit fabrication ? polymers, SOG, SiLK, etc.

Magnetic disk coatings - magnetic particle suspensions, head lubricants, etc.

Flat screen display coatings. - Antireflection coatings, conductive oxide, etc.

Compact Disks ? DVD, CD ROM, etc.

Television tube phosphor and antireflection coatings.

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스핀코팅 Quality 결정단계

APT Gernan의 Spincoater는 축적된 다년간의Know-how를 바탕으로 최고의 성능을 발휘하도록 설계되었습니다

스핀코팅 프로세스는 4단계로 나눌수 있습니다.

스테이지 3 (flow controlled) 과 스테이지 4 (evaporation controlled) 가 마지막 코팅두께의 유니포머티에
가장 중요한 단계라 핳수 있습니다.

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스테이지 1: The first stage is the deposition of the coating fluid onto the wafer or substrate.
It can be done using a nozzle that pours the coating solution out, or it could be sprayed onto the surface, etc.

Usually this dispense stage provides a substantial excess of coating solution compared to the amount that will ultimately be required in the final coating thickness.

For many solutions it is often beneficial to dispense through a sub micron sized filter to eliminate particles that could lead to flaws.

Another potentially important issue is whether the solution wets the surface completely during this dispense stage. If not, then incomplete coverage can result.

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스테이지 2: The second stage is when the substrate is accelerated up to its final, desired, rotation speed.
This stage is usually characterized by aggressive fluid expulsion from the wafer surface by the rotational motion.

Because of the initial depth of fluid on the wafer surface, spiral vortices may briefly be present during this stage; these would form as a result of the twisting motion caused by the inertia that the top of the fluid layer exerts while the wafer below rotates faster and faster.

Eventually, the fluid is thin enough to be completely co-rotating with the wafer and any evidence of fluid thickness differences is gone.
Ultimately, the wafer reaches its desired speed and the fluid is thin enough that the viscous shear drag exactly balances the rotational accelerations.

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스테이지 3: The third stage is when the substrate is spinning at a constant rate and fluid viscous forces dominate fluid thinning behavior.
This stage is characterized by gradual fluid thinning.
Fluid thinning is generally quite uniform, though with solutions containing volatile solvents, it is often possible to see interference colors "spinning off", and doing so progressively more slowly as the coating thickness is reduced.

Edge effects are often seen because the fluid flows uniformly outward, but must form droplets at the edge to be flung off.
Thus, depending on the surface tension, viscosity, rotation rate, etc., there may be a small bead of coating thickness difference around the rim of the final wafer.

Mathematical treatments of the flow behavior show that if the liquid exhibits Newtonian viscosity (i.e. is linear) and if the fluid thickness is initially uniform across the wafer (albeit rather thick), then the fluid thickness profile at any following time will also be uniform --- leading to a uniform final coating (under ideal circumstances).

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스테이지 4: The fourth stage is when the substrate is spinning at a constant rate and solvent evaporation dominates the coating thinning behavior.
As the prior stage advances, the fluid thickness reaches a point where the viscosity effects yield only rather minor net fluid flow.

At this point, the evaporation of any volatile solvent species will become the dominant process occurring in the coating. In fact, at this point the coating effectively "gels" because as these solvents are removed the viscosity of the remaining solution will likely rise -- effectively freezing the coating in place. (This behavior was used in the seminal work of Meyerhofer (J. Appl. Phys. 49 (1978) 3993) where he quantified the coating thickness dependence on spin speed and viscosity and its relationship to the evaporation rate.)

After spinning is stopped many applications require that heat treatment or "firing" of the coating be performed (as for "spin-on-glass" or sol-gel coatings). On the other hand, photoresists usually undergo other processes, depending on the desired application/use.

Clearly stages 3 and 4 describe two processes that must be occurring simultaneously throughout all times (viscous flow and evaporation). However, at an engineering level the viscous flow effects dominate early on while the evaporation processes dominate later.

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Fluid Flow Complications
The flow behavior described above ignores several effects that are important for many coating solutions.

As noted above, the evaporation step is critical in defining what the final coating thickness will be. But, evaporation occur -- by necessity -- from the top surface, and only some of the solution components are volatile enough to evaporate to any substantial degree.

Thus, there will necessarily be an enrichment of the non-volatile components in the surface layer of the coating solution during the spinning process. The figure at right illustrates that concept.

One of the key consequences is that this surface layer will very likely have a higher viscosity than the unmodified starting solution (this may simply be due to the higher concentration, but might also occur because of cross-linking effects, etc).

With a higher viscosity, it will then impede the flow characteristics set out above, making it a difficult differential equation to solve directly. And, this surface layer may have the secondary result of reducing the evaporation rate. So both the evaporation and flow processes are coupled through the behavior of the "skin" that develops on the top of the outwardly flowing solution during spin coating.

Another important effect is that some solutions are not "Newtonian" in their viscosity/shear-rate relationships. Some solutions change viscosity depending on what shear rate is used, thus depending on distance from the center, the shear rate will be different and thus the flow behavior.

This can give radial thickness variation that varies rather smoothly in a radial sense, as pointed out by Britten and Thomas [J. Appl. Phys. 71 (1992) 972-979].

Air Flow Effects Important for Spin Coating

Air Flow Basics (Ideal Case)
The image at the right, from Millsaps and Pohlhausen, [J. Aeronautical Sci., (1952) 120-126] shows a schematic of the ideal airflow field above an infinitely large spinning disk. At the surface of the disk there is a "no-slip" condition so the contacting air must be exactly co-rotating --- hence the flow vectors pointing essentially tangentially to any point at a given radius (and proportional to the distance from the center).

At moderate distances from the surface a centripetal acceleration must be provided by the viscous effects; this condition is thus maintained only when some outward radial air flow is also occurring. This outward flow is balanced by some minor downdraft over the entire wafer.

This is a steady state configuration and does not include inertial effects included in the "spin-up" stages. This air flow pattern also only hold true so long as the flow is laminar. A "boundary layer" of uniform thickness thus exists over the entire surface area of the spinning wafer: it is through this boundary layer that evaporating solvent must diffuse.

Because the boundary layer is constant in thickness over the wafer then the evaporation rate as a function of position is predicted to also be constant.

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Air Flow Complications
The steady flow field described above is limited to cases where the flow is laminar and where it is "steady". In fact, except for very large wafers, most spinning conditions DO satisfy the constraint of having laminar flow.

However, there can be un-steady oscillating instabilities in the boundary layer near the surface of the wafer. These form spiral shaped waves or rolls that are called "Ekman" spirals. Wahal, et al [Applied Physics Letters 62 (1993) 2584-6] have experimentally observed Ekman spirals (shown in the figure at right) for nominally laminar conditions in spin coating.

They claim that these instabilities can lead to coating thickness variations, but have not explained WHY that would be the case.


APT Spincoater의 특장점

APT SPIN150i
 
  • Ideal for Process R&D
  • Material Size: Ø6”or 5”square
  • RPM: 1 ~12,000 rpm
    (for mechanical/non-vacuumchuckswe recommend max. 3.000rpm)
  • program:unlimited programs with max.unlimited step
  • Accuracy (+/- 1 rpm),Digital control Stepping Motor
  • Chemical resistant NPP Bowl
  • Digital Process Controller-easy step-by-step
  • Programmable Digital Acceleration/Deacceleration 30,000 rpm/sec
  • Clear Lid with 19mm center hole
  • Drain Port, Nitrogen Purge Port
  • Specially designed Bowl for Anti-Reflection & Reducing the the airflow to zero
  • Safety Door Interlock:Rotating stops when opening door
  • German-APT Made CE-approved
  • 2chuck :Fragment ,4-6''wafer
  • 용도 : High Uniformity 요구되는웨이퍼 PR 코팅,유기박막코팅,
    ITO Glass 박막코팅



  • 작동 모습




    - APT Spincoater의 특징
    Wafer uniformity의 향상을 위한 2 Point

    1. acceleration during ramp up: Changing the acceleration resulted in more smooth resist films,
    i.e. “comet” tails disappeared when the acceleration during ramp up was increased to 30,000 rpm/second, with a final spinning speed of 12,000 rpm.

    2. Airflow in the spinbowl: Reducing the the airflow to zero resulted in circular uniformal resist pattern on the wafer

 

 
 

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