Seagate Design Group
Roughness Reduction of Transducer Elements in Magnetic Recorder
Using Chemical Mechanical Planarization/Polishing (CMP)
Alan Bagwell
Anthony Lindert
Loc Nguyen
Greg Rayner
Industrial Mentor:
Faculty Advisor:
Dr. Vince Engelkes
Prof. C. Daniel Frisbie
Seagate Technology
University of Minnesota
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Abstract
The objective of the design is to reduce the RMS roughness value of the
transducer of the recording head from the current value of 10Å to 2Å. Reducing
the roughness will allow the flight height, currently at 10nm, to be set to a lower
value. Decreasing the flight height will improve reliability and performance of the
read and write processes. Chemical mechanical planarization/polishing is the
best solution in terms of cost, time, ease of implementation, and achieving final
RMS roughness value of 2Å. Slurry and abrasive choice is the main focus for
improving the CMP process.
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Introduction
The goal of this project is to design a fabrication process to reduce the
roughness of the transducer, which contains the reader and writer elements of
the recording head. Currently the transducer region (Figure 1) has a 10Å root
mean square (RMS) roughness value measured with an Atomic Force
Microscope (AFM)1. RMS roughness is a surface roughness statistical value for
the differences in surface height from an average surface height value2. The
objective is to reduce the RMS roughness value from 10Å to 2Å.
Figure 1. Hard drive recording slider (left) and cross sectional view of the transducer
(right). Images from “Materials for Electronics” by Dr. Marcus Mooney from Seagate
Technology.
Roughness reduction is crucial in improving the performance of the reader
and writer functions. Current clearance between the reader/writer and the
magnetic recording platter is 10 nm; this is also known as “flight height” 1.
1
2
Dr. Vince Engelkes from Seagate Technology.
ASM Handbook: Volume 18. ASM International, 1992, page 332.
2
Reducing the roughness of the transducer will allow the clearance distance or
flying height to be reduced. This will increase the signal-to-noise ratio of the
reader/writer elements and the magnetic data on the platter, leading to
improvement in reliability and performance. Reducing the transducer roughness
will also increase the life of the hard drive, because it decreases the probability of
collision between the recording head and the media3.
3
Effects of Media Burnish Capability to Head-Disk Clearance. Li, Chen, Liu, Gao, and Demczyk.
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Possible Solutions
The main objective for this design is to reduce the roughness of the
transducer area from 10Å to 2Å RMS roughness value. One possible solution is
to modify and/or add an additional chemical mechanical planarization/polishing
(CMP) step; CMP is currently used in Seagate’s wafer fabrication process. CMP
is defined as:
“A process that uses an abrasive, corrosive slurry to physically grind flat
and chemically remove the microscopic topographic features on a wafer
so that subsequent processes can begin from a flat surface4.”
The abrasive is the mechanical part in CMP and is usually diamond, cerium, or
alumina particles. The corrosive slurry is the chemical part of CMP and is usually
a liquid with low or high pH value5.
Since Seagate already uses some form of CMP in their fabrication
process and already has the necessary equipment, the benefits of this process
are that it will be easy to implement and will have a low cost. Because there are
a number of different materials with different moduli, the disadvantage will be
difficulty in obtaining a controlled flat surface. Softer materials lapped or have a
higher removal rate than harder materials, thus leading to a higher roughness
value. Literature shows that reduction in RMS roughness value below 1Å is
possible through CMP of a single material. Further research in the area of slurry
4
www.appliedmaterials.com/products/cmp_4.html. 3/2/2009
“Processing considerations for CMP on thin-film head wafers” by Ming Jiang. Solid-State
Technology. September 2004.
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type, polishing pad, lapping plate speed, and wafer pressure is needed to
accomplish the project goal.
Another possible solution is a process known as “Spin-on Glass” (SOG).
SOG is a process that uses a liquid form of glass to fill small gaps, cracks, and/or
voids between metal layers6. This process is widely used in the fabrication of
circuit boards. According to Honeywell Electronic Materials, SOG using
siloxanes have the ability to fill gaps or voids as small as 1Å.
SOG process involves pouring glass liquid over a spinning wafer after
each deposition to fill in voids. This ensures a planarized wafer at the end of
deposition fabrication, which can lead to a more uniform polishing or smaller
RMS roughness value from CMP. Lapping and removal rate will be more
consistent if the surface is filled compared to a surface with crack and voids. The
advantages of SOG are that it is relatively easy to implement and low cost. The
only cost for this process is the simple tool to hold and spin the wafer, liquid
glass, and the time to perform the process after each deposition stage. The key
disadvantages of SOG are possible material contaminations and thus a possible
reduction in magnetic and electrical properties of the reader/writer operations.
The third possible solution is to use a focused ion beam (FIB) to mill and
polish the transducer region. FIB is similar to a scanning electron microscope
(SEM) but uses gallium ions rather than electrons for imaging7. Using a focused
beam of ions, FIB can also be used to mill or cut away unwanted material with
superior accuracy. The advantage of using a FIB for this design is a nearly
“Optimization of Spin-On-Glass Process for Multilevel Metal Interconnects”. Aric C. Madayag
and Zhiping Zhou. Georgia Institute of Technology
7 http://www.fei.com/products/types/focused-ion-beam-tools.aspx
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perfect polish or sub-angstrom RMS roughness value. The disadvantages of FIB
are high machine cost, time, and possible re-deposition and contamination.
Milling time can be extensive depending on the amount of material that needs
removing. The other concern is the removed material might be re-deposited
back onto the surface. This can leads to a decrease in the optimal function of the
reader and writer elements.
The last possible solution for the design is material change; changing the
current materials of the reader/writer elements with materials of similar moduli,
but still keeping the same magnetic and electrical properties. Materials with the
same modulus will have a more uniform lap rate compared to a wafer with a wide
range of moduli. The advantage of material change is better polishing potential.
The disadvantage of material change is possible reduction in magnetic and
electrical properties.
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Idea Selection
Four weighting factors were considered in the design: cost, time, ease of
implementation, and final roughness outcome. Cost factored in any additional
monetary needed for extra equipment or supply; it has the lowest weighting
factor of 10%. Time accounts for extra time needed for the process; it has a
weighting factor of 20%. Ease of implementation refers to how well the selected
process can be implemented into the current fabrication process; it has a
weighting factor of 30%. Roughness outcome factored in how well the selected
process can achieve the final RMS roughness value of 2Å. Ease of
implementation and final roughness outcome were given a higher weighting
factor because they are part of the design requirements. A rating factor, from 1
to 4, was also given to each process in each of the weighting factor categories; 1
is worst and 4 is best.
Table 1 summarizes the different possible processes for the design and
the weighting factors. CMP and material change will not require additional
equipment or extra processing time, thus they have a high rating factor for both
cost and time categories. SOG received a 3 for rating factor in the cost and time
categories because it incurs minor costs associated with equipment, liquid glass
supply, and extra processing steps. FIB received a 1 rating for cost and time
categories because of the extensive equipment cost and lengthy milling time. In
the ease of implementation category, CMP received the highest rating because
CMP is currently used in Seagate fabrication process, while material change has
the lowest rating because it will require a propagated change in the overall
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fabrication process. For roughness category, FIB received a 4 because it can
achieve a near perfect flatness; other processes received a 2 because each
process is capable of achieving ideal polished and flatness.
Table 1. Idea selection matrix with weighting factors and total score of CMP, FIB, SOG,
and material change processes.
Idea Selection Process
CMP FIB SOG
$$S Cost
(10%)
Time
(20%)
Ease of
Implementation
(30%)
Roughness
(40%)
Total
Material
Change
4
1
3
4
3
1
3
4
4
2
3
1
2
4
2
2
3
2.5
2.6
2.3
According to the selection matrix, CMP is the best solution with a total score of 3.
Although chemical mechanical planarization is the current final planarization
process in use at Seagate, it is a complicated procedure with many possible
methods of improving and optimizing the process to meet the roughness goal of
2Å. As previously stated, CMP consists of a pad which is spun against the
substrate and a chemical slurry containing abrasive particles used to
simultaneously soften the substrate through chemical means and then polish it
flat with mechanical abrasion.
The process control mechanisms in CMP are extensive. There are
multiple pressure controls, rotator speed settings, slurry release rate, processing
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temperature, polishing time, slurry type, abrasive type and size, and plate or pad
type. Each of these will affect the final outcome of the sample in term of
planarity, uniformity, roughness, and polished.
With all the different input parameters, the slurry itself is the most
important parameter in reducing the substrate roughness. The size and
composition of the abrasive particles as well as their average shape has a large
impact on final roughness. A proper balanced of chemical and particles’ sizes
and shapes for slurry is essential to achieve the design goal of 2Å RMS
roughness value. Modification of the slurry will be the main focus for this project.
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Product Design Specifications (PDS)
Roughness Reduction of Transducer Elements in Magnetic Recorder Using
Chemical Mechanical Planarization/Polishing (CMP)
Goal: Reduce current transducer RMS roughness value of 10Å to 2Å.
Technical Requirements:

Current transducer RMS roughness is 10Å; ideal RMS roughness should
be 2Å.

Slider and transducer must be planarized.

Zero reduction in current magnetic and electrical properties of reader and
writer elements. No contamination and/or negative reaction from slurry or
abrasive after CMP.

Product life must last at least 5 years or a mean time between failures
(MTBF) > 1 million hours usage.
Production Requirements:

Ease of mass production and fabrication.

Cost conscious.
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