PMGI and LOR Resists: FAQs

What is PMGI?
PMGI is an alkaline soluble polymer derived from PMMA. It is formulated for spin coating in a solvent primarily composed of cyclopentanone, for use as a photoresist in multilayer lithography when an undercut layer is needed, as in liftoff applications. It is used to create the retrograde profile, which provides a clean liftoff with no tearing, ripping or shredding of the deposited film. PMGI has an actinic spectral response at wavelengths from 240nm to 290nm.

How are PMGI and LOR products used?
There are different ways to use PMGI. It may be used as the bottom layer of a 2 layer system or also between other layers in a trilayer system. After the top imaging resist is exposed and developed, additional development removes the PMGI directly beneath the new imaging resist window and also develops PMGI laterally. This lateral undercutting of the top imaging resist leaves an overhang feature in the imaging resist.

Will I need an adhesion promoter for PMGI and LOR?
PMGI and LOR typically requires no adhesion promoter. As with any resist, the softbake temperature should exceed the Tg (glass transition temperature) of the resist resin in order to maximize adhesion. The Tg of PMGI is a function of molecular weight and is typically about 180-190C. Therefore, to obtain the best adhesion, the softbake temperature should be greater than 185C.

How thick can PMGI be coated?
PMGI and LOR resists are available in a range of concentrations designed for spin coating from very thin, (<50nm) to relatively thick (>8µm) in a single step. For thick films, the bake should be adjusted to accommodate the solvent evaporation. A ramped softbake is recommended for coatings above about 5 or 6 microns to prevent micro bubbles.

How can I defeat edge bead or cobwebbing?
Both of these problems can be overcome by using MCC's edge bead remover product EBR PG. It should be noted that acetone is not compatible with PMGI and LOR and will not remove edge bead. EBR PG is a universal formulation and will also remove other conventional resists and anti reflective coating materials. This permits a single edge bead remover to be used for many materials.

Will an interfacial layer form using PMGI and LOR?
If PMGI and LOR are used as the bottom layer of a resist stack, and the solvent has been removed by the softbake, no or neglible intermixing with the top resist occurs, primarily because PMGI is insoluble in conventional photoresist solvents. Thus no interfacial layer is formed. If PMGI and LOR are used as a middle or top layer, it is possible that intermixing can occur due to penetration by the PMGI and LOR solvent which is capable of dissolving most resist chemistries.

How do I expose PMGI?
PMGI can be exposed with different optical processes using DUV (240 to 290nm) flood exposure, as described in "Can I reproducibly control the undercut?" PMGI is also sensitive to electron beam irradiation and is currently used with e beam exposure tools.

What is a typical exposure dose?
The dose required to flood expose PMGI is dependent upon thickness and exposure source output as well as other process variables. Typical optical doses are in the 1 to 5 Joule/cm2 range. Typical e beam doses are reported to be in the 30 to 50 µC/cm2 range, or about 3 to 5 times faster than PMMA.

Is a post exposure bake required?
No post exposure bake is required prior to development. Following development, a subsequent bake can be used to reflow the PMGI as for air bridge and micro lens applications.

Can I reproducibly control the undercut?
Yes. There are 4 different variables that can be used to "tune" the process to give the desired undercut. They are: exposure (or lack of exposure), softbake temperature/time, developer type/normality and the PMGI resin composition. It is the last parameter, the resin, that makes PMGI unique and permits a broad range of highly reproducible undercut rates.

How do I decide if LOR A or LOR B resists are better for my application?
LOR A resists have lower dissolution/undercut rates in aqueous developers than LOR B resists. Therefore they are designed for more aggressive developers such as 2.38% TMAH (0.26N) such as Shipley's CD 26 & JSR's NMD-3. Because of LOR A's lower undercut rates they are better suited for high resolution lift-off processes where fine control of the undercut profile is required. LOR B resists are optimized for less aggressive developers such as 0.24N TMAH developers such as Shipley's MF-319 and Clariant's AZ 400 K 1:4. The relatively high dissolution/undercut rates make them better suited for thick film and large feature deposition processes.

How thick must the PMGI and LOR layer be for my deposition process?
The PMGI and LOR layer, which is the bottom layer in a bi-layer stack, should be greater than the target deposition thickness. This will allow discontinuity of the deposited film and a subsequent clean lift-off. If the PMGI and LOR layer is not thicker than the subsequent film deposition, the film will be continuous, which will result in tearing of the film during lift-off. Ina typical process the PMGI and LOR film is 1.33X the target deposition thickness.

What is the minimum feature size I can obtain with an LOR bi-layer lift-off process?
LOR A resists are currently used with advanced I-line resists in 0.35um high volume production applications. Superb control of the undercut profile can be obtained with optimized prebake and develop processes. We also offer a line of PMGI resists, which have lower dissolution (undercut) rates than LOR A. These resists are currently used with deep UV resists in 0.25um high volume production applications.

There are 2 different ways to optically expose PMGI.

1. Coat and bake the PMGI.
2. Flood expose the entire film.
3. Coat the imaging resist and process it normally.
4. Develop the PMGI at the same time as the imaging resist.

A flood exposure of the entire film as described above, will result in a larger undercut since all PMGI beneath the imaging resist has undergone scission and has a higher dissolution rate. OR

1. Coat and bake PMGI.
2. Coat, bake, image and develop the top resist.
3. Flood expose the PMGI through the developed window in the top resist
4. Develop the PMGI at the same time as the imaging resist.

A flood exposure of PMGI through a top imaged and developed resist will result in a lesser undercut than if the entire PMGI film was flood exposed. The exposed regions develop out at a high rate, decreasing as the developer laterally reaches the unexposed region of PMGI.

To reduce the undercut even further, eliminate any exposure of the PMGI. The intrinsically high dissolution rate of unexposed PMGI will permit some undercut even though it has not been exposed.

To obtain a very large undercut without exposure, the NANO LOR resist products can be used.

The amount of undercut is also influenced by the softbake time and temperature. PMGI should be baked at a temperature of 185C or greater, up to about 250C to drive off the maximum amount of solvent and densify the film. It can also be baked at a temperature lower than 185C. As the softbake temperature and time are reduced, more solvent is retained in the film. This results in a larger amount of undercut due to the increased dissolution rate.

Some developers are more aggressive than others and result in different undercut rates during the development. For example, Shipley Microposit Developer or AZ Developer, containing sodium silicates and phosphates, have a very low development rate. TMAH (tetra methyl ammonium hydroxide) - metal ion free - based developers, such as Shipley Microposit Developer CD26 are more aggressive and will result in higher undercut rates. The same is true for the metal ion bearing sodium hydroxide and potassium hydroxide developers such as the Shipley Microposit 350 and 450 series.

To maximize the development selectivity between the imaging resist and PMGI, a 2 developer process can be used. The imaging resist can be developed to completion in the recommended imaging resist developer. The PMGI can then be developed in a TEAH (tetra ethyl ammonium hydroxide) developer such as the MCC 101 Developer. This developer permits dissolution and undercut of PMGI while dissolving the imaging resist at a slower rate than the first developer used.

PMGI Resin
Just as the undercut rate can be controlled with the parameters discussed above, the type of PMGI resin used will also affect undercut. MCC offers PMGI formulations made from resins of differing dissolution rates. There are slow, medium and fast resins offered.

What resolution can I attain with PMGI?
Resolution is a function of the top imaging resist. The PMGI contributes the undercut needed for a discontinuous deposited film prior to lift off which can be adjusted to the desired geometry. PMGI is currently used in conjunction with DUV resists to achieve 0.25µm geometries. It is also used in resist stacks where 0.1µm gates have been imaged.

What is the best developer for PMGI?
The best developer for PMGI is dictated by the application requirements. Some developers have a high dissolution rate. Other developers have a low dissolution rate. PMGI can be used with both metal ion bearing and metal ion free developers. For development that is more selective for PMGI than the imaging resist, the MCC's 101 Developer is used. This allows for controlled development of each resist layer.

How can I strip PMGI and LOR?
Typical resist strippers can be used for PMGI and LOR. Remover PG from MCC or Remover 1165 from Shipley can be used. For other non metallized substrates, oxygen plasma or Piranha can be used. The removal rate in plasma is roughly twice that of conventional novolak based resists.

Can I use PMGI as an imaging resist?
PMGI is not usually used as an imaging resist for optically exposed lift off applications, unless an isotropic slope is needed. It can be used as the imaging resist when undercut cannot be used, such as in the manufacture of T gates with e beam exposure.

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