Polymer Dielectrics for Multichip
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`Module Packaging
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`PHILIP GARROU, SENIOR MEMBER, IEEE
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`Invited Paper
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`To keep pace with the advances, in IC performance packaging
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`and interconnect technology has had to respond with a revolu-
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`tionary packaging approach, dubbed multichip module (MCM).
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`The performance enchancements seen from this technology come
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`in part from the thin film polymeric dielectrics that are used in
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`their fabrication. Polymer performance is based on the complex
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`interrelationship between such properties as adhesion, stress,
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`moisture absorption, and thermal and chemical stability and the
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`inherent electrical and mechanical properties of the polymer,
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`Several commercial materials based on polyimides (PI’s) or
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`benzocyclobutenes (BCB ’s) are available. This paper reviews these
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`materials and how their properties affect the overall performance
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`and reliability of such MCM structures.
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`I.
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`INTRODUCTION
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`As semiconductor manufacturers move toward the billion-
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`transistor chip and system manufacturers strive for the
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`hand-held mainframe, many have come to the conclusion
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`that the performance advantages inherent in advanced de-
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`vices cannot be exploited in advanced systems if we con-
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`tinue to use “conventional” packaging and interconnect [1].
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`Semiconductor device advances are directly related to
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`a continuing decrease in feature size. Feature sizes have
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`gone from ca. 1 pm in VLSI devices to submicron (<
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`0.5 pm) features in ULSI devices. The smaller feature sizes
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`provide increased gate density, increased gates per chip,
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`and increased clock rates. With reduced feature size, each
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`device has reduced parasitics, allowing faster switching and
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`shorter gate-to-gate distances, reducing interconnect delays.
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`Such benefits are offset by an increase in the number of
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`I/O’s and in the power that is dissipated per chip [2].
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`Over the last 25 years electronic packaging has remained
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`somewhat stagnant, and advances have been evolutionary
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`not revolutionary. In general, individually packaged chips in
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`plastic or ceramic have been interconnected by through hole
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`or surface mount attachment techniques on a single-layer or
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`multilayer printed wiring boards. Those technologies served
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`the industry well for many years [3]. For VLSI devices
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`Manuscript received January 31, 1992; revised May 21, 1992.
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`The author is with Central Research, Dow Chemical Company, 6100
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`Fairview Road, Charlotte, NC 28210.
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`IEEE Log Number 9206221.,
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`and beyond, however, such technologies become inefficient
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`and limit performance. For example, even today, for an
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`advanced CMOS microprocessor subsystem it is common
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`to find cases where the capacitive load imposed by the
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`packaging adds as much as 4 ns to the cycle time [4].
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`This is easy to understand if one considers conven-
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`tional single-chip packaging. A 1 cm VLSI chip having
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`400 peripheral leads on 4 mil pitch would require a single-
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`chip package 5 cm on a side to fan out to the pitch required
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`for PWB (printed wiring board) attachment.
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`To fully utilize high-speed devices in the future, inter-
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`connect
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`paths, lower capacitive load, and reduced circuit noise. A
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`paradigm shift in packaging approach was needed to pro-
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`duce such density gains in interconnect. The revolutionary
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`technology that evolved to meet this objective has been
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`dubbed MCM-D, or multichip module deposited. It involves
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`interconnecting multiple bare dies on structures fabricated
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`by multilayering and patterning thin films of conductor
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`metallization and low dielectric constant “deposited” poly-
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`mer on a base substrate which may or may not contain
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`noncritical interconnect pathways within it [5].
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`A printed wiring board can also be used as a substrate
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`for the interconnect of bare chips (“chip-on-board”). The
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`limitation is the relatively large line and via dimensions
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`required in PWB fabrication. In order to obtain 1000 in/sq.
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`in. of wiring (easily obtainable on an MCM having by two
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`signal layers with 25 um lines on 50 pm pitch), a PWB
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`interconnect scheme would require more than 14 layers at
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`state-of-the-art feature sizes [6]. The comparison between
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`wiring density availability on PWB versus MCM has been
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`extensively studied by Messner [7], who has concluded
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`that MCM interconnection will be cost-effective as well
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`as performance driven.
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`Another metric that has been used to compare the relative
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`densities available from various technologies is the active
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`silicon area. Here, packaging efficiency is defined as the
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`ratio of substrate area to area occupied by semiconductor.
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`Reche [8] has plotted this efficiency ratio for several
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`packaging techniques (Fig. 1).
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`PROCEEDINGS OF THE IEEE, VOL. 80, NO. 12, DECEMBER 1992
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`0018-9219/92303.00 © 1992 IEEE
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`SAMSUNG ET AL. EXHIBIT 1051
`Page 1 of 13
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`'IlIble 1 US. Equipment Output and MCM Penetration (BPA Consultants [6])
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`1995 Unit Ouput
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`160
`26 000
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`0000
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`0000
`400
`4400
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`1 900
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`Supercomputers
`Mainframes
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`Workstations and Top PC
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`Portable Communications
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`Avionic Systems
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`High Perfonnanoe Test
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`Equipment Type
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`1995 MCM Penetration
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`20%
`5 %
`12%
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`SE REDUCTION
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`WEIGHT REDIICTION
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`SIGNAL DEAV REDUCTION
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`JUNCTION raursr-wruns
`REOUCYION
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`RH.lABIl.l'l'Y IMPROVED
`HHLDOOSY
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`UFECVQECOST
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`COMPARED TO
`SURFACE MOUNT PCI
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`EMPARED YO
`TNIIU-HOLE FCI
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`IX
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`3!
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`20 c4
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`5‘
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`mt
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`A!
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`BETTE
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`EITER
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`ADVANTAGE
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`Fig.2. MCM advantages over SMT and through-hole PWB
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`packaging [14].
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`l
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`LINEWIDTH (|lII)
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`‘W Pwl
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`IO mlcrom HDMI
`25 rnlemno HDMI
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`EFFICIENCY (I)
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`Fig. 1. MCM (HDMI) efficiency advantage versus alternative
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`packaging techniques [8].
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`It is becoming generally accepted that MCM technology
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`delivers unquestioned superiority in electrical performance
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`and interconnect density, while equally important gains
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`are made in size and weight and significant advances are
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`expected in reliability. It is for these reasons that multichip
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`packaging has developed into the single most active packag-
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`ing thrust in the 1990’s [9]. Recent govemment—sponsored
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`studies on the impact of MCM packaging and interconnect
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`technology on system performance have resulted in a wider
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`appreciation of these facts [10], [11].
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`Table 1 depicts projected MCM usage in selected appli-
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`cations as ascertained by BPA consultants [12]. Thin-film
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`MCM use is also projected for advanced telecommunication
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`applications and for 1990’s consumer electronics applica-
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`tions such as HDTV and automotive navigation systems
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`[13].
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`Size, weight, and reliability are important drivers for
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`introduction of MCM technology into military and avionics
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`application areas. Hagge and coworkers at Rockwell [14],
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`[15] have shown that MCM technology offers an improve-
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`ment by a factor of 2 to 5 over surface mount technology
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`(Fig. 2) for a wide range of airborne electronic applications.
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`A working example is shown in Fig. 3, where Him-
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`mel and Licari compare the transformation of a Hughes
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`system from 700 sq. in. to 80 sq. in. by replacing conven-
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`tional technology with MCM technology (identified by the
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`Hughes internal acronym HDMI (high density microelec-
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`tronic interconnect)) [16].
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`Speed is often cited as the most compelling long—term
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`reason for acceptance of MCM-D technology in computer
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`and telecommunication applications. Clock rates of 40 to
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`50 MHz are often quoted as the threshold above which
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`device performance is significantly impaired and thus MCM
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`GARROU: POLYMER DIELECTRICS
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`'F$flIH% mDHMHRm
`LIKPSGVEIIHPVT
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`Fig. 3. MCM (HDMI) size advantage versus DI? and SMT im-
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`plementation [16].
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`packaging schemes are essential [4]. It has been projected
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`that by the mid-1990’s 18% of CMOS logic and most
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`leading edge computers will be operating at 50 MHZ and
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`above [12].
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`second
`terabit per
`Osaki of NTT has noted that
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`throughputs could be required during the implementation of
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`broadband ISDN telecom networks. Such large-capacity
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`high-speed switching and transmission systems will
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`require MCM packaging technologies to implement signal
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`switching [17]. AT&T MCM technology has been in
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`production for their switching and transmission equipment
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`in their Merrimack Valley facility since 1987 [18].
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`Thin-filrn MCM technology has been implemented
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`by mainframe manufacturers in systems
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`such as
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`IBM 390/ES9000 [19],
`the NEC SX3 [20],
`the Hitachi
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`M880 [21], and the DEC VAX-9000 [22].
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`As is the case in IC device performance, feature size and
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`the resulting density improvements are the main factors
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`leading to improved electrical performance in these sys-
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`tems. However, such technology was initially implemented
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`because the interconnect density inherent
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`1943
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`SAMSUNG ET AL. EXHIBIT 1051
`Page 2 of 13
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`Table 2 Properties of Polymer Dielectrics
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`Electrical Properties“)
`Photo-
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`Physical/Mechanical Properties“)
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`Breakdown
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`CTE
`Tensile Strength
`Volt V/cm
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`(3123(PP‘“) Tg(°C) Ref. MP“ Elongation (%)
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`E’ '
`5’
`Material
`Vendor
`Sensitive
`x 105
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`DUPONT
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`HITACHI
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`.002
`.002
`.
`.002
`.
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`.003
`.003
`.002
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`PI-2545
`3.5
`40
`105
`>400
`35, 20
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`15
`133
`>320
`40
`PI-2555
`3.3
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`PI-2611D
`2.9(z)
`25
`350
`20(x,y) >400
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`>100(z)
`3.9(x,y)
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`15
`130
`40
`310
`PI—2722
`3.3
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`10
`133
`45-58
`>350
`PIO-13
`3.4
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`PIQ-L100
`3.2(z)
`35
`385
`8(x,y)
`410
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`10
`124
`40
`— -—
`PL-2315
`3.3
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`ClBA-
`56
`140
`PROB-400
`3.0
`39
`350
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`GEIGY
`30
`145
`45
`280
`'Il0RAY
`UR-3800
`3.3
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`NAT’L
`7
`31
`154
`EL-5010
`3.2
`38
`214
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`STARCH
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`4
`>350
`b
`119
`CEMOTA
`55
`>2
`.005
`IP-200
`2.9
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`DOW
`>4
`.002
`BCB-13005
`2.7
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`3.3
`44, 58
`8(0)
`52(c)
`>350
`85
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`(a) Dielectric constant is dependent on humidity and frequency. Values given at 1 kHz where available. See references for exact measurement conditions.
`(b) Manufacturers data sheets.
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`(c) For antioxidant grade XU71988.
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`>2
`>2
`>2
`2.4
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`3
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`3.4
`3.3
`3.1
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`3.0
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`2.4
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`8.9
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`.
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`.
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`29
`29
`29
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`".=~-©-°-O-""2
`
`ODA
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`
`0
`
`--N
`
`-fig H.’
`H
`
`n
`
`;;'r
`‘H
`
`n
`
`Polynmic Acid
`
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`Potymtug
`
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`
`Fig. 4. Chemical reactions to form polyimide.
`
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`0"
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`PMDA
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`‘
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`Commercially viable materials are derived from the reaction
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`of an aromatic dianhydride with an aromatic diamine [26],
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`[27] as shown in Fig. 4. Products are supplied as soluble
`
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`polyamic acid fl’AA)
`intermediates, which upon curing
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`eliminate water. PAA solutions are thermally unstable and
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`must be stored at 0° C. The curing reaction typically requires
`
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`350°—450°C+.
`Volksen and coworkers have discussed the undesirable
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`characteristics of PAA’s such as their high curing tem-
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`peratures, reactivity with copper metallizations, and low
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`planarization capabilities and have shown that polyamic
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`esters such as II resolve many of these problems (28):
`
`PROCEEDINGS OF THE IEEE, VOL 80, NO. 12, DECEMBER 1992
`
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`technique provided efficient signal redistribution from the
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`wiring grid of the ceramic substrate to the wiring inter-
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`connection grid on the semiconductor chip, reducing the
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`number of cofired ceramic layers necessary to interconnect
`
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`the high I/O count chips.
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`
`Each company has its own acronym for this technology
`
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`(i.e., AVP and POLYHIC at AT&T [23], [18] silicon-on-
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`silicon at Rockwell [14], [15]), HDMI at Hughes [16],
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`etc.) and its own modifications to the overall fabrication
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`process, such as substrate material (silicon Versus cofired
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`ceramic versus metal), conductor (Cu versus Al versus Au),
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`via interconnection technique (unfilled versus filled versus
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`plated up posts), die attach technique (wire bond versus
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`TAB (tape automated bonding) versus flip chip) to name
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`just a few [24]. However, one commonality among nearly
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`all the current approaches is the use of polymeric insulator
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`to separate the conductor traces.
`
`II. POLYMERIC THIN-FILM DIELECTRIC LAYERS
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`The widespread use of polymers as insulating layers in
`
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`microelectronic structures is relatively recent [25]. The suit-
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`ability of a specific polymeric material is highly dependent
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`on the process chosen to fabricate the structure and the
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`intended application of the module. The recent increase in
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`MCM activity has resulted in the proliferation of polymeric
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`materials for this market. General properties for the leading
`
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`dielectric candidates (materials most frequently cited in the
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`
`
`
`
`
`literature) are compared in Table 2.
`
`III. POLYMERIC MATERIALS
`
`
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`
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`
`
`A. Polyimides
`
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`
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`Polyimides are a class of polymers containing the func-
`tional unit I:
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`
`SAMSUNG ET AL. EXHIBIT 1051
`Page 3 of 13
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`

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`rv. "" @ ' ©.‘@ @‘:
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`Developed by Hughes and commercialized by National
`
`
`
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`
`
`the Thermid PI’s, V, consist of low-molecular-
`Starch,
`
`
`
`
`
`
`
`weight PI oligomers terminated in acetylenic end groups:
`
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`V. -'=b1L§0<I§O1§00E};5*
`
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`The acetylene groups cross-link during cure without the
`
`
`
`
`evolution of water (31).
`
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`
`
`Polyphenylquinoxalines (PPQ’s), VI, commercialized by
`
`
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`
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`
`
`
`
`Cemota under the trade name Syntorg are fully cyclized
`
`
`
`
`
`
`and stable at room temperature (32):
`
`
`
`
`Table 3 Pyralin Product Line
`
`
`
`
`CHEIIISTRY TYPE
`
`
`
`
`
`
`
`
`
`
`
`
`
`111
`PI-2575
`Pl-555
`PI-2701
`Pl~27ZZ
`
`
`I
`
`P1 7545
`
`
`
`
`
`
`
`Type I PMDA/ODA
`
`
`
`
`Type III BTDAIODA/m-PDA
`
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`
`
`2) Photosensitive Polyimides; Photosensitive PI’s contain
`
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`photoreactive groups that lead to photocrosslinking with
`
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`adjacent polymer chains when exposed to UV light. This
`
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`leads to a differential solubility between the exposed and
`
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`
`
`unexposed portions of the film, and,
`thus, development
`
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`
`
`inherently
`like a negative photoresist. PI’s that are not
`
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`photosensitive are modified by “covalent” (VII) or “ionic”
`
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`(VIII) attachment of photosensitive groups to the parent PI
`molecules:
`
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`
`
`
`
`
`
`©Cova|ontbondi1utypo
`
`
`
`I000‘
`
`000'!
`
`moo‘
`Inn:
`+n.n-An-M-t.~.
`'
`®ooc/‘"‘ooo®
`'“’ ®0oc
`®!PI'ntnsIIIti1itIDG'IID
`
`com-M1»
`ooo® »
`
`Type v BPDAIPDA
`
`
`
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`
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`
`
`
`
`Such materials are becoming commercially available.
`
`
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`
`
`1) Commercially Available Polyimides: The Pyralin series
`
`
`
`
`
`
`
`
`
`of PI products
`from DuPont are based on several
`
`
`
`
`
`
`
`amine/dianhydride chemistries
`shown in Table 3.
`as
`
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`
`
`
`Typel chemistry is based on PMDA/ODA,
`type III on
`
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`
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`BTDA/ODA/m-PDA, and type V on PBDA/p-PDA [29].
`
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`
`PIQ polyimides were introduced by Hitachi Chemical in
`
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`the early 1980’s as an interlayer dielectric for LSI chips
`
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`[30]. Structure III was introduced into the PI chain in order
`
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`
`to increase thermal stability. PIQ polyimides have structures
`
`
`
`
`exemplified by PIQ-L100 (IV).
`
`
`
`VII.
`
`VIII.
`
`
`
`® Ion bonding type
`0
`.
`,c+H-Ar.
`
`
`
`,_
`
`n+M ®
`
`
`
`9
`WC \
`
`Ar
`
`‘fume’ ‘cow
`
`GARROU: POLYMER DIELECTRICS
`
`
`
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`
`
`
`SAMSUNG ET AL. EXHIBIT 1051
`Page 4 of 13
`
`

`
`Table 4 Comparison of Properties for Cured Photosensitive Films
`
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`
`
`Type
`
`Covalent
`
`Ionic
`
`
`(a) Adhesion to Si wafer after 20 hours of PCI‘.
`
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`
`
`Elongation (%)
`
`
`<1
`10
`
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`
`Kojima and coworkers (33) have compared the properties
`
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`
`
`of “covalent” and “ionic” photosensitive PI’s for the same
`
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`
`diamine/diacid combinations as shown in Table 4. While
`the covalent materials show better resolution in thicker
`
`
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`layers, the ionic materials reveal vastly superior mechanical
`
`properties.
`
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`
`
`PI’s such as IX, derived from BTDA and certain ortho-
`
`
`
`
`
`
`
`alkyl substituted aromatic amines, are inherently photosen-
`
`
`sitive [.34]:
`
`IX.
`
`en.
`
`b‘
`
`3
`
`n
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`Several processing steps can be omitted when using
`
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`photoimagible polymers as shown in Fig. 5. The use of
`
`
`
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`
`
`
`photosensitive polyimides has been extensively detailed by
`
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`
`
`companies such as Boeing [35], NTI‘ [36], NEC [37],
`
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`
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`Toshiba [38], and Mitsubishi [39]. Toray Photoneece pho-
`
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`
`
`
`tosensitive PI’s and the Hitachi photosensitive products
`
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`
`
`
`(designated by a PI. suffix) are of the ionic variety [40].
`
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`
`
`Dupont photosensitive products are based on type III chem-
`
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`
`
`
`istry and have been of the “covalent” variety. OCG (Olin-
`
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`
`
`Ciba Geigy) 400 series Probamides are of the inherently
`
`
`
`photosensitive (IX) type.
`
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`
`
`3) Low-CTE Polyimides: Detailed studies have been car-
`
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`
`
`
`ried out to understand the relationship between PI structure
`
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`
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`and the coefficient of thermal expansion (CTE) [41]—[42].
`
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`
`PI’s having rigid backbones reveal very high modulus
`
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`
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`and low biaxial (in plane) CTE values. These values are
`
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`
`
`highly anisotropic resulting in Z axis CTE’s reportedly
`
`
`
`
`> 100 ppm [43].
`
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`
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`
`
`
`Commercially, low—CTE PI products are exemplified by
`
`
`
`
`
`
`
`Pyralin type V products and PIQ-L100 (IV).
`
`
`
`B. Benzocyclobutenes
`
`
`
`
`
`
`Benzocyclobutenes, X, commercialized by Dow Chemi-
`
`
`
`
`
`
`
`
`cal under the trade name Cyclotene, polymerize thermally
`
`
`
`
`
`
`
`without
`the evolution of by-products. Siloxy containing
`
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`
`
`
`
`
`DVS-BCB, XI, is the first BCB introduced for application
`
`
`
`
`
`
`
`in microelectroniw [44]—[48]. BCB's are thermally stable
`
`
`
`
`
`
`
`
`at room temperature and cure at 220°—Z50° C.
`
`
`
`
`
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`
`
`A photosensitive BCB product has recently been intro-
`
`
`duced [49].
`
`
`X. @R@
`
`
`
`
`
`Tensile Strength (MPa)
`
`
`<10
`116
`
`
`
`
`
`
`Peel St:rength(“)
`
`:_(2.Lcm)__
`0
`
`260
`
`
`
`
`
`
`UVMQUA
`llllllllll
`
`Fig. 5. Processing steps for feature generation in photosensitive
`
`
`
`
`
`
`
`
`versus nonphotosensitive PI.
`
`
`
`
`
`
`
`
`
`
`XI.
`
`
`
`
`
`
`
`
`
`IV. POLYMER PERFORMANCE
`
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`
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`
`
`
`
`There is good agreement that it is essential to evaluate
`
`
`
`
`
`
`
`the following criteria when choosing a polymeric dielectric
`
`
`
`
`
`
`
`
`
`
`for the fabrication of a reliable thin—film structure [1], [3].
`
`
`
`
`
`
`
`
`
`As stated previously, the best material will be determined
`
`
`
`
`
`
`
`
`by the processes chosen to fabricate the structure.
`
`
`
`
`A. Processing Properties
`
`
`
`
`
`
`
`1) Dielectric Application: Organic dielectrics are typi-
`
`
`
`
`
`
`
`
`
`
`cally applied by spin coating. The polymer needs to achieve
`
`
`
`
`
`
`
`
`the desired thickness in the fewest coating applications
`
`
`
`
`
`
`
`
`possible, to exhibit thickness uniformity across the wafer,
`
`
`
`
`
`
`
`
`
`and to produce a smooth, planar, pinhole-free film. The
`
`
`
`
`
`
`
`
`need for planarization is described separately below. Cured
`
`
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`
`
`
`
`
`
`
`film thickness is a function of both spin speed and spin
`
`
`
`
`
`
`
`
`
`
`time. Nonuniformity across the wafer can result if the spin
`
`PROCEEDINGS OF THE IEEE, VOL. 80, NO. 12, DECEMBER 1992
`
`
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`
`
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`
`
`SAMSUNG ET AL. EXHIBIT 1051
`Page 5 of 13
`
`

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`
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`process [58] has been detailed where the time of the cure
`
`
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`
`
`
`
`
`operation has been reduced to < 15 minutes by processing
`
`
`
`
`
`
`
`
`
`
`the substrates in a belt furnace that allows access to rapid
`
`
`
`
`
`heat up and cool down.
`
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`
`
`
`
`
`3) Etching: Etching of the polymer dielectric is neces-
`
`
`
`
`
`
`
`
`
`
`sary to create the vias that interconnect the power, ground,
`
`
`
`
`
`
`
`
`and signal
`layers. For polyimides, both “wet” chemical
`
`
`
`
`
`
`
`
`etching and “dry” plasma etching have been reported.
`
`
`
`
`
`
`
`
`
`The use of photoimagible polymers avoids this need, as
`
`
`
`
`
`
`
`
`
`was discussed earlier. Only partially cured PI films can
`
`
`
`
`
`
`
`
`
`be wet etched. Strong base etchants for wet processing
`
`
`
`
`
`
`
`
`
`of partially cured PIQ have been described [59]. The
`
`
`
`
`
`
`
`
`problems encountered during the base etching of polyirnide
`
`
`
`
`
`
`
`
`
`films have been detailed [60]. The low CTE polyimides
`
`
`
`
`
`
`
`
`
`are reportedly more resistant to wet chemical etching. In
`
`
`
`
`
`
`
`
`
`general, the thicker layers of dielectric required for MCM
`
`
`
`
`
`
`
`
`
`fabrication preclude the use of wet etch techniques. Inoue
`
`
`
`
`
`
`
`
`
`
`
`
`
`[21] has detailed a wet etch process for 8 pm of PI used
`
`
`
`
`
`
`
`
`in the HITAC M-880 microchip carrier. Although well
`
`
`
`
`
`
`
`
`
`controlled,
`the process is complex and includes the need
`
`
`
`
`
`
`
`
`
`for Cr barrier metallization (for Al conductors) and Ar
`
`
`back-sputter descumming.
`
`
`
`
`
`
`
`
`
`
`
`Dry plasma processing can also be used to etch vias in
`
`
`
`
`
`
`
`polymer dielectrics. Depending on the MCM construction
`
`
`
`
`
`
`
`
`
`approach, isotropic or anisotropic etching is used to produce
`
`
`
`
`
`
`
`
`
`
`sloped or straight walled vias [61], [62]. The final shape,
`
`
`
`
`
`
`
`
`
`
`profile, and size of the etched via are determined by
`
`
`
`
`
`
`
`
`the masking technique employed and the RIE parameters.
`
`
`
`
`
`
`
`
`
`PI’s can be etched in 02 plasma, but the incorporation
`
`
`
`
`
`
`
`
`
`of fluorine containing etch gases allows better control
`
`
`
`
`
`
`
`
`
`
`of sidewall slope and up to five times faster processing
`
`
`
`
`
`
`
`
`
`
`
`
`[63], [64]. PI can be masked by a thick layer of positive
`
`
`
`
`
`
`
`
`photoresist although hard masks are sometimes used to
`
`
`
`
`
`
`
`
`
`avoid enlargement of the vias during etching [65]. DVS-
`
`
`
`
`
`
`
`
`
`
`BCB is highly resistant to wet chemical etching. Its silicon
`
`1947
`
`
`
`Fig, 6. Spin coating parameters for DVS-BCB application.
`
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`
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`
`
`
`
`
`
`
`
`speed is either too slow or too fast [50]. A plot of coating
`
`
`
`
`
`
`
`
`
`thickness versus spin speed is shown for DVS-BCB in
`
`
`Fig. 6.
`
`
`
`
`
`
`
`
`It is important to remove organic contaminants from the
`
`
`
`
`
`
`
`
`surface prior to application since spin-on polymers tend to
`
`
`
`
`
`
`
`
`pull away from these areas leaving “pinholes” [51].
`
`
`
`
`
`
`
`
`
`
`There is very little published literature on spray coating of
`
`
`
`
`
`
`
`
`
`dielectric layers. Spray deposition of thin coatings of highly
`
`
`
`
`
`
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`viscous materials is a complex process. The thickness, uni-
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`formity, and texture of the sprayed surface are influenced by
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`the viscoelastic properties of the polymer, the concentration
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`of the polymer in solvent, and the critical surface tension
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`at the interface [52], [53]. AT&T POLYHIC substrates are
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`manufactured using a spray coating process.
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`2) Curing: During Pl curing (400°C+) two processes
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`take place, the loss of N-methyl pyrolidone solvent and
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`the elimination of water. Total cure times of 4 to 6 hours
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`are typical. It is recommended that curing be performed in
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`an inert environment. Improper or incomplete curing can
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`have a pronounced effect on the proprieties of a PI film.
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`Monitoring of the degree of cure using IR spectroscopy
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`has demonstrated that the rate of reaction is dependent on
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`both temperature and thickness [54]. PI curing can also
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`be followed by monitoring electrical property changes by
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`microdielectrometry [55].
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`Heacock has studied the hydrolysis of PMDA/ODA, the
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`reverse of the imidization reaction, at elevated temperature
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`and humidity [56]. PI hydrolysis has also been discussed
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`in detail by Volksen [28].
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`BCB curing is a purely thermal process. No catalysts are
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`required and no violates are produced. BCB’s must be cured
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`in a low-oxygen (< 100 ppm 02) environment to prevent
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`oxidation of the structure. Oxidation negatively impacts the
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`properties of the resultant film [44]. Curing in a box oven
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`requires 4 to 5 hours. A recent rapid thermal curing (RTC)
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`GARROU: POLYMER DIELECTRICS
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`SAMSUNG ET AL. EXHIBIT 1051
`Page 6 of 13
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`

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`ElalrRoll(Inpmlpulu)
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`1 0 0 0
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`Flulnel
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`Z 0 II 0
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`(In nhllemz)
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`Fig. 7. Etch rate versus fluence for several polymer dielectrics.
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`content requires fluorine containing plasmas [44],
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`Metal, SiO2, or photoresist etch masks can be used.
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`Laser processing has also been shown to be a viable
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`technique for via formation [67]. A detailed comparison
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`of direct write, scanning, and projection laser ablation
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`techniques has been published [64]. The ablation rate for
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`various polymer dielectrics is shown in Fig. 7.
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`[46].
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`B. Pabzmer Electrical Properties
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`1) Dielectric Constant: Dielectric constant (c’) is the most
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`critical electrical parameter for a microelectronic polymer.
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`The lower the dielectric constant,
`the faster the signal
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`propagation velocity, as described by
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`(1)
`Vp = ex/c7
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`where Vp is the velocity of propagation, 6' is the dielectric
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`constant of the insulator, and c is the speed of light.
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`A lower dielectric constant allows signal
`traces to be
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`designed wider and the dielectric thickness decreased.
`It also allows one to maintain the same characteristic
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`impedance while lowering the line resistance and cross talk.
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`Good dielectric performance is obtained from a material
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`whose dielectric constant is invariant with frequency and
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`temperature.
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`Dielectric constants of typical MCM polymers are shown
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`in Table 2. Polyimides have dielectric constants in the range
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`of 3.2 to 3.4. Low CTE Pl’s reveal highly anisotropic
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`dielectric constants (estimated 3.0(z), 3.8(:r7, y)) [43]. DVS-
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`BCB has an isotropic dielectric constant of 2.7.
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`2) Dissipation Factor.’ Dissipation factor (e”) and loss
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`tangent (e”/e’) are also important electrical parameters,
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`especially at high frequencies. bow values are indicative of
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`minimal conversions of electrical energy to heat and little
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`overall power loss.
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`3) Dielectric Strength: Dielectric strength/breakdown
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`voltage defines the voltage that causes current
`to flow
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`through the insulator. The materials listed in Table 2 have
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`values in the range of 106 V/cm.
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`C. Physical, Mechanical Properties
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`1) Water Absorption: Water absorption impacts both the
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`electrical properties and the processibility of MCM di-
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`electrics. Absorption of water, with a dielectric constant of
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`78, raises the dielectric constant of an insulating polymer
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`and thus affects circuit performance.
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`1948
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`l POLYMER
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`_L
`I
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`Doom ot Planarlzauon (Don a I - 'I_ x 100
`‘.
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`Fig. 8. Planarization of a metal feature.
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`Absorbed moisture can also cause severe damage to
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`structures during fabrication. Uncontrolled outgassing of
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`moisture from underlying layers of dielectric during sub-
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`sequent high-temperature processing can result
`in blis-
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`tering and delamination of the thin-film structures being
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`fabricated. [50], [63], [68]. Bakeout cycles are routinely
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`employed during the fabrication of MCM’s with polymers
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`that exhibit significant moisture absorption. Gridded ground
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`planes and in some cases gridded signal planes are used to
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`allow moisture to escape [69]. It has been reported that PI
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`moisture absorption increases after aging at 85°C/85% RH
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`[70].
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`Moisture uptake is specific to the polymer in question
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`and the conditions of the experiment. Reported water ab-
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`sorption values, with conditions where available, are given
`in Table 5.
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`2) Planarization: Relief existing on a surface prior to
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`photolithography will affect the resolution and quality of
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`subsequent conductor traces. One of the main functions of
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`the insulator layer is to planarize the underlying topography
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`and, thus, provide a flat focal plane for the next layer. Lack
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`of topographic planarity can lead to subsequent nonuniform
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`metal thickness (poor step coverage). These thinned and
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`weakened areas are susceptible to cracking [71].
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`Rothman,
`in her classic paper, defined the degree of
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`planarization (DOP) for a polymer dielectric as shown in
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`Fig. 8 [72].
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`Senturia and coworkers [73] have shown that the DOP
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`obtained