Polymer Dielectrics for Multichip
`Module Packaging
`
`PHILIP GARROU, SENIOR MEMBER, IEEE
`Invited Paper
`
`To keep pace with the advances, in IC pegormance packaging
`and interconnect technology has had to respond with a revolu-
`tionary packaging approach, dubbed multichip module (MCM).
`The performance enchancements seen from this technology come
`in part from the thin film polymeric dielectrics that are used in
`their fabrication. Polymer performance is based on the complex
`interrelationship between such properties as adhesion, stress,
`moisture absorption, and thermal and chemical stability and the
`inherent electrical and mechanical properties of the polymer.
`Several commercial materials based on polyimides (PI’S) or
`bemocyclobutenes (BCB’s) are available. This paper reviews these
`materials and how their properties affect the overall performance
`and reliability of such MCM structures.
`
`I. INTRODUC~ION
`As semiconductor manufacturers move toward the billion-
`transistor chip and system manufacturers strive for the
`hand-held mainframe, many have come to the conclusion
`that the performance advantages inherent in advanced de-
`vices cannot be exploited in advanced systems if we con-
`tinue to use “conventional” packaging and interconnect [ 11.
`Semiconductor device advances are directly related to
`a continuing decrease in feature size. Feature sizes have
`gone from ca. 1 pm in VLSI devices to submicron (<
`0.5 pm) features in ULSI devices. The smaller feature sizes
`provide increased gate density, increased gates per chip,
`and increased clock rates. With reduced feature size, each
`device has reduced parasitics, allowing faster switching and
`shorter gate-to-gate distances, reducing interconnect delays.
`Such benefits are offset by an increase in the number of
`I/O’s and in the power that is dissipated per chip [2].
`Over the last 25 years electronic packaging has remained
`somewhat stagnant, and advances have been evolutionary
`not revolutionary. In general, individually packaged chips in
`plastic or ceramic have been interconnected by through hole
`or surface mount attachment techniques on a single-layer or
`multilayer printed wiring boards. Those technologies served
`the industry well for many years [3]. For VLSI devices
`
`Manuscript received January 31, 1992; revised May 21, 1992.
`The author is with Central Research, Dow Chemical Company, 6100
`Fairview Road, Charlotte, NC 28210.
`IEEE Log Number 9206221..
`
`and beyond, however, such technologies become inefficient
`and limit performance. For example, even today, for an
`advanced CMOS microprocessor subsystem it is common
`to find cases where the capacitive load imposed by the
`packaging adds as much as 4 ns to the cycle time [4].
`This is easy to understand if one considers conven-
`tional single-chip packaging. A 1 cm VLSI chip having
`400 peripheral leads on 4 mil pitch would require a single-
`chip package 5 cm on a side to fan out to the pitch required
`for PWB (printed wiring board) attachment.
`To fully utilize high-speed devices in the future, inter-
`connect technologies will need to deliver shorter signal
`paths, lower capacitive load, and reduced circuit noise. A
`paradigm shift in packaging approach was needed to pro-
`duce such density gains in interconnect. The revolutionary
`technology that evolved to meet this objective has been
`dubbed MCM-D, or multichip module deposited. It involves
`interconnecting multiple bare dies on structures fabricated
`by multilayering and patterning thin films of conductor
`metallization and low dielectric constant “deposited” poly-
`mer on a base substrate which may or may not contain
`noncritical interconnect pathways within it [5].
`A printed wiring board can also be used as a substrate
`for the interconnect of bare chips (“chip-on-board”). The
`limitation is the relatively large line and via dimensions
`required in PWB fabrication. In order to obtain 1000 in/sq.
`in. of wiring (easily obtainable on an MCM having by two
`signal layers with 25 pm lines on 50 pm pitch), a PWB
`interconnect scheme would require more than 14 layers at
`state-of-the-art feature sizes [6]. The comparison between
`wiring density availability on PWB versus MCM has been
`extensively studied by Messner [7], who has concluded
`that MCM interconnection will be cost-effective as well
`as performance driven.
`Another metric that has been used to compare the relative
`densities available from various technologies is the active
`silicon area. Here, packaging efficiency is defined as the
`ratio of substrate area to area occupied by semiconductor.
`Reche [8] has plotted this efficiency ratio for several
`packaging techniques (Fig. 1).
`
`1942
`
`PROCEEDINGS OF THE IEEE, VOL. 80, NO. 12, DECEMBER 1992
`
`0018-9219/92$03.00 0 1992 IEEE
`
`MICRON ET AL. EXHIBIT 1051
`Page 1 of 13
`
`

`
`Table 1 U S . Equipment Output and MCM Penetration (BPA Consultants [6])
`
`Equipment Type
`
`1995 Unit Ouput
`
`1995 MCM Penetration
`
`Supercomputers
`Mainframes
`Workstations and Top PC
`Portable Communications
`Avionic Systems
`High Performance Test
`
`160
`26 000
`7 500 OOO
`4000000
`4 400
`1 900
`
`20%
`5%
`12%
`4%
`10%
`5%
`
`COMPARED TO
`SURFACE MOUNT PCB
`
`COMPARED TO
`TnRU-mKE PC0
`
`26 mICmn* HDMl
`
`0 REU*BILITTlYPROVED
`
`5x
`
`4x
`
`U
`
`U(
`m e
`
`I ox
`
`6X
`¶x
`
`loco
`
`4x
`
`ADVANTAGE
`
`0
`
`SQEREDUCMm
`
`0 WEIGHT REDUCTION
`
`0 SIGNAL DUAY REDUCTION
`
`0 JUNCllON TEMPERATURE
`~l"
`
`LINEWIDTH (um)
`
`60
`
`0
`0
`
`60
`EFFICIENCY (%)
`
`Fig. 1. MCM (HDMI) efficiency advantage versus altemative
`packaging techniques [8].
`
`0 euImcosJ
`
`10
`
`0 UFECYWUMT
`
`C 0 M P t " E
`
`BmER
`
`C O M P n m M
`
`DErrEn
`
`Fig. 2.
`MCM advantages over SMT and through-hole PWB
`packaging [14].
`
`It is becoming generally accepted that MCM technology
`delivers unquestioned superiority in electrical performance
`and interconnect density, while equally important gains
`are made in size and weight and significant advances are
`expected in reliability. It is for these reasons that multichip
`packaging has developed into the single most active packag-
`ing thrust in the 1990's [9]. Recent government-sponsored
`studies on the impact of MCM packaging and interconnect
`technology on system performance have resulted in a wider
`appreciation of these facts [lo], [ll].
`Table 1 depicts projected MCM usage in selected appli-
`cations as ascertained by BPA consultants [12]. Thin-film
`MCM use is also projected for advanced telecommunication
`applications and for 1990's consumer electronics applica-
`tions such as HDTV and automotive navigation systems
`~ 3 1 .
`Size, weight, and reliability are important drivers for
`introduction of MCM technology into military and avionics
`application areas. Hagge and coworkers at Rockwell [14],
`[15] have shown that MCM technology offers an improve-
`ment by a factor of 2 to 5 over surface mount technology
`(Fig. 2) for a wide range of airborne electronic applications.
`A working example is shown in Fig. 3, where Him-
`me1 and Licari compare the transformation of a Hughes
`system from 700 sq. in. to 80 sq. in. by replacing conven-
`tional technology with MCM technology (identified by the
`Hughes internal acronym HDMI (high density microelec-
`tronic interconnect)) [ 161.
`Speed is often cited as the most compelling long-term
`reason for acceptance of MCM-D technology in computer
`and telecommunication applications. Clock rates of 40 to
`50 MHz are often quoted as the threshold above which
`device performance is significantly impaired and thus MCM
`
`W
`
`Fig. 3. MCM (HDMI) size advantage versus DIP and SMT im-
`plementation [16].
`
`packaging schemes are essential [4]. It has been projected
`that by the mid-1990's 18% of CMOS logic and most
`leading edge computers will be operating at 50 MHz and
`above [12].
`Osaki of NTT has noted
`terabit per second
`that
`throughputs could be required during the implementation of
`broadband ISDN telecom networks. Such large-capacity
`high-speed switching and
`transmission systems will
`require MCM packaging technologies to implement signal
`switching [17]. AT&T MCM technology has been in
`production for their switching and transmission equipment
`in their Merrimack Valley facility since 1987 [18].
`Thin-film MCM technology has been
`implemented
`by mainframe manufacturers in systems such as the
`IBM 390/ES9000 [19], the NEC SX3 [20], the Hitachi
`M880 [21], and the DEC VAX-9000 [22].
`As is the case in IC device performance, feature size and
`the resulting density improvements are the main factors
`leading to improved electrical performance in these sys-
`tems. However, such technology was initially implemented
`because the interconnect density inherent in the MCM
`
`GARROU: POLYMER DIELECTRICS
`
`1943
`
`MICRON ET AL. EXHIBIT 1051
`Page 2 of 13
`
`

`
`Table 2 Properties of Polymer Dielectrics
`
`Electrical Properties(a)
`
`
`
`Vendor Vendor
`
`
`
`Material Material
`
`
`Photo- Photo-
`
`Sensitive Sensitive
`
`
`
`,I ,I
`
`E"
`E"
`
`
`Breakdown Breakdown
`
`Volt V/cm Volt V/cm
`x lo6
`x 106
`
`
`
` G:::::: G::::::
`
`
`cTE cTE
`@pm) T g ( " c ) GPa
`@pm) To("C) GPa
`
`Physicalh4echanical Propertieda)
`
`
`Tensile Strength Tensile Strength
`
`MPa MPa
`
`
`
`Elongation (%) Elongation (%)
`
`
`
`Ref. Ref.
`
`DUPONT
`
`PI-2545
`PI-2555
`PI-2611D
`
`HITACHI
`
`PI-2722
`PIQ-13
`PIQ-L100
`PL-2315
`PROB-400
`UR-3800
`
`:!&
`TORAY
`
`:$zcH EL-5010
`
`35, 20 >400
`3.5
`>320
`40
`3.3
`20(x,y) >400
`2.9(z)
`>100(Z)
`3.9(X,Y)
`310
`40
`15
`b
`3.3
`42, b
`10
`45-58 >350
`3.4
`42, b
`35
`410
`3.2(z)
`8(x,y)
`--
`40
`10
`b
`3.3
`56
`b
`39
`350
`3.0
`30
`40, b
`45
`280
`3.3
`38
`214
`7
`31
`3.2
`4
`,005
`55
`>350
`>2
`2.9
`b
`IP-200
`CEMOTA
`44, 58
`8(c)
`3.3
`52(c) >350
`>4
`,002
`2.7
`BCB-13005
`DOW
`(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.
`(c) For antioxidant grade XU71988.
`
`,002
`,002
`,002
`
`.002
`,002
`,002
`,003
`,003
`,002
`
`>2
`>2
`>2
`
`2.4
`3
`
`3.4
`3.3
`3.1
`
`+
`
`+
`+
`+
`
`40
`15
`25
`
`29
`29
`29
`
`3.0
`2.4
`8.9
`
`--
`
`3.5
`
`3.4
`
`105
`133
`350
`
`130
`133
`385
`124
`140
`145
`154
`119
`85
`
`technique provided efficient signal redistribution from the
`wiring grid of the ceramic substrate to the wiring inter-
`connection grid on the semiconductor chip, reducing the
`number of cofired ceramic layers necessary to interconnect
`the high 1/0 count chips.
`Each company has its own acronym for this technology
`(i.e., AVP and POLYHIC at AT&T [23], [18] silicon-on-
`silicon at Rockwell [14], [15]), HDMI at Hughes [16],
`etc.) and its own modifications to the overall fabrication
`process, such as substrate material (silicon versus cofired
`ceramic versus metal), conductor (Cu versus Al versus Au),
`via interconnection technique (unfilled versus filled versus
`plated up posts), die attach technique (wire bond versus
`TAB (tape automated bonding) versus flip chip) to name
`just a few [24]. However, one commonality among nearly
`all the current approaches is the use of polymeric insulator
`to separate the conductor traces.
`
`11. POLYMERIC THIN-FILM DIELECTRIC LAYERS
`The widespread use of polymers as insulating layers in
`microelectronic structures is relatively recent [25]. The suit-
`ability of a specific polymeric material is highly dependent
`on the process chosen to fabricate the structure and the
`intended application of the module. The recent increase in
`MCM activity has resulted in the proliferation of polymeric
`materials for this market. General properties for the leading
`dielectric candidates (materials most frequently cited in the
`literature) are compared in Table 2.
`
`111. POLYMERIC MATERIALS
`
`A. Polyimides
`Polyimides are a class of polymers containing the func-
`tional unit I:
`
`PMDA
`
`ODA 1
`
`Pdyimidr
`
`Polyomic Acid
`
`Fig. 4.
`
`Chemical reactions to form polyimide.
`
`I.
`
`Commercially viable materials are derived from the reaction
`of an aromatic dianhydride with an aromatic diamine [26],
`[27] as shown in Fig. 4. Products are supplied as soluble
`polyamic acid (PAA) intermediates, which upon curing
`eliminate water. PAA solutions are thermally unstable and
`must be stored at 0°C. The curing reaction typically requires
`350" -450"CS.
`Volksen and coworkers have discussed the undesirable
`characteristics of PAA's such as their high curing tem-
`peratures, reactivity with copper metallizations, and low
`planarization capabilities and have shown that polyamic
`esters such as I1 resolve many of these problems (28):
`
`1944
`
`PROCEEDINGS OF THE IEEE, VOL. 80, NO. 12, DECEMBER 1992
`
`MICRON ET AL. EXHIBIT 1051
`Page 3 of 13
`
`

`
`Table 3 Pyralin Product Line
`CHEMISTRY TYPB
`
`m
`I
`PI2545 PI-2525
`PI-2555
`PI-2701
`P I - m
`
`V
`
`2611
`
`Standard
`
`Photoswitive
`
`L o w m
`
`Developed by Hughes and commercialized by National
`Starch, the Thermid PI’s, V, consist of low-molecular-
`weight PI oligomers terminated in acetylenic end groups:
`
`Type1 PMDNODA
`
`The acetylene groups cross-link during cure without the
`evolution of water (31).
`Polyphenylquinoxalines (PPQ’s), VI, commercialized by
`Cemota under the trade name Syntorg are fully cyclized
`and stable at room temperature (32):
`
`Type III BTDAIODAIm-PDA
`
`VI.
`
`TypeV BPDAIPDA
`
`Such materials are becoming commercially available.
`1) Commercially Available Polyimides: The Pyralin series
`of PI products from DuPont are based on several
`in Table 3.
`amine/dianhydride chemistries as shown
`Type I chemistry is based on PMDA/ODA, type 111 on
`BTDA/ODA/m-PDA, and type V on PBDA/p-PDA [29].
`PIQ polyimides were introduced by Hitachi Chemical in
`the early 1980’s as an interlayer dielectric for LSI chips
`[30]. Structure 111 was introduced into the PI chain in order
`to increase thermal stability. PIQ polyimides have structures
`exemplified by PIQ-L100 (IV).
`
`2) Photosensitive Polyimides: Photosensitive PI’s contain
`photoreactive groups that lead to photocrosslinking with
`adjacent polymer chains when exposed to UV light. This
`leads to a differential solubility between the exposed and
`unexposed portions of the film, and, thus, development
`like a negative photoresist. PI’s that are not inherently
`photosensitive are modified by “covalent” (VII) or “ionic”
`(VIII) attachment of photosensitive groups to the parent PI
`molecules:
`
`OCOVak3fltbonclinotype
`
`0 Ion bonding type
`
`111.
`
`0
`
`0
`
`
`
`GARROU: POLYMER DIELECTRICS
`
`1945
`
`MICRON ET AL. EXHIBIT 1051
`Page 4 of 13
`
`

`
`Table 4 Comparison of Properties for Cured Photosensitive Films
`
`Type
`
`Elongation (%)
`
`Tensile Strength (MPa)
`
`Strength(a)
`
`I
`
`,
`
` \
`
`<1
`Covalent
`Ionic
`10
`(a) Adhesion to Si wafer after 20 hours of PCT.
`
`<10
`116
`
`0
`260
`
`Kojima and coworkers (33) have compared the properties
`of “covalent” and “ionic” photosensitive PI’s for the same
`diamine f diacid combinations as shown in Table 4. While
`the covalent materials show better resolution in thicker
`layers, the ionic materials reveal vastly superior mechanical
`properties.
`PI’s such as IX, derived from BTDA and certain ortho-
`alkyl substituted aromatic amines, are inherently photosen-
`sitive [34]:
`
`plw.
`
`I N
`
`Fig. 5. Processing steps for feature generation in photosensitive
`versus nonphotosensitive PI.
`
`Several processing steps can be omitted when using
`photoimagible polymers as shown in Fig. 5. The use of
`photosensitive polyimides has been extensively detailed by
`companies such as Boeing [35], NTT [36], NEC [37],
`Toshiba [38], and Mitsubishi [39]. Toray Photoneece pho-
`tosensitive PI’s and the Hitachi photosensitive products
`(designated by a PL suffix) are of the ionic variety [40].
`Dupont photosensitive products are based on type I11 chem-
`istry and have been of the “covalent” variety. OCG (Olin-
`Ciba Geigy) 400 series Probamides are of the inherently
`photosensitive (IX) type.
`3) Low-CTE Polyimides: Detailed studies have been car-
`ried out to understand the relationship between PI structure
`and the coefficient of thermal expansion (CTE) [41]-[42].
`PI’s having rigid backbones reveal very high modulus
`and low biaxial (in plane) CTE values. These values are
`highly anisotropic resulting in 2 axis CTE’s reportedly
`> 100 ppm [43].
`Commercially, low-CTE PI products are exemplified by
`Pyralin type V products and PIQ-L100 (IV).
`
`B. Benzocyclobutenes
`Bemocyclobutenes, X, commercialized by Dow Chemi-
`cal under the trade name Cyclotene, polymerize thermally
`without the evolution of by-products. Siloxy containing
`DVS-BCB, XI, is the first BCB introduced for application
`in microelectronics [44]-[48]. BCB’s are thermally stable
`at room temperature and cure at 22Oo-25O0 C.
`A photosensitive BCB product has recently been intro-
`duced [49].
`
`X.
`
`1946
`
`-- .
`”.%
`
`XI.
`
`S , Q
`bw
`
`&<.
`
`Iv. POLYMER PERFORMANCE
`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 [l], [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 fo; planarization is described separately below. Cured
`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
`
`MICRON ET AL. EXHIBIT 1051
`Page 5 of 13
`
`

`
`‘ e
`
`Fig. 6. Spin coating parameters for DVS-BCB application.
`
`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
`viscous materials is a complex process. The thickness, uni-
`formity, and texture of the sprayed surface are influenced by
`the viscoelastic properties of the polymer, the concentration
`of the polymer in solvent, and the critical surface tension
`at the interface [52], [53]. AT&T POLYHIC substrates are
`manufactured using a spray coating process.
`2) Curing: During PI curing (40O”Ct) two processes
`take place, the loss of N-methyl pyrolidone solvent and
`the elimination of water. Total cure times of 4 to 6 hours
`are typical. It is recommended that curing be performed in
`an inert environment. Improper or incomplete curing can
`have a pronounced effect on the proprieties of a PI film.
`Monitoring of the degree of cure using IR spectroscopy
`has demonstrated that the rate of reaction is dependent on
`both temperature and thickness [54]. PI curing can also
`be followed by monitoring electrical property changes by
`microdielectrometry [55].
`Heacock has studied the hydrolysis of PMDA/ODA, the
`reverse of the imidization reaction, at elevated temperature
`and humidity [56]. PI hydrolysis has also been discussed
`in detail by Volksen [28].
`BCB curing is a purely thermal process. No catalysts are
`required and no violates are produced. BCB’s must be cured
`in a low-oxygen (< 100 ppm 0 2 ) environment to prevent
`oxidation of the structure. Oxidation negatively impacts the
`properties of the resultant film [44]. Curing in a box oven
`requires 4 to 5 hours. A recent rapid thermal curing (RTC)
`
`process [58] has been detailed where the time of the cure
`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.
`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 polyimide
`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 bamer metallization (for A1 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 0 2 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
`
`GARROU: POLYMER DIELECTRICS
`
`1947
`
`MICRON ET AL. EXHIBIT 1051
`Page 6 of 13
`
`

`
`0
`
`2 0 0 0
`1000
`Fluence (In mJlcm2)
`
`Fig. 7. Etch rate versus fluence for several polymer dielectrics.
`
`content requires fluorine containing plasmas [44], [46].
`Metal, SiOz, or photoresist etch masks can be used.
`Laser processing has also been shown to be a viable
`technique for via formation [67]. A detailed comparison
`of direct write, scanning, and projection laser ablation
`techniques has been published [64]. The ablation rate for
`various polymer dielectrics is shown in Fig. 7.
`
`B. Polymer Electrical Properties
`1) Dielectric Constant: Dielectric constant ( E ’ ) is the most
`critical electrical parameter for a microelectronic polymer.
`The lower the dielectric constant, the faster the signal
`propagation velocity, as described by
`v p = c d 2
`(1)
`where V p is the velocity of propagation, E’ is the dielectric
`constant of the insulator, and c is the speed of light.
`A lower dielectric constant allows signal traces to be
`designed wider and the dielectric thickness decreased.
`It also allows one to maintain the same characteristic
`impedance while lowering the line resistance and cross talk.
`Good dielectric performance is obtained from a material
`whose dielectric constant is invariant with frequency and
`temperature.
`Dielectric constants of typical MCM polymers are shown
`in Table 2. Polyimides have dielectric constants in the range
`of 3.2 to 3.4. Low CTE PI’S reveal highly anisotropic
`dielectric constants (estimated 3.0(z), 3.8(z, y)) [43]. DVS-
`BCB has an isotropic dielectric constant of 2.7.
`2) Dissipation Factor: Dissipation factor ( E ” ) and loss
`tangent (?/E’)
`are also important electrical parameters,
`especially at high frequencies. Low values are indicative of
`minimal conversions of electrical energy to heat and little
`overall power loss.
`3) Dielectric Strength: Dielectric strengthbreakdown
`voltage defines the voltage that causes current to flow
`through the insulator. The materials listed in Table 2 have
`values in the range of 106 V/cm.
`
`C. Physical, Mechanical Properties
`1) Water Absorption: Water absorption impacts both the
`electrical properties and the processibility of MCM di-
`electrics. Absorption of water, with a dielectric constant of
`78, raises the dielectric constant of an insulating polymer
`and thus affects circuit performance.
`
`t t
`
`
`
`
`1 21 1, 1 21 1,
`
`t t t t
`Degreo of Planarlzalton (Ooq 8 I -
`
`
`
`t POLYMER t POLYMER
`
`x 100
`
`1
`
`Fig. 8. Planarization of a metal feature.
`
`Absorbed moisture can also cause severe damage to
`structures during fabrication. Uncontrolled outgassing of
`moisture from underlying layers of dielectric during sub-
`sequent high-temperature processing can result in blis-
`tering and delamination of the thin-film structures being
`fabricated. [50], [63], [68]. Bakeout cycles are routinely
`employed during the fabrication of MCM’s with polymers
`that exhibit significant moisture absorption. Gridded ground
`planes and in some cases gridded signal planes are used to
`allow moisture to escape [69]. It has been reported that PI
`moisture absorption increases after aging at 85OC/85% RH
`[701.
`Moisture uptake is specific to the polymer in question
`and the conditions of the experiment. Reported water ab-
`sorption values, with conditions where available, are given
`in Table 5.
`2) Planarization: Relief existing on a surface prior to
`photolithography will affect the resolution and quality of
`subsequent conductor traces. One of the main functions of
`the insulator layer is to planarize the underlying topography
`and, thus, provide a flat focal plane for the next layer. Lack
`of topographic planarity can lead to subsequent nonuniform
`metal thickness (poor step coverage). These thinned and
`weakened areas are susceptible to cracking [71].
`Rothman, in her classic paper, defined the degree of
`planarization (DOP) for a polymer dielectric as shown in
`Fig. 8 [72].
`Senturia and coworkers [73] have shown that the DOP
`obtained by fully cured films is the composite of two
`processes: drying and curing. During drying, loss of pla-
`narization occurs from shrinkage when the polymer can no
`longer flow. For polyimides, imidization and its associated
`water loss which occur during curing cause further shrink-
`age, which adds to planarization loss. Shrinkage is reported
`to be as high as 50+% depending on the polyimide. M,
`also plays a role in planarization; the lower the M,,
`the
`better the planarization.
`DVS-BCB exhibits < 5% shrinkage on cure and has a rel-
`atively low M, as applied, resulting in > 90% planarization
`from a single application. Low M, Thermids also exhibit
`excellent planarization.
`Fig. 9 reveals the degree of planarization versus the
`number of coating applications needed if the initial coating
`gives DOP = P as calculated by Chao [74].
`
`1948
`
`PROCEEDINGS OF THE IEEE, VOL. 80, NO. 12, DECEMBER 1992
`
`MICRON ET AL. EXHIBIT 1051
`Page 7 of 13
`
`

`
`Table 5 Polymer Dielectric Water Uptake
`Polymer
`Condition
`
`Water Uptake (%)
`
`Reference
`
`1.7
`1.75, 2 - 3
`0.4
`0.5
`2-3
`3.0
`1.1
`2.3
`0.5
`1.3
`1.4
`1.0
`0.8
`5.0
`1.1
`0.3
`0.23
`0.023
`
`(68)
`(29,g)
`(68)
`(29,g)
`(9)
`(g)
`(68)
`(42)
`(68)
`(42)
`(68)
`(31)
`(62)
`(9)
`(g)
`(68)
`(44)
`(76)
`
`PI-2555
`
`PI-2611D
`
`PI-2545
`PI-2722
`PIQ-13
`
`PIQ-L100
`
`EL-50 10
`
`IP-200
`PROB-400
`UR-3800
`BCB-13005
`
`a
`b
`a
`b
`C
`h
`a
`C
`a
`C
`a
`C
`C
`h
`e
`a
`d
`f
`
`(a) 2 hour water boil.
`(b) 1 hour at 50% RH.
`(c) Unknown.
`(d) 24 hour water boil.
`(e) 24 hour RT immersion.
`(f) 168 hour, 85OC, 85% RH.
`(g) Manufacturer’s data sheets.
`(h) 100% RH.
`(i) PCT.
`
`Table 6 Polymer Dielectric Planarization
`
`Polymer
`
`Planarization(”) (%)
`
`Reference
`
`t291
`30
`PI-2525
`
`P I
`21
`PI-2545
`18, 25 - 28
`PI-2611D
`t291, [621
`28
`PI-2722
`(b)
`t 621
`90
`EL-5010
`t621
`25
`IP-200
`25
`PIQ-13
`@)
`70
`PIQ-6700
`@)
`25 - 28
`t681
`PIQ-L100
`t521, t441
`90+, 95
`BCB-13005
`(a) One coating, dielectic/feature height >2:1, see reference for
`feature line width and spacings.
`(b) Manufacturers data sheets.
`
`-
`
`2
`
`0
`
`6
`8
`1 0 1 2 1 4
`4
`NUMBER OF COATINGS n
`Fig. 9. Degree of planarization versus number of coatings (initial
`coating yields DOP = P) [74].
`
`Thin lines and narrow spaces are easier to planarize [72].
`This is depicted in Fig. 10, where loss of planarization with
`increasing conductor width is clearly evident [75]. Reported
`planarization data are compiled in Table 6.
`3) Chemical Resistance: Polymer dielectrics must be re-
`sistant to the processing and cleaning chemicals used in
`the fabrication of the structures. The usual manifestation of
`poor chemical resistance is swelling, cracking, or crazing
`
`of the polymer layer. Thermosets are generally much more
`resistant than thermoplastic materials. Most of the materials
`listed in Table 2 are reported to be resistant to normal
`processing chemicals [68], [76].
`4) Mechanical Properties: The mechanical properties of
`a polymer describe how the polymer responds to stresses
`imposed upon it. Although there are many measures of
`mechanical performance, the properties reported for MCM
`polymer dielectrics usually include tensile strength (max-
`imum tensile stress that a material can withstand before
`irreversible yielding), tensile and flexural modulus (ratio
`of tensile or flexural stress to strain), and the elongation
`or strain at break (the amount the film yields before it
`ruptures).
`Polymers fall into four different classes. Soft, weak
`polymers have low moduli and low elongations; hard brittle
`
`GARROU: POLYMER DIELECTRICS
`
`1949
`
`MICRON ET AL. EXHIBIT 1051
`Page 8 of 13
`
`

`
`I
`
`DVS-BCB
`
`100
`
`CI t
`
`1
`
`100
`10
`1000
`Feature Width (pm)
`
`10000
`
`L95 um
`m(1) coat of DVS-BCB (4.5 um)
`o(2) Coats of PI-2555 (8.9 um Total)
`~ ( 2 ) Coats of PIQ-Ll10 (7.5 urn Total)
`
`Fig. 10. Planarization versus feature width 17.51.
`
`polymers have high moduli and low strain at break; hard
`strong polymers have high moduli and moderate stress
`at break; and hard tough polymers have high moduli
`and high strain at break. There are very few literature
`data on the correlation between performance or reliability
`of an MCM structure and the systematic modification
`of such mechanical properties for a given polymer or
`polymers.
`Mechanical properties are known to degrade under ther-
`mal aging, and during processing (solvent exposure, oxygen
`plasmas, etc.). For instance, cracking of PI films during RIE
`(reactive ion etching) has been observed at ion energies as
`low as 50 eV [77].
`5) Reliability Issues: Although there are no MIL-STD’s
`for MCM’s fabricated using organic dielectrics, it is com-
`monly accepted that such structures should pass temperature
`cycling, thermal shock, accelerated aging, and thermal stor-
`age testing. Test structures of Cu/PI have been reported to
`pass such testing protocol [78]-[SO]. In addition it has been
`suggested that MCM dielectrics pass THB (temperature-
`humidity-bias) testing on triple track structures [81]. Such
`structures allow measurement of leakage current and con-
`ductor corrosion [82].
`Roy [83] has recently reported that Cr/Cu/Cr PI struc-
`tures pass MIL-STD-883C. From HAST (highly acceler-
`ated stress testing) tests performed on such structures, he
`predicts > 20 years lifetime in field service. Aluminum
`structures showed failure after 1000 hours of 150°C high-
`temperature storage. The PI that was tested was not iden-
`tified.
`to
`intimately related
`The following properties are
`the reliability of a
`thin-film polymer MCM struc-
`tures.
`
`6) Thermal, Thermooxidative Stability: A material is ther-
`mally stable at a given temperature when its properties at
`that temperature do not change. Thermal stability for an
`MCM polymer dielectric needs to be in excess of the tem-
`peratures experienced during subsequent processing and/or
`repair steps. In general, the dielectric will be subjected to
`temperatures less than 35OOC during chip attach or other
`assembly processes (63).
`Thermal stability is best determined from isothermal
`thermogravametric analysis (TGA) on thicknesses similar
`to those used to fabricate the structures. Isothermal data are
`required since thermal degradation is limited by solid-state
`diffusion and therefore is thickness dependent [44], [45].
`Fully aromatic PI’S are stable in air and nitrogen at
`400°C. DVS-BCB undergoes 1% weight loss per hour at
`350°C under nitrogen.
`Polyimides that are not fully aromatic and BCB’s are
`inherently more susceptible to oxidation due to the presence
`of benzylic and/or aliphatic functionality in the cured films.
`The use of antioxidants increases the stability of DVS-BCB
`in air at high temperatures [58], [84].
`The PPQ/Cu interface reveals degradation in air above
`200” C. This has been interpreted as copper catalyzed degra-
`dation [85].
`7) T,: The glass transition temperature (Tg) of a polymer
`is the temperature where the material exhibits significant
`deformation in response to an external load. The Tg of a
`polyme