Metallizing Techniques

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102 R.E. Southward, D.M. Stoakley/ Progress in Organic Coatings 41 (2001) 99119

functionalities, which means that adhesion of a silver layeron a polymer surface can be a substantial problem as men-tioned before [1719,3235]. Thus, in selected applicationsit may be necessary to protect the surface of a silver mirrorwith an appropriate top-coating. Generating sufcient ad-hesion between the metal and the base polymer has provento be a more challenging problem that has not been solvedfor traditional deposition techniques as mentioned earlier.

2.2. First metallization studies

To our knowledge, the rst work on in situ reductionof silver(I) in polyimide lms was reported in the patentliterature by Endrey [13] in 1963 using silver(I) carboxy-late complexes. This effort is disappointing since much is

Scheme 2.

claimed in the midst of a dearth of characterization data. Alarge number of silver(I) salts of oxy acids of carbon, e.g.,formic, acetic, propionic, succinic, oxalic, maleic, furmaric,citric, etc., were said to have been added to organic solutionsof a variety of polyimides. However, in the four examplesgiven in the patent, the silver salt was either silver ac-etate or silver caprylate, the organic solvent was dimethyl-formamide (DMF), and in every case excess pyridine wasadded to aid in dissolution of the silver carboxylate com-plex and to keep the poly(amic acid) from gelling. Onlytwo poly(amic acids) were reported in the examples: theones formed from pyromellitic dianhydride (PMDA) with4,4 -diaminodiphenylmethaneand with m-phenylenediamine(see Scheme 2 for monomer structures). Endrey states thataddition of the silver salts to the poly(amic acid) leads to a


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R.E. Southward, D.M. Stoakley / Progress in Organic Coatings 41 (2001) 99119 103

metathesis reaction which gives a poly-silver(I) salt of theamic acid. This is reasonable since an amic acid carboxylategroup is less basic than an acetate or caprylate group. Suchcomplexation tends to give a gelled system since it is es-tablished that silver carboxylate complexes are dimeric in-volving the coordination of two carboxylate groups witheach silver(I) ion [36,37]. Gelation was broken down byoverwhelming the system with a silver(I) complexing agent,specically pyridine. Pyridine is well known to form a lin-ear bis complex with silver(I) [38]. Endrey states that moresolvent or pyridine or a -ketonic type compound such asethyl acetoacetate are advised to clear any gel or insolublematter that may form in the polyamide-acid salt solution.Specically, a clear viscous dope was formed with rapidstirring upon the addition of a solution of silver(I) acetate,dissolved in a large excess of pyridine, to a DMF solutionof the poly(amic acid). The polyamide-acid salt was thencast onto a glass plate and thermally cured in a vacuum ovento 300 C where the lm was held for 30 min. A polyimide

lmwas produced with embedded metallic silverparticles asshown by X-ray diffraction. Some lms were visually clearwhich was interpreted to mean that the particles thereindo not have dimensions greater than 0.8 mm. Initially, themetallized lms were not conductive. However, upon fur-ther heating in nitrogen for several hours at 275 C an elec-trically conducting surface was achieved. No measurementor mention of the reectivity of the lms was made, andno mechanical or thermal data were mentioned. No journalpublications by Endrey or others at DuPont elaborating thework in the patent have appeared. No examples of lms pre-pared with -diketonic-type compounds were presentedin the examples, even though compounds were claimed to

be solubilizing, gel-preventing ligands. Thus, there is sub-stantial uncertainty as to the full range of properties of theEndrey lms.

2.3. Silver(I) nitrate studies

A surprising aspect of the Endrey patent is that silver(I)nitrate is not mentioned as a source of homogeneous sil-ver(I) polymer solutions and lms. Silver(I) nitrate is avail-able in high purity at a modest cost; it is soluble in DMAc, N -methylpyrrolidinone (NMP), and DMF, which are com-mon and useful solvents for the management of poly(amicacid) precursors. The rst report of the use of silver(I)nitrate in a poly(amic acid) was by St. Clair and Taylor[39] in 1983 with additional, but limited, silver(I) nitratestudies appearing through 1992 [3943]. These investiga-tors examined silver(I) nitrate as the source of silver(0) inlms of the traditional poly(amic acids) BDTA/4,4 -ODA,BDSDA/4,4 -ODA, and PMDA/4,4 -ODA (see Scheme 2 forstructures). Thermal curing of the doped amic acid precur-sors gave lms which varied in reectivity with some beingverbally described as very reective. BTDA/4,4 -ODAlms were prepared with silver(I) nitrate at 1.3, 4.0, 5.3, and10.0% silver. In general, silver(I) nitrate lms were found

not to be electrically conductive at the surface; they are notconductive in the bulk since concentrations are below thepercolation threshold. Unfortunately, quantitative data wereminimal. The lack of surface conductivity was ascribed toa polymer overlayer which could be removed partially bysputtering with argon. It was claimed that if silver(I) nitratewas added in the absence of light, a conductive lm resulted[44]. No conrmation of this observation has appeared inthe literature, and we have had no experience which wouldsuggest an explanation for this result. The only quantitativereectance work with silver(I) nitrate found that the reec-tivity was maximized in BTDA/4,4 -ODA when the silver(0)concentration was ca. 5% and the lm was post-cured orsintered at 300 C for 8 h [43]. (Above ca. 5% silver the lmsare seriously degraded on thermal curing.) In this singlecase the reectivity was maximized at 48%. It is clear fromthermal and mechanical data that silver(I) nitrate adverselyaffects polyimide properties at concentrations greater thanca. 45% silver. T g for a 4.0% silverBTDA/ODA lm was

320

C while that for the parent polyimide was 286

C; fora 5.3% lm T g was 332 C. This suggests that the presenceof silver(0) and/or silver(I) in the cured polyimide inducescrosslinking. The mechanical properties were degraded withthe tensile strength of the 4.0% doped lm at room tempera-ture being 12.0ksi and the modulus being 215 ksi comparedwith 16.5 and 284 ksi, respectively, for the undoped poly-imide. The presence of silver(0) in polyimide lm reducesthermal degradative stability in air. The 10% weight lossvalue using dynamic thermal gravimetric analysis (TGA)was ca. 400 C compared with ca. 550 C for the parent lm.Thus, it appears that silver metal catalyzes oxidative degra-dation as would be expected since there is a rich literature

on silver catalyzed organic reactions [31]. Often the lmswere brittle and could not be removed from the castingplates without disintegrating. Generally, lms had lowertensile strengths and higher moduli when these could bemeasured. XPS data showed that there was much more sil-ver on the air side of the lm for BTDA/4,4 -ODA than withsilver(I) nitrate in BDSDA/4,4 -ODA even though T g forthe latter polyimide is 70 C less than for BTDA/4,4 -ODA.It was suggested that the coordination of silver to thethioether groups may account for this reduced migration.Comparison of binding energies and concentrations at themetallized air side surface for C, N, and O of the parent andmetallized BTDA/4,4 -ODA lms showed little difference.This nding was interpreted to mean that even thoughthere is considerably more silver on the air side, it mustbe either segregated on the polyimide surface or present asa very thin lm . . . . The persistence of polyimide nearthe surface prevents the silver particles from coming intocontact to form a conductive layer.

Independent of the above work, Auerbach [45] in 1984reported in a brief communication the use of silver(I) nitratein a poly(amic acid), the classic and prototypical poly(amicacid) synthesized from PMDA and 4,4 -ODA, which led toconductive lms with NMP as the solvent.


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Reduction of silver(I) triuoroacetate in BTDA/4,4 -ODAdoes not lead to any change in T g, and the mechanical prop-erties of the AgCF 3CO2 silvered lms are similar to thoseof the parent polyimide. These ndings are in stark contrastto the observations with silver(I) nitrate in BTDA/4,4 -ODAwhere T g is elevated by ca. 50 C and mechanical propertiesare degraded. SEM reveals that the 6.0% lm of Table 1has surface silver particles sizes of 125200 nm and that theparticles, while partially interconnected, are overall isolatedfrom one another, thus explaining the lack of conductiv-ity. When a 10.7% lm (not listed in Table 1) is cured at135 C for 1 h, heated over 4 h to 300 C, and just reaches300 C, the particle morphology is well established, and areectivity of 81% is achieved. The predominate particlesizes are in the range 125200nm, similar to the 6.0% lmbut less densely distributed. After remaining at 300 C for7 h, there is still a preponderance of isolated particles, andthe lm is not conductive; however, the average particlesize increases to 150300 nm. The shapes of the particles

after 7 h at 300

C suggest that there is an increase in sizedue to sintering. Several apparent necks are visible in themicrographs. Thus, particle size appears to be function of concentration and the details of the cure cycle. Additionalstudies with silver(I) triuoroacetate with poly(amic acid)sare clearly warranted and may yield exceptional lms. Inparticular, as will be seen in systems discussed later, higherconcentrations of silver (ca. 13%) and longer cure timesmay lead to lms which are both highly reective andconductive.

Table 2Selected reectivity, resistivity, and thermal data for polyimides doped with [(HFA)(1,5-COD)Ag] (cured at 100 C for 1 h, 200 C for 1 h, 300 C for 1h)

Filma Polymer:dopant moleratio (ca. %Ag) b

Percent of reectivity f

(at 531nm)Resistivity( /sq)d

T g ( C) 10% weight loss e

20 45 70 Parent Doped Parent Doped

BTDA/4,4 -ODA 2:1 (10% Ag) 65 57 45 >10 11 270 265 609 493BTDA/4,4 -ODA 4:1 (5.3% Ag) 60 50 32 NC 270 265 609 493BTDA/4,4 -ODA 8:1 (2.7% Ag) 53 44 22 NC 270 263 609 481BTDA/4,4 -ASD 2:1 (9.7% Ag) 55 51 49 >10 11 270 268 608 516BTDA/4,4 -ASD 4:1 (5.1% Ag) 52 40 31 NC 270 270 608 482BTDA/4,4 -ASD 8:1 (2.6% Ag) 46 36 18 NC 270 262 608 478DSO2DA/4,4 -ODA 2:1 c c c NC 296 293 584 474BTDA/DDSO2 4:1 c c c NC 273 249 619 586BTDA/4,4 -ODA/4,4 -ASD 2:1 (9.8% Ag) 60 52 48 NC 270 272 608 502

BTDA/APB 2:1 (8.5% Ag) 56 45 24 NC 276 280 638 478ODPA/4,4 -ODA 2:1 49 c c NC 264 268 c c

BDSDA/4,4 -ODA 2:1 (7.4 % Ag) 4 3 9 320 214 221 598 502BDSDA/4,4 -ASD 2:1 (7.2 % Ag) 4 3 7 10 4 218 221 566 469BPADA/4,4 -ODA 2:1 (xx% Ag) 55 c c >1011 219 227 c c

BPADA-4,4 -ASD 2:1 (xx% Ag) 5 c c 104 (50) 217 225 584 463

a All lms were cast and cured on glass plates.b Ag is calculated on the basis of only silver(0) and polyimide remaining in the cured lms. Ligands and ligand fragments are assumed to be

volatilized from the lm.c Not reported.d Four-point probe.e Heating rate not reported.f Reectivity data arc relative to a Perkin-Elmer polished aluminum optical mirror set at 100%.

2.5. (1,1,1,5,5,5-Hexauoroacetylacetonato)( 4-1,5-cyclooctadiene)silver(I) studies

From 19941998, Taylor and coworkers [5054] publis-hed noteworthy single-stage self-metallization work using(1,1,1,5,5,5-hexauoroacetylacetonato)(1,5-cyclooctadiene)silver(I), Ag(HFA)(COD), as the metallic silver metal pre-cursor (this complex is commercially available). Pertinentresults are summarized in Tables 2 and 3. In the solidstate, Ag(HFA)(COD) is a dimeric complex, with unusualbridging -diketonate ligands [55] as shown below.

The complex is soluble in DMAc and in poly(amic acid)matrices. In DMAc, Ag(HFA)(COD) may well be mono-meric since oxygen bridging acetylacetonate ligands arenot common, and high concentrations of a polar donor sol-vent such as DMAc may solvate and stabilize monomericstructures. The rst [5052] poly(amic acid) systems which


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Table 3Mechanical data for selected polyimide lms of Table 2 doped with [(HFA)(1,5-COD)Ag] (cured at 100 C for 1 h, 200 C for 1h, 300 C for 1h)

Filma Polymer:dopant mole ratio (ca. %Ag) b Tensile strength (ksi) Modulus (ksi)

Parent Doped Parent Doped

BTDA/4,4 -ODA 2:1 (10% Ag) 17.9 11.8 445 317BTDA/4,4 -ASD 2:1 (9.7% Ag) 16.5 15.1 379 343

BTDA/4,4 -ODA/44 -ASD 2:1 (9.8% Ag) 17.3 16.1 409 413BDSDA/4,4 -ODA 2:1 (7.4% Ag) 12.7 6.10 209 153

a All lms were cast and cured on glass plates.b Ag is calculated on the basis of only silver(0) and polyimide remaining in the cured lms. Ligands and ligand fragments are assumed to be

volatilized from the lm.

were doped with Ag(HFA)(COD) to develop reective lmsincluded: BTDA/4,4 -ODA, BTDA/4,4 -ASD, BDSDA/ 4,4 -ODA, BDSDA/4,4 -ASD, BTDA/APB, BTDA/DDSO 2 ,DSO2DA/4,4 -ODA, andtherandomcopolymerBTDA/4,4 -ODA/4,4 -ASD (see Scheme 2 for structures). Thermalcuring to 300 C (usually in air) of these poly(amic acid)

lms doped with Ag(HFA)(COD) (ca. 319% silver) gavemetallized lms which had air side reectivities of 465%.Most lms were not conductive. For all samples cured onglass plates, XPS revealed that the air side of the lm had1050 times more silver than the glass side; the glass sidesof the lms were never reective.

The specular reectance was highest (65%) with BTDA/ 4,4 -ODA lms, with the reectivities for BTDA/4,4 -ASD,BTDA/3,3 -APB, and the BTDA/4,4 -ODA/4,4 -ASD co-polymer lms being in the range 5560%. For all lms thereectivity decreased noticeably with increasing angle of incidence.

For the 2:1 (polyimide repeat unit to silver ratio)

BTDA/4,4-ODA and BTDA/4,4 -ASD lms, the theoreticalweight percent silver based on polyimide and silver alone(i.e., assuming that the HFA and COD ligands were lostfrom the system in some form) remaining after thermalcuring should have been 10.0 and 9.7%, respectively. Theamount of silver actually found after thermal curing was8.4 and 8.1%, respectively. This is in rough agreement withthat calculated on the basis of only metallic silver and poly-imide remaining after curing. The values found are lowerthan those calculated in part due to residual HFA in someform residing in the nal polyimide as indicated by the factthat uorine was found to be 2.2 and 3.6%, respectively.The analytical results indicate clearly that the metallic silverformed in the lms does not catalyze oxidative degradationof the polyimide during thermal curing. Such degradationwould not be surprising since silver is a well-establishedoxidation catalyst for organic molecules.

At an 8:1 ratio, the two cured BTDA/4,4 -ODA andBTDA/4,4 -ASD lms have calculated silver concentrationsof 2.7 and 2.6%, respectively. Surprisingly, the reectiv-ities did not diminish in proportion to the reduction of silver and remained relatively high at 53 and 46%. Noneof the BTDA/4,4 -ODA lms was reported to be elec-trically conductive; however, the BDSDA/4,4 -ODA and

BTDA/4,4 -ASD lms were surface conductive with sheetresistivities of 320 and 1 .34 .2 10 4 / sq as taken fromthe oven, and after polishing 3 and 20 /sq, respectively.The authors note that the lms with the greatest conductiv-ity were the ones with the poorest reectivities, which theyascribed to increased surface roughness of the conductive

silver(0) layer. XPS data for the 2:1 BDSDA/4,4 -ODA lmshowed extensive formation of sulfuroxygen linkages inthe sulfoxide/sulfone/sulfate region. Thus, it is clear thatsulfur is being oxidized, at least at the surface. The forma-tion of SO linkages in the polymer backbone could alsocontribute to the somewhat lower silver values found fromelemental analysis data. Milling experiments would be in-teresting to see if there was signicant sulfur oxidation inthe bulk of the lm. The authors do not make clear whetherit is silver(I) or silver metal that is causing the oxidationof sulfur. They state silver also appears to interact withthioether groups in the polymer backbone thereby causingoxidation to a sulfone-like moiety [54]. It is conceivable

that Ag(I) is actually oxidizing the sulde to a sulfoxide orsulfone, and that this is the mechanism for the productionof metallic silver. Further studies with sulde-containingpolyimides and other silver(I) additives should be pursued.

As might be expected, the presence of metallic silver ad-versely affects the thermal stability of the polymers in air asseen from the data for 10% weight loss in Table 3. Thermalstability (10% weight loss) in air for silvered lms is lowerby ca. 100 C than that for the parentpolyimide. Nonetheless,the metallized polyimide lms still have a wide thermal userange. The authors found that within experimental error T gdid notappear to change . . . [54]. Mechanical properties forthe Ag(HFA)(COD) lms were often similar to those of theparent polyimide for several systems in as seen in Table 3.Indeed, all of the lms could be folded on themselves with-out fracture. Thus, Ag(HFA)(COD) gives lms with muchbetter thermal properties ( T g and 10% weight loss) andmechanical properties (tensile strength and modulus) thansilver(I) nitrate. These results, coupled with the silver(I) tri-uoroacetate lms discussed above, demonstrate that ligandeffects are pronounced in the fabrication of metallized lms.Taylor and coworkers [51] found that curing in nitrogendid not alter signicantly the properties of metallized lmsprepared with Ag(HFA)(COD) and BTDA/4,4 -ODA.


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The SEMs for the sulfur-containing BDSDA/4,4 -ODA(2:1) and 4,4 -ASD (2:1) lms showed much larger silverparticles (100500 nm) and less uniformity among parti-cle distribution than that observed for the correspondingBTDA lms with particle sizes ca. 100 nm and uniformlydispersed at the surface. From TEM the thickness of the 2:1BTDA/4,4 -ODA silver layer was 70 nm whereas for thecorresponding sulfur-containing BDSDA/4,4 -ODA lm itwas 140 nm. These observations seem to be reversed withsilver(I) nitrate as the dopant as cited earlier. In the nitratecase, Taylor and coworkers found from XPS data that theBTDA/4,4 -ODA lm had much more silver on the airside than the BDSDA/4,4 -ODA analog. This variance wasnot addressed by the authors. The BTDA/4,4 -ODA silverlm (2:1), which is not conductive, clearly has silver par-ticles which are surrounded by polymer which insulate themetal clusters from one another. This was demonstrated bymicroscopy data which showed that the silver(0) layer wasnot continuous and from the XPS which showed the atom

percents of carbon, nitrogen, and oxygen to be near 59, 6.4,and 14%, respectively. These atom percentages are approx-imately what is expected in a parent BTDA/4,4 -ODA lm.Thus, silver(0) is not catalyzing surface polyimide oxida-tive degradation, and silver(0) migration is not sufcient tocover the surface so that particles are in contact to makethe lm conductive.

Several conclusions can be drawn from this 19941996Ag(HFA)(COD) work. First, it is possible to developa uniform surface layer of silver metal, which exhibitsmodest reectivity, on a polymer lm by thermal reduc-tion/decomposition of a dissolved silver(I) coordinationcompound. Secondly, lms that were found to be highly

reective were not conductive, whereas the lms whichexhibited no metallic reectivity were conductive. Thereappears to be an inverse relationship between conductivityand reectivity. Thirdly, the presence of sulfur linkages inthe polyimide repeat unit leads to larger surface particles of silver; however, the larger the surface particles, the lower isthe specular reectivity. The authors conclude that the devel-opment of the surface layer is a complex and subtle functionof numerous variables including polymer structure, thermalcure cycle, the amounts of oxygen and moisture in the curecycle atmosphere, the solvent, and the coordination environ-ment of the silver. Even though there are a multiplicity of variables affecting metallization, they felt able to hypothe-size that electrically conductive lms (which were not spec-ularly reective), would result if the polyimide exhibited lowT g (ca. 200 C) and contained sulfur and that specularlyreective lms (which were not conductive) would result if sulfur were absent in the polyimide regardless of T g.

The conclusions of the authors were further supported intwo subsequent papers [53,54] where three additional met-allized polyimides (BPADA/4,4 -ODA, BPADA-4,4 -ASD,and ODPA/4,4 -ODA see Scheme 2) were studied withAg(HFA)(COD) and previously studied systems were ex-amined further. Table 2 shows that for the reectivity and

resistivity data for these additional polyimides. Consistentwith their hypothesis the new sulfur-containing polyimideBPADA-4,4 -ASD with a low T g (217 C) exhibited lowresistivity and was not reective; on the other hand, thenon-sulfur-containing analog, BPADA/4,4 -ODA, had areectivity of 55%, but was not conductive. This was con-sistent with the previous ndings that conductive lmshave low reectivities. Comparison of BTDA/APB andBDSDA/APB lms via TEM showed again that the con-ductive sulfur-based lm had larger and more irregularsilver particles at the surface. This was also true for theBDADA/4,4 -ODA and BDADA/4,4 -ASD pair where theconductive silvered BDADA-4,4 -ASD lm showed largerand less uniform metal particle size. Ion milling Augerspectra for silvered conductive BDSDA/ASD and silverednon-conductive BTDA/4,4 -ODA lms show that the silverconcentration increased 100% after milling for a short time.This indicates that there is a greater excess of polyimideor polyimide fragments very close to the surface. (This

was also seen by Southward et al. with (triuoroacetyl-acetonato)silver(I) in BTDA/4,4 -ODA; vide infra.)The suggestions of the Taylor group concerning the

importance of sulfur in polymers and the glass transitiontemperatures are interesting, but the study of additional sys-tems and more characterization data are desirable. There areseveral matters which require attention. First, it is importantto follow the production of silver metal during the thermalcure. This can be done by looking at the development of specular reectivity and X-ray diffraction peaks for f.c.c.silver as a function of time and temperature during the curecycle. X-ray diffraction gives information both with regardto the formation of a metallic silver phase and to the size

of the crystallites. The mechanism of silver(I) reductionremains unclear. The electron for silver(I) reduction mightcome from the ligand, from solvent, from polymer, or fromoxygen in Ag 2O formed in situ during the cure cycle. If theelectron comes from the polymer, which seems entirely pos-sible, then changing polymer structure could have a domi-nant effect, that is more important than T g, on the resultinglms in terms of reectivity, particle size, and conductivity,T g values for the nal polyimides may well have little ef-fect on the overall migration processes since migration mayoccur for the most part while the polymer is predominantlyin the amic acid form adulterated with residual solvent. T gvalues for poly(amic acids) are much lower than for thecorresponding polyimides, and even lower where plasti-cized with solvent and the silver(I) additives. Thus, silver(0)atom/cluster migration may be completed by the time thatmobility would be governed by T g of the nal polyimide.Secondly, it is essential to have both SEM and TEM micro-graphs as a function of the cure cycle because a lm whichis very low in reectivity may have a similar amount of sur-face silver as one high in reectivity with the reduced reec-tivity being due to degradation contaminants or roughnessat the surface. Microscopy would also give denitive infor-mation concerning the details of silver(0) aggregation. It is


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conceivable that conductivity is governed by both silver(0)aggregation and surface degradation of residual polyimidewhich allows the silver particles to come into contact.Indeed, we have found this to be the case in the (triu-oroacetylacetonato)silver(I) BTDA/4,4 -ODA system dis-cussed below. Thirdly, it is a very appealing idea to havelow valent sulfur in the poly(amic acid)-polyimide systemssince sulfur could be the source of the electron for silver(I)reduction, the sulde then being oxidized to a sulfoxide andsulfone. Thus, oxidized polyimide as a sulfoxide or sulfonewould be entirely stable, i.e., we would have the polymerintimately involved in electron transfer to form silver metal,yet the polyimide would not suffer any essential damageas the polymer oxidation product itself would be a stablepolyimide.

2.6. Studies with (1,1,1,5,5,5-hexauoroacetylacetonato)-silver(I) prepared in situ

One drawback to the use of Ag(HFA)(COD) is itsinstability. The complex loses olen slowly when exposedto air over long periods of time and should be stored ina tightly closed container and kept in a refrigerator ac-cording to Partenheimer and Johnson [56], who reported itssynthesis. Also, Ag(HFA)(COD), alone and in curing lms,has a obnoxious stench associated with the readily liberated1,5-cyclooctadiene ligand. Furthermore, when we dupli-cated the work of Rubira et al. [50,51], we found that themetallizing lms (ca. 225 cm 2 in area) detached from theglass plate, except at the very edges, at ca. 200 C during thethermal cure and inated to form a single dome-like struc-ture. Ination is presumably due to the release of volatile

solvent and/or gases from ligand decomposition. Thesegases create pressure against the exible mixed poly(amicacid)-polyimide lm. Ination of the silverpolymer lm isfacilitated by partial metallization of the glass side of thelm which decreases adhesion of the curing doped lmsto the glass plate. Thus, large area lms prepared withAg(HFA)(COD) give unsightly and deleterious crease linesas the weight of the metallizing lm cannot be supportedby the pressure of the escaping gases. However, reectivi-ties of ca. 65% with the Ag(HFA)(COD)BTDA/4,4 -ODAsystem were encouraging, and AgHFA-based complexesseemed to warrant further study.

Since silver(I) complexes are often unstable both ther-mally and photolytically, Southward et al. [47,5759] sug-gested that better metallized lms might be realized if thesilver(I) complex could be freshly prepared for each lmpreparation. Further, they reasoned that this could be donemost easily by preparing the complex in situ in the samesolvent in which the poly(amic acid) was dissolved (usuallyDMAc). An example of such an in situ complex prepara-tion was described earlier and summarized in Scheme 1 forAg(HFA)(COD). The initial work of Southwardet al. [47,57]involved preparing AgHFA without COD, trialkarylphos-phines, or any other stabilizing ligands apart from what

interaction the solvent, DMAc, or polymer donor groupsmight play in coordinating to the silver(I)-HFA species.

Southward et al. [47,5759] rst studied BTDA/4,4 -ODAmetallization with the in situ preparation of (hexauo-roacetylacetonato)silver(I), without the COD ligand, inDMAc (see Scheme 1). It was found that silver(I) acetate,which is insoluble in DMAc, dissolved within minutes af-ter adding ca. 1.1 equivalents of hexauoroacetylacetone(HFAH). Addition of BTDA/4,4 -ODA (1215% by weightin DMAc) to the solution of AgHFA gave a clear, homo-geneous, yellow silver(I)-doped solution. Doped poly(amicacid) lms were cast at thicknesses (ca. 375500 mm)to give cured metallized lms near 25 mm. Section A of Table 4 shows that lms produced with 5.312.8% silver arevery reective, with the maximum reectivity being signif-icantly greater than that reported by Taylor and coworkerswith the isolated Ag(HFA)(COD) complex. The reectivitydecreases with increasing angle of incidence, which may bemostly determined by the presence of signicant amounts

of polymer at the surface which strongly absorbs in the visi-ble. T g for the metallized lms is close to that for the parentpolymer as are the moduli and tensile strengths. This indi-cates that there is minimal damage to the essential polyimidestructure when silver(I) undergoes reduction upon heatingto 300 C. None of the lms was conductive even when thesilver concentration was increased to ca. 18%. Curing undernitrogen gave essentially similar lms as seen in Section B of Table 4. Also, the use of silver(I) uoride rather than silver(I)acetate gave similar lms as seen in Section D of Table 4.

SEMs showed that there were uniform globular particlesof silver metal formed at the air side surface of the lmswith particle sizes of ca. 6070nm. This is very similar

to the result reported by Taylor and coworkers using solidAg(HFA)(COD), but better reectivity was realized. Theparticle morphology is similar to what occurs via the islandgrowth mechanism with vapor deposition techniques. Thelack of conductivity is consistent with polyimide separat-ing the metal particles. SEM data shows clearly that silverparticles are not in contact. Holding the lm at the nal300 C cure temperature for 7 h does not induce conductiv-ity. Southward et al. with Taylor and coworkers suggest thatthere is an overlayer of polyimide at the surface. XPS dataalways show substantial C, N, and O at the surface in a atomratio similar to the parent polymer. This overlayer wouldaccount in part for the angle dependence of the reectiv-ity as the pathlength through absorbing surface polyimideincreases with the angle of incidence. Also, the absorptionof light by any overlayer and near surface polyimide wouldlimit the maximum reectivity, and the observation of ca.80% reectivity may not be limited by silver particle mor-phology but by absorbing surface polyimide. Many experi-ments with varied concentrations and cure cycles never leadto a reectivity higher than 82%. Surface metal morphologydoes not seem to be the main problem in limiting reectivitysince commercial glass silvered mirrors have a similar sur-face roughness. Interestingly, TEM data for a 9.9% AgHFA


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silvered lm showed that adjacent to the air-side-metallizedsurface there are several hundred nanometers which are sub-stantially free of silver(0) particles; this depletion zone,for which we have no explanation at present, was alsoobserved in Ag(HFA)(COD)-metallized BTDA/4,4 -ODAlms reported by Rubira et al. [50,51]. Observation of thedepletion zone indicates that the in situ method of gener-ating AgHFA is essentially equivalent to adding the solidAg(HFA)(COD) complex, except that better reectivitieswere observed. The TEM micrograph clearly indicates thatthere is no continuity of the metal surface. SEMs of theglass side of the lms always showed less silver with moreirregular particle sizes. The glass side for this 9.9% silverlm is much richer in uorine than the air side via XPS,18.3 and 3.2%, respectively. XPS data show that the uorineis most likely due to CF 3 groups with a C 1s binding en-ergy of ca. 293 eV and a uorine binding energy of 688 eV.There is no evidence for an inorganic uoride such as thatwhich might arise from the formation of silver(I) uoride.

Thermal data for the silvered lms (Section A of Table 4) show that metallization of the polyimide does notdo damage to the bulk polymer properties. T g for all lms,regardless of concentration of silver, is very close to thatof the parent. Similarly, the coefcients of linear expansionare the same as that for the parent, consistent with nanome-ter size silver particles which are not in contact with oneanother. One thermal property of the metallized lms thatis degraded is the temperature at which 10% weight loss isobserved. Weight loss in air for silvered lms is ca. 150 Clower than for the parent; the metallized lms are ca. 100 Cmore stable in a nitrogen atmosphere. The thermal stabilityof the silvered lms is also shown by elemental analysis

data (Section A of Table 4). The silver found in the metal-lized lms is close to that calculated for the silver additivedecomposing to silver metal and volatile ligand productswhich exit the lm on heating. The correspondence of calculated and found silver clearly indicates no massivepolyimide oxidative degradation as might be expected.

As mentioned above, when a BTDA/4,4 -ODA-AgHFAresin is cast and thermally cured on a glass plate, the met-allizing lm begins to lift uniformly from glass plate inthe temperature regime where silver(I) is visually seen toundergo reduction (ca. 180200 C). Indeed, the TGA curvefor the 1,5-cyclooctadiene adduct of (hexauoroacetylace-tonato)silver(I) [60,61] shows early loss of COD followedby reductive decomposition of AgHFA over the temperaturerange 160200 C. This is the same range where the lminates and lifts from the plate. The lm remains rmly ad-hered at the edges, and thus the curing lm forms a dome-likestructure above the glass plate. The detachment of the lmfrom the glass plate is presumably due to two effects. First,gas evolution arising from the release of residual solventand/or gaseous products creates pressure on the glass side of the lm. Normally, such gases permeate through an undopedBTDA/4,4 -ODA polymer which adheres rmly to the glassplate throughout the entire cure cycle. This adhesion is

presumably due to the fact that the neat BTDA/4,4 -ODApolymer bonds strongly to the glass surface via reactionin part with surface SiOH entities. Thus, no bubbling isobserved for silver-free lms. However, and secondly, theformation of silver metal at the glasspolymer interfaceeliminates the strong adhesion between the glass plateand the curing polymer since silver(0) is a passive andnon-oxophilic metal. Thus, there is little tendency for silverto adhere to the glass via AgOSi bonds, and the curingpolymer lm rises from the glass plate with the increas-ing gas pressure since the polymer is still very exibleat 200 C as the partially imidized poly(amic acid) whichis plasticized with residual solvent and silver(I) additive.Eventually, at temperatures beginning near 225 C the lmrelaxes back onto the glass plate, so that if one had notobserved the lm having been inated-off the plate duringthe cure cycle, one would not have known that the lm hadbeen anything but resting at against the plate during theentire cure cycle. This detachment is a serious problem for

coatings which are to be cured in place. Detachment is nota problem for lms which are ca. 225 cm 2 in area or smallerand are to be used apart from the substrate on which theywere cured. However, lms with larger areas lift from theplate in sections which leaves crease lines. This detachmentproblem was been solved by Southward et al. by simplycasting silver(I)-BTDA/4,4 -ODA lms on undoped, fullyimidized parent BTDA/4,4 -ODA lm bases. Data for rep-resentative lm-on-lm composite lms are shown inSection C of Table 4. These composite lms remain rmlyadhered to the glass base and give smooth and uniformlyat-metallized panels. Furthermore, there is outstandingadhesion both at the polymerpolymer interface and at the

metalpolymer interface. The lms have strongly adheredsurface metal layers and are completely stable to removalof silver by a variety of adhesive tapes as per the ASTMtesting protocol. The polymerpolymer interface never ex-hibited any signs of separating even with soaking in waterfor months. While fully imidized BTDA/4,4 -ODA is gen-erally insoluble in DMAc and other organic solvents, it maybe that the strong polymerpolymer interface is due to thefact that on standing for 18 h at room temperature beforebeing subjected to thermal curing a very small portion of the doped BTDA/4,4 -ODA lm dissolves into the base lmto give excellent adhesion, perhaps involving transimidiza-tion between the two layers. Since T g for BTDA/4,4 -ODAis ca. 275 C, and the composite-metallized lms are curedsome 25 C above T g , the two polymer lms may also un-dergo thermal welding. Studies by Kramer et al. [34] onPMDA/4,4 -ODA, which has a T g of 380 C, showed thata second PMDA/4,4 -ODA lm cast on a fully imidizedPMDA/4,4 -ODA base lm gave a very sharp interfacialboundary, and the layers were easily peeled apart. Theyconcluded that strong interfacial bonding occurred onlywhen an inter-penetration layer of more than 50 nm oc-curred; with a diffusional distance of 200 nm the compositelm exhibited the same strength as the bulk material. TEM


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data, presented later in this paper, show a distinct boundarybetween the silver-containing BTDA/4,4 -ODA layer andthe BTDA/4,4 -ODA base layer. Thus, it appears that thereis minimal diffusion of silver atoms and clusters into thebase BTDA/4,4 -ODA lm. The mechanical and thermalcharacteristics for the lm-on-lm samples are essen-tially those of the parent polyimide. This lm-on-lmapproach minimizes the silver required for the formation of a reective surface.

Sawada and Ando [62] have been interested in thesynthesis and properties of polyimides derived from2,2 -bis(triuoromethyl)-4,4 -diaminobiphenyl (TFDB) be-cause of their potential for optical communication appli-cations. For example, the polyimides derived from thedianhydrides PMDA or 6FDA and TFDB as the diamineshows high transparency over 400800 nm (visible) andlow optical transmission losses in the near-infrared region,as well as low a dielectric constants, low refractive in-dices, and low water absorption. Waveguides constructed of

these materials have optical losses of less than 0.3 dB/cmat 1300 nm, and the increase in optical loss was less than5% after heating at 380 C for 1 h and at 85 C with a rela-tive humidity of 85% for over 200 h. The incorporation of nanometer-sized metal particles into uorinated polyimidesto effect property modication was pursued.

Silver(I) acetylacetonate with hexauoroacetylacetonewas added to the poly(amic acid) of PMDA/TFDB to givea homogeneous solution from which lms were cast. TheHFAH was added to improve the solubility of the metallicdopant. No quantitative data were given with regard to theamount of HFAH added, and thus their experiment cannotbe duplicated from their published report. Thus, the active

dopant species is (hexauoroacetylacetonato)silver(I) asthe acetylacetonate ligand would be exchanged via protontransfer from the much more acidic HFAH to the basicacetylacetonate ligand. PMDA/TFDB lms were cast asdoped poly(amic acid) solutions and cured on fused silica.All lms were cured under nitrogen from 70 to either 300or 350 C at a heating rate of 4 K/min. (This is rapid com-pared with previously described work.) Ten percent weightloss was done only under nitrogen and was ca. 100 C lessthan that of the parent polymer. Glass transition tempera-tures were not determined. The silvered lm was said tobe fragile although no mechanical measurements werereported. After curing to 300 C the lm appeared reddishbrown on both the air and silica sides. However, after cur-ing to 350 C a lustrous metallic surface appeared. Thelms were not conductive, and no specular reectivity mea-surements were made. The goal appeared to be the prepa-ration of lms in which there is a uniform distribution of nanometer-sized metal particles evenly dispersed through-out the lm. X-ray diffraction patterns for lms cured to300 and 350 C were sharp and indicated only f.c.c. silvermetal. From the Scherrer equation the crystallite size of the350 C lms was ca. 17 nm. No peaks for silver(I) oxideor silver(I) compounds were observed. TEM data showed

silver particle sizes from 10 to 30 nm uniformly dispersedthroughout the lm. It was especially interesting to notethat at the air side surface of the 350 C lm there was anca. 30 nm band of silver particles ca. 50 nm below the lmsurface. Such an overlayer has been suggested the work of Taylor and coworkers and Southward et al. explaining thelack of surface conductivity and the decreasing reectivitiesof non-conducting lms with increasing angle of incidence.

The work presented to this point with an HFA complex of silver(I) was encouraging with regard to preparing silveredlms which had metallic-like specular reectivity, but dis-couraging with regard to preparing lms with even modestsurface conductivity. However, on making a subtle changeto from hexauoroacetylacetone to triuoroacetylacetone asthe -diketonate ligand source dramatic metallization resultswere eventually achieved as described next.

2.7. (1,1,1-Triuoroacetylacetonato)silver(I) studies

In 1997, Southward et al. [63,64] reported the initialmetallization studies with (triuoroacetylacetonato)silver(I),AgTFA, prepared via the in situ approach used for AgHFA.AgTFA was rst isolated in 1981 by Wenzel and Siever[65]. However, it proved extremely difcult to prepare in areproducible manner and is photolytically unstable. Thus,with AgTFA an in situ preparation for each individual lmpreparation was invaluable, assuring that lm variation wasnot a function of degraded complex.

As selected data in Table 5 show, the rst AgTFA lmshad only modest specular reectivity (2952%) relative tothe AgHFA analogs discussed earlier for which specular re-ectivities at similar silver concentrations were in the range

7582%. All AgTFA lms cured only to 300

C for 1h hada metallic green luster and showed reectivities at 20 Cin 3035% range. However, while reectivities were disap-pointing, for the rst time consistent surface conductivitywas obtained. Conductivity for AgTFA lms is a functionof silver concentration and the thermal cure cycle; only athigher silver concentrations, ca. 12% or higher, and highernal cure or post-cure temperatures of 340 C do the lmsbecome conductive as taken from the oven. (None of theearlier lms was cured for longer than 1 h at the nal tem-perature of 300 or 340 C.) They also exhibited an increasein specular reectivity, which now did not diminish as afunction of increasing angle of incidence. This is to be con-trasted with the AgHFA analogs for which surface conduc-tivity was never observed even with heating for 7 h in air at300 C; the same lack of conductivity was true for metallizedlms prepared with the isolable Ag(HFA)(COD) complex.All AgTFA-metallized lms are exible and can be creasedtightly without breaking. The surface silver is well adheredto the polymer as observed in analogous AgHFA systems.

The linear coefcients of thermal expansion (CTE) forthe 10.6, 13.3 and 12.0% AgTFA lms of Table 5 are 28,33 and 36 ppm/K, respectively. These are lower than theCTE for the parent polymer at 43 ppm/K and lower than


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Fig. 2. SEM of selected silver-polyimide lms: (top) Ag(CF 3CO2)-BTDA/4,4 -ODA 10.7% silvered lm cured to 300 C for 1 h (air side), R = 65%(97% polished), not conductive; (middle) AgHFA-BTDA/4,4 -ODA 10.7% silvered lm cured to 300 C for 7 h (air side), R = 62% (the maximum Rwas 75% after curing for 1 h at 300 C.), not conductive; (bottom) AgTFA-BTDA/4,4 -ODA 12.8% silvered lm cured on a BTDA/ODA base to 300 Cfor 7 h (air side), R = 95%, resistivity < 0.1 /sq.

by more than a few degrees. This suggests that the bulk polymer structure is not compromised by the reduction of silver(I) and the formation of silver(0). However, the for-mation of metal clusters in the bulk of the polymer as wellas on the surface diminishes the high temperature thermaloxidative stability of the hybrid lm. While in a nitrogen

Table 7X-ray photoelectron spectroscopic surface composition for selected (1,1,1-triuoro-2,4-pentanedionato)silver(I)-BTDA/ODA (10.7% Ag) lms of Fig. 1

Wt.% of silver

Thermalhistory a

Reectivity(%) at 20

Resistivity( /sq)b

Filmsurface

Ag Relative atom percent

F C O N

Parent 300 for 1 h NA NA Air 0 0 78 16 5.410.7 275 for 0 h 20 NC Air 2.8 0.0 78 15 4.410.7 300 for 0 h 28 NC Air 4.0 0.47 78 13 4.510.7 300 for 5 h 79 PC Air (glass) 27 (3.7) 0.33 (4.5) 45 (69) 26 (19) 1.6 (3.1)

a All cure cycles involved heating from 22 to 135 C over 20 min and holding for 1 h followed by heating over 240min to 300 C and holding at300 C for varying times. Samples were removed at various times in the cure.

b Four-point probe. PC: partially conductive with some but not all portions of the lm registering conductivity; NC: not conductive; NA: not applicable.

atmosphere the temperature at which there is 10% weightloss is not vastly different from that of the undoped poly-mer, in air there is a reduction in stability with 10% weightloss temperature, i.e., ca. 150200 C lower than the control.Nonetheless, the thermal stability of the mirrored lms inair is more than adequate for most purposes.


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Fig. 3. Development of reectivity as a function of temperature/time for 13% silver(0) BTDA/4,4 -ODA lms with varying silver(I) additives. Thesilver(I) precursors are listed at the right of the plot. Time zero is after holding the lm at 135 C for 1 h; from 135 to 300 C the temperature increase0.69 C/min; at 300 C the temperature remained constant.

The linear CTE for the three lms of Table 6 are 3334 ppm/K. These are signicantly lower than for the parentpolymer at 43 ppm/K and than for the analogous AgHFAlms at 43, 44 and 43 ppm/K for 5.0. 7.4 and 9.9% silver,respectively (Section A of Table 4). Thus, as mentionedpreviously, in the conductive AgTFA lms we may be seeing

a hybrid value for the CTE reecting the fact the surfacesilver aggregates are in contact with one another. Again, thevariation in CTE values between the AgTFA and AgHFAsystems demonstrates the pronounced differences that canarise due to subtle ligand effects.

For metallized lms of Table 6 the tensile strengthsare, within experimental error, those of the parent. Themodulus for the two lms cast on a glass plate are ele-vated ca. 5% while the value for the silvered lm cast ona BTDA/4,4 -ODA base is virtually the same as for theundoped polyimide.

The AgTFA complex is introduced to the poly(amicacid) resin via an in situ synthesis. While the preparation

of solid AgTFA has been reported in the literature [65], itis unstable and difcult to prepare reproducibly. Thus, wechose to synthesize AgTFA in poly(amic acid) solutionsvia the in situ reaction of silver(I) acetate, AgOAc, andTFAH, both of which can be obtained in pure and stableform. Combining AgOAc, TFAH, and BTDA/4,4 -ODA inDMAc under ambient conditions leads to formation of ahomogeneous solution of the AgTFA complex. The basicityof the acetate anion and the large formation constants as-sociated with metal -diketonate complexes lead to protontransfer from TFAH to the acetate anion giving AgTFA and

acetic acid. Even though AgTFA is not isolated, we can beassured that this complex is formed. First, Ag AgOAc byitself is not soluble in DMAc. Second, when the poly(amicacid) is added to AgOAc alone, immediate polymer gela-tion occurs. The pathway for gelation is deprotonation of the aromatic carboxyl groups of the poly(amic acid) via

transfer to the more basic acetate ions of AgOAc. Carboxy-late groups of the polymer then coordinate to Ag(I) ions toform the extended gel network. It is well known that silvercarboxylate complexes are dimeric involving the coordina-tion of two carboxylate groups [36,37]. If TFAH is added toAgOAc before the addition of the poly(amic acid), no gela-tion occurs, and the insoluble AgOAc dissolves. We choseAgOAc as the precursor to formation of the in situ AgTFAcomplex because the acetate salt is readily available in highpurity, is thermally and photochemically stable, and is nothygroscopic. We found that TFAH/Ag(I) ratios of 1.35:1rather than 1:1 gave doped solutions which were somewhatless viscous and more easily processed.

In summary, BTDA/ODA polyimidelms can be preparedfrom single phase silver(I) acetate-1,1,1-triuoro-2,4-penta-nedione-BTDA/ODA solutions cast and cured either onglass plates or a parent polyimide base. Depending onconcentration and thermal conditions, metallized lms canbe fabricated with excellent specular reectivity, surfaceconductivity, outstanding metalpolymer adhesion, andintact mechanical characteristics. This lm-on-lm app-roach minimizes the silver required for the formation of areective surface and assures composite polymer propertieswhich are those of the parent polyimide.


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3. General conclusions

It now appears certain any silver(I) compound whichis soluble in a solution of a poly(amic acid) will lead tothe incorporation of metallic silver in a polyimide lmafter an appropriate thermal treatment. However, there arepronounced ligand and anion effects on the reduction of silver(I) and subsequent aggregation of silver(0) particles,and therefore, the quality of the silver surface with regard tospecular reectivity and electrical conductivity varies enor-mously. These effects are illustrated in part in Figs. 2 and 3.The SEM micrograph of Fig. 2 shows the prominent effectthat ligand variation has on silver particle morphology atthe reective air side surface of the lm. Fig. 3 shows theemergence of the air side reective surface (in percent spec-ular reectivity) as a function of cure time and temperature.It is apparent from the curves that the triuoroacetato andhexauoroacetylacetonato complexes of silver(I) metallizemuch earlier than the triuoroacetylacetonato complex, and

that the former two additives lead to larger discrete sil-ver particles at the surface (Fig. 2). However, these largerparticles are isolated from one another by the dielectric poly-imide and hence are not conductive. The (triuoroacetyl-acetonato)silver(I) complex with BTDA/4,4 -ODA givesmuch smaller particles throughout most of the cure cycle,which is clear from TEM micrographs and line broaderX-ray diffraction patterns which are not shown [72]. Thesesmaller silver(0) particles appear to catalyze surface poly-imide degradation after the lm is at 300 C for ca. 23 h.This polymer degradation process allows metal particles tocome into contact giving a conductive surface. Figs. 2 and3 are simply meant to reinforce the point that metallization

effects are not well understood. The mechanism of silver(I)reduction and factors affecting aggregation and migrationremain unclear; the roles of ligand, solvent, and polymerfunctionality in the reduction and aggregation processesneed systematic study. Nonetheless, synthetic pathwaysnow exist to fabricate-metallized lms and coatings frompassive metals.

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