Contrast Enhancement Materials
CEM Theory
DEFINITION:
CEM is a photo-bleachable material, which is initially opaque to the exposure wavelength(s), but becomes nearly transparent (»90% transmission) upon exposure. (Figure 1) CEM is applied to the photoresist surface. After conventional exposure, the layer is removed with DI water pre-wet and the resist developed in the ordinary way. Because of the presence of the bleachable material the contrast of the illumination that reaches the photoresist is increased. The enhancement of the contrast depends on the photochemical properties of the CEM and the dose required to expose the resist.
FUNCTION
The CEM is spin coated on top of positive photoresist and then exposed. During exposure, the aerial image from the mask hits the CEM layer, where the regions of higher intensity (open areas of the mask) are bleached at a faster rate than the lower intensity regions (closed or dark areas of the mask). By adjusting the bleaching dynamics so that the absorption of the CEM layer is sufficiently high and the photospeeds of the CEM and resist layers are properly matched, it is possible to completely expose the underlying photoresist in the light areas before the CEM is bleached through in the dark areas. Thus, during exposure an in-situ “conformal contact mask” is formed in the CEM layer. The net effect is a higher contrast level of the aerial image used to expose the photoresist. The enhancement of the contrast depends on the photochemical properties of the CEM and the dose required to expose the resist.
Other conditions such as light scattering and second order aerial image effects also contribute to degradation of desired photoresist performance. With the proper match of CEM and exposure parameters the CEM layer will absorb, in the dark areas, all the light from light scattering and second order aerial image effects before they reach the resist surface. The benefit is much straighter or vertical sidewalls and the elimination of rounding or pointed edges at the tops of features.
Figure 2: Thin Coat of CEM is spun on the resist.
The CEM bleaches (becomes more transparent) with exposure.
The CEM behaves like a conformal mask atop the photoresist.
Sequence of steps in the CEM process
- Spin coat positive photoresist on primed wafers
- Softbake photoresist according to standard process
- No Barrier coat required for 365iS and 365HR
- Spin coat CEM material
- Expose wafer
- Strip CEM using a DI water pre-wet
- Post Exposure bake according to standard process (if required)
- Develop photoresist according to standard process
CEM - Features/Benefits:
- Generate Vertical Profiles
- Improve sidewall angles and eliminate “T” top profiles for thick resist films such as Bump and MEMS
- Simple/Low Defect Process
- Increase Exposure and Develop Latitudes
- Reduce/Eliminate Proximity and Interference Effects
- Increase Depth of Focus (DOF) Latitude
- Typical 20-40% increase in DOF
- Extend resolution limit
- Extend life of exposure tool
- Reduce Rework and Increase Yields
- Avoid costly capital expense
- Produce and maintain “Lift-off” or “T” top profiles for specific metal lift-off applications
- More reliable and cost effective than off-axis illumination and phase-shift mask techniques
- Reduce Costs
CEM Product Line
I-line
Barrier coat materials
** Barrier coat not required
CEM Application processes*
- Manufacturing GaAs microwave ICs, esp. metal lift-off level processing
- MESFET gate processing. Also reentrant Lift Off profiles
- Electro-optic or optoelectronic devices
- Analog devices
- Semiconductor lasers
- Wireless & telecom products
- Metal 1/metal 2 - 0.8 um metal lines
- HBT
*Taken from a recent customer survey
Conclusions
CEM Increases your process window
- Increased DOF
- Increased Resolution
- Improved Exposure and Develop Latitude
Use of CEM can save on capital investment
- Extension of stepper limitation
- Added production process latitude
CEM is a simple production process
- Only 1-2 extra processing steps
- Increase in yields due to increased process latitude