CA1092009A - Etchant and process of etching - Google Patents

Abstract

ETCHANT AND PROCESS
Abstract of the Disclosure This invention is directed to an etchant and to a process for its use. The etchant comprises a synergistic combination of hydrogen peroxide and molybdenum as oxidants in acidic solution.
The combination of oxidants provides for sustained etching at an exalted rate. The etchant is useful for etching metals, especially copper and its alloys, and is particularly useful in the manufacture of printed circuit boards.

Description

-- . l ~31Z~)~9 Background of the Invention .
1. Introduction This invention relates to etchants comprising a syner-gistic combination of hydrogen peroxide and molybdenum as oxi-dants in acidic solution.

2. Description of the Prior Art The activation of relatively dilute mineral acids with an oxygen release compound is known in the art and described, for example, in Plating, "Surface Treatment of Metals with Per~
oxygen Compounds", Vol. 42, pg 561 (1955). In the Plating publi- -cation, it is taught that mineral acids such as sulfurie acid, nitric acid, acetie acid and phosphorie aeid may be aetivated with oxygen release eompounds sueh as hydrogen peroxide or-other peroxy, persulfate or perborate eompounds. The oxygen release eompound enhances the etch rate thereby permitting a decrease in the concentration of aeid.
Aetivation of dilute sulfurie aeid with hydrogen perox-ide is most attraetive due to low cost and ease of eleetrolytic copper recovery from the spent etchant prior to disposal. How-ever, the utility of such hydrogen peroxide etching solutions is reduced somewhat due to eatalyzed deeomposition of hydrogen ' peroxide eaused by etched metal ions or other transition metal ions in solution and a slow eteh rate, typieally 1.4 mils of i copper from 1 ounce eopper elad laminate in 10-25 minutes at ,l 120F. To enchance eteh rate, more eoncentrated solutions of i ,:
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hydrogen peroxide have been used, but concentrated peroxide solutions are hazardous to health and safety. In addition, such etchants have not been successfully used with tin dissimilar metal etch resists because they attack tin, especially immersion tin.
To limit metal ion catalyzed decomposition of hydrogen peroxide, a number of stabilizers are used in the prior art. A
variety of such stabilizers is disclosed in U.S. Patents Nos.

3,293,093; 3,341,384; 3,407,141 and 3,668,131. A preferred class of stabilizers is disclosed in U.S. Patent No. 3,801,512.
The stabilizers disclosed in said patent are the arylsulfonic acids. `~
Though stabilized acid solutions activated with hydro-gen peroxide are improved over their unstabilized counterparts, nonetheless they still possess disadvantages which limit their use, particularly for the manufacture of printed circuit boards. ~ -Summary of the Invention In accordance with the subject invention, a new etchant `
is provided which comprises a synergistic combination of oxidants ~;
; 20 comprising hydrogen peroxide and a molybdenum compound in an acidic solution. The hydrogen peroxide provides sustained etching-with the advantages and disadvantages described above. In the ` acid/peroxide environment, the molybdenum etches at a substan-tially greater rate and therefore exalts the rate to a desirable level for commercial use. The molybdenum cannot be used as a sole oxidant because it is rapidly depleted as it is reduced to a lower valent form as etching proceeds. It is therefore incap-able of providing sustained etching. Though not wishing to be bound by theory, it is believed that the combination of the peroxide and ,:

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the molybdenum 1~ a Qynergistlc . comblnatlon becau~e both are believed to etch though the peroxide in the acid environ-ment is believed to provide the secondary function of oxidizing Imolybdenum to a higher valence capable of etching metals in the 5 ll acidified peroxide environment.

¦ Description of the Preferred Embodiments I
¦ It is known that molybdenum has a valence of +6 in its l most stable valence form but is reducible to the lower valence ¦Iforms of any of +5, +4, +3 or ~2. It is also known that Imolybdenum~ in acidified peroxide environments,forms permolybdate.
Encyclopedia ~ritannica, Vol. 15,1959 Edition r pp 682-683 ~
Recognizing the above, it is beiieved that etching takes place through several mechanisms. First, the permolybdate and the ~ -hexavalent molybdenum oxidize the metal etched with concomitant i 15 '' reduction of the lybdenum to a lower valence state which may be ! any one or more of the +5 to +2 forms. The peroxide also oxidizes the metal etched in accordance with known reactions. In addition, it is believed that the peroxide performs the additional role of reoxidizing the reduced molybdenum back to hexavalent 20 1 form which in turn is converted to permolybdate. Therefore, ,- , I . .
moly~denum available for etching is not consumed as etching , proceeds and need only be replenished in incidental amounts sufficient to compensate for that lost through drag-out. As to ~; I the peroxide, it is consumed and should be replaced in amounts 25 Il approxlmately sufficient to replace that consumed by etching and by reoxidizing the molybdenum.
The peroxide content of the etchant may vary within relatively broad limits such as from 0.1 to 10 moles per liter ..

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~2~9 of solution. However, because peroxide i5 a relatively dangerous material to handle, its concentration is preferably maintained relatively low and preferably varies between about 0.5 and 2.5 ~'moles per liter.
As above, the molybdenum compound is added to the etchant as a synergistic co-oxidant with the hydrogen peroxide to exalt the etch rate. The particular molybdenum compound used does not appear to be critical provided it is sufficiently soluble in l solution and is oxidized to a form that etches metal in the etchant environment. Preferably, the molybdenum is added as hexavalent molybdenum but may be added in a lower valence form as the peroxide will oxidize it to its higher valence form. The preferred molybdenum compound is sodium molybdate. Other useful molybdenum co~pounds include ammonium molybdate and molybdic acid.
; The concentration of the molybdenum compound is not critical, it having been found that for immersion etching, the etch rate increases with small additions of molybdenum and levels off as concentration increases. For etching copper, some exaltation of the etch rate occurs when the molybdenum concentra-tion is as low as 0.01 moles per liter (as molybdenum metal) and rapidly increases as the concentration increases to about 0.1 to 0.15 moles per liter. Thereafter, the etch rate continues to increase, but at a less rapid rate to a maximum ra~e obtained at be~ween about 0.4 to 1.0 moles per liter dependent upon numerous 25 ~ variables such as temperature, peroxide content, dissolved metal content and the like. For spray etching, an exalted rate occurs at higher concentrations of molybdenum than for immersion etching.
Based upon the above, i~ should be apparent that the molybdenum content may vary within broad limits provided its 3 concentration is sufficient to exalt the rate to that desired.
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.. , _5_ . i 1~2~9 Preferably, the concentration varies between 0.01 and 1.0 moles per liter and more preferably between .02 and 0.5 moles per liter.
With regard to the above concentrations for both the peroxide and the molybdenum, it has been found that to obtain satisfactory etching, the molar ratio of the peroxide to the molybdenum should be at least l:l and more preferably, at least 1.2:1. The upper limit to this ratio is of less importance.
The acid used is in part dependent upon the metal to be etched. For example, for etching copper, sulfuric acid is preferred because it is economical, yields copper in a con-veniently recoverable form and provides overall satisfactory results. As further example, hydrochloric acid would be pre-ferred for titanium. For slow or mild etching, a weak acid such as acetic or phosphoric might be used. In addition, the acids may be used in various combinations with each other. For pur-, poses herein, the preferred acid is sulfuric. The concentration of the acid may vary within broad limits provided it is present in an amount sufficient to reduce the pH of the etchant to below 7. In this respect, it has been found that for the etching of ' copper using sulfuric acid as the acid, etch rate is not apprec-iably increased as the acid concentration increases above 0.1 N.
Preferably, the acid content of the etchant varies between about .OlN and 2.5N but more preferably varies between about 0.5 and 1.5N.

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0~9 It 18 of lntere~t to note that ln an etchant stabilized wlth an arylsul~onlc acld, a8 the concentration Or molybdenum lncrease~ up to about 0.75 moles per llter, the normal peroxlde ¦ con~umptlon 1~ reduced. Thl~ 18 ~uggestive that the molybdenum 5 11 18 the prlmary etchant and the peroxide ConBumptiOn i8 not due to etchlng, but rather to oxldatlon Or the reduced rorm Or ¦l ~olybdenum.
¦ In additlon to ~xaltlng rate, molybdenum appears to ¦ lnhlbit attack on solder plate in contact wlth the metal etched.
, Thls 1~ true only rOr those etchants utllyzlng an acld th~
itselr does not normally attack solder plate. For example, molybdenum will lnhlblt attack on aolder plate ln an etchant u~lng ~ulruric acid but wlll not lnhiblt attack on aold~
~ plate 1~ nltric acld 1~ ~ubatltuted for the ~ulrurlc acid.

In accordance wlth the most prererred embodlment Or the !
lnvention, the etchant i8 stabillzed to prevent catalytic I decomposition Or the peroxlde caused by metal lmpurities. Though most stabillzerc known to the art may be u~ed, the prererred stabllizers ar~ arylsul~onlc acids or salts thereor as diacl~ ed ln the absve-noted U. S. Patent No. 3,801,512. A prererred ~tabllizer i8 phenol ~ul~onic acid. Other stabllizers that may be u~ed lnclude Yulroaallcyllc acid and toluene ~ulfonlc ' acld. The concentratlon Or the ~ulronlc acld may vary from a~
,l low a~ 1 gram par llter Or solution to the solu~lllty limlt of 1 the ~tablllzer but prererably varie~ between 3 and 30 g~ M8 per llter o~ ~olution.
Using the etchant Or thls invention to etch copper, at 120~F, the bath wlll dla001v~ about 70 to 80 gram~ of copper per lltor. Malntalnlng the hydrogen peroxlde concentratlon ' withln 50-100~ o~ lts lnltial bath makeup and at all timeo in ' . I , Il -7-!
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~9~ 9 an amount sufficient to maintain a ratio of the peroxide to molybdenum of at least 1:1 permits continued operation until saturation with dissolved copper. Allowing the bath to cool to room temperature induces crystallization of copper compounds which may be recovered by filtration dependent upon the acid used. Readjustment in the concentration of the components of ; the etch solution makes the same suitable for reuse.
It is of interest to notethat as etching proceeds, at the interface of the etchant and copper where etching is taking place, a deep blue layer forms which is the color of molybdenum in a lower valence form. This is suggestive that molybdenum has been reduced by oxidation of the copper layer and dis`solution of the same. This deep blue color will persist at the interface unless the etchant is stirred. If stirred, the color is lost which is further suggestive that the reduced molybdenum has been reoxidized to hexavalent form by the perox-` ide. Moreover, if the peroxide isnot present in solution, etch-ing will proceed but only for a short period of time and there-after etching stops. While etching is taking place, the entire solution becomes deep blue establishing that molybdenum does etch copper but is not reoxidized in the absence of the peroxide.
The etchants of this invention are used for the same purposes as similar etchants of the prior art. They are particu-larly useful for the manufacture of printed circuit boards and for chemical milling. Procedures for the formation of printed circuit boards are described in numerous publications, for : .': , example, in Coombs, PRINTED CIRCUITS HANDBOOK, McGraw-Hill Pub-lishing Company, New York, 1967, Chapter 5.
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In a typlcal process for the manuracture Or prlnt~d clrcult boards, a aultable base material ls selected ~uch as copper clad epoxy. Hole~ are drilled at approprlate locatlons on the board and the walls Or the hole~ are metal}lzed such as wlth copper to provlde electrical contact between the two ~urraces o~ the ba~e materlal. Methods ror metalllzation are ; known and include the ~tep~ or cleanlng, catalyzlng and ~lectro-less copper depo~ltion.
Fbllowlng metalllzatlon o~ the hole~, utlllzing a proce~
known a~ pattern platlng, a conductor pattern 18 rormed on the copper by appllcatlon o~ an organlc resi~t materlal whlch may b~ either a photoreslst or a screen re~l~t dependent upon de~lgn and derinitlon. The re~ist coat~ the copper that i8 not part Or the conductor pattern and leaves the copper bare ln a conductor ~pattern. The thlckness Or the conductor pattern i8 then increased using electrole~s and/or electrolytlc copper platlng procedures. Followlng the ~tep~ Or copper platlng, a di~slmllar metal etch re~ist such aa ~older i~ applled over ~ th0 copper in the conductor pattern to protect the same fro~
~ub~equently applied etchant3, the organlc re~i~t i~ removed exposing the ~Iwanted copper (not part of the conductor pattern), and the unwanted copper 1~ di~solved with a suitable etchant for the copper ~uch ao that dlsclo~ed herein whlle the conductor pattern 1~ protected rrom the etchant by the dl~lmllar metal etch re~ist. Typlcally, the di~slmllar metal ~tch r~ist 1 3elected not only for lt~ ablllty to with~tand the etchant, but al~o ror lts ~olderablllty 80 the electriaal connectlons can be soldercd directly to th~ etch re~lst.
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An alternative to pattern p~ tlng 1~ a proces3 known a~ panel platlng. Thi~ procedure 18 Blmllar to pattern platlng except that the copper 1~ built up to full thlckne~s prlor to appllcation Or the re~lst. The remalnlng step~ are e3~entlally the same. The proce~s 1~ le~ deslrable a~ it lnvolves deposltion and removal o~ larger quantltle~ o~ copper and hence, jl is less economlcal.
Th~ lnventlon will be better under~tood by rererence to ~ the ~ollowlng examples:

I Example~ 1-7 ` Con~tituent 1 2 3 4 5 6 7 Sulruric acid (50~)(1) ml 200 200 200 200200 200 200 Hydrogen Peroxlde (35%) ml100 100 100 100100 100 100 ; Sodlum Molybdate (gm) 0 1 5 10 20 50 100 Phenolsul~onlc acid gm 16 16 16 16 16 16 16 Water to 1 llter Tlme(2) to etch copper , at 75F (min) 73 4 31 26 24 11 11 I at 120F (mln) 13 6 ll 3 3 5 8 ¦ Hydrogen peroxide l, con~umptlon during etching(percent) 7.33.43.3 5.6 4.26.4 11 : .
J~attery grad~ or better.
' (2)~y immer~lon o~ 1.4 mll copper clad epoxy ~ubstrate ; 25 l mea~urlng 3"x3" ln etchant.

The above illu~trate~ the er~cts of molybdenum concen--~ tratlon both on ~tch rate ancl peroxlde con~umptlon.
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1,, ~IÇ92 Example 8 The etchant or Example 5 was used to contlnuously etch copper untll the dl~solved copper content exceeded 70 gram~ per ~ llter, During thi~ tl~e, addltlon~ of peroxlde and sulrurlc acld l, were made as needed ln order to achleve an acceptable etch rate at 120F. Therearter, the bath was cooled to about 70F and allowed to stand overnlght, durln~ whlch perlod Or tlme, crystal~
ormed ln ~lgnl~icant quantlty which were belleved to be copper sulrate pentahydrate.

1 ample~ 9 & 10 Using Example 5 as a base formulatlon, other aclds were ~ubstltuted ~or sulrurlc acld. The aclds used, the a~ounts and the result~ obtalned are a~ set rorth ln the ~ollowlng Il table:
Tlme to etch 1 gm copper (mln) Acid Amt(ml) at 75F at 120F
~ Hydrochloric (37~) 16618 6 1/2 : 1 Methane ~ulfonic (50%) 188 9 2 , From the above,lt 1~ apparent that oth~r acld~ may b~
.:: 20 ~ubstltuted ror 8ul~urlc rOr purpo~ 0~ etchlng copper.

Example 11 ~: ~ A prlnted circult board 1~ prepared from a coppar clad epoxy G-10 ~ub~trate using the rollowlng ~equencé o~ step~:
I~ (a) Clean t~ ~ubatrate and drlll hole3 at approprlate s~ 25 ~l locatlon~, (b) Metalllze the walls Or the holes by electroles~
platlng lncludlng the 3tepA Or catalysi~ wlth a cataly~t ~uch as Catalyst 6F Or Shlpl~y Company Inc., accelerata wl~h a mlld acld ,l solutlon and alectrol~31y plate copper such as wlth electrole~
copper CP-74 Or Shipley Company Inc.

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(c) Apply a po~ltlve working photoreslst such as AZ-ll9, I expo~e and develop.
: I (d) Electroplate copper to ~ull d031red thlcknes~.
' J (e) Electroplate ~older over the expo~ed copper.
(r) Remove the photore~iAt by di~solutlon ln a 3ultable solvent thereror.
(g) Remove exposed copper by immer~lon Or the board ln tha etchant Or Example 5 until all unwanted copper 1 dls~olved.

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Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An etchant comprising an acid solution of an oxidant combination of a peroxide and molybdenum, the peroxide being present in an amount sufficient to oxidize the molybdenum and provide sustained etching and the molybdenum being present in an amount sufficient to exalt the etch rate to a rate in excess of that achievable by the peroxide alone.

2. The etchant of Claim 1 where the peroxide to molybdenum ratio 18 at least 1 to 1.

3. The etchant of Claim 2 where the concentration of the peroxide varies between 0.1 and 10 moles per liter and the molybdenum concentration varies between 0.01 and 1.0 moles per liter.

4. The etchant of Claim 2 where the concentration of the peroxide varies between 0.5 and 2.5 moles per liter and the concentration of the molybdenum varies between 0.02 and 0.5 moles per liter.

5. The etchant of Claim 2 where the peroxide is hydrogen peroxide.

6. The etchant of Claim 5 where the acid is sulfuric acid.

7. An etchant comprising an acid solution of an oxidizing combination of hydrogen peroxide and molybdenum, the peroxide being present in an amount of from about 0.1 to 10 moles per liter and the molybdenum being present in an amount of from about 0.01 to 1 moles per liter, the ratio of the peroxide to the molybdenum being at least 1 to 1.

8. The etchant of Claim 7 where the acid is sulfuric acid.

9. The etchant of Claim 8 where the concentration of the acid varies between 0.01 and 2.5 Normal, the concentration of the peroxide varies between 0.1 and 10 moles per liter and the concentration of the molybdenum varies between 0.01 and 1.0 moles per liter.

10. The etchant of Claim 9 where the concentration of the acid varies between 0.5 and 1.5 Normal, the concentration of the peroxide varies between 0.5 and 2.5 moles per liter, the concentration of the molybdenum varies between 0.02 and 0.5 moles per liter and the ratio of the peroxide to the molybdenum is at least 1.2 to 1.

11. An etchant comprising an oxidant combination in a sulfuric acid solution, said oxidant combination comprising hydrogen peroxide and molybdenum, the sulfuric acid concentra-tion varying between 0.01 and 2.5 moles per liter, the hydrogen peroxide concentration varying between about 0.1 and 10 moles per liter, the molybdenum concentration varying between about 0.02 and 0.5 moles per liter and the ratio of the peroxide to the molybdenum being at least 1 to 1.

12, The etchant of Claim 11 where the concentration of the acid varies between 0.5 and 1.5 Normal, the concentration of the peroxide varies between 0.5 and 2.5 moles per liter, the concentration of the molybdenum varies between 0.02 and 0.5 moles per liter and the ratio of the peroxide to the Molybdenum is at least 1.2 to 1.

13. A process for etching a metal comprising contact of the metal with the etchant of Claim 1.

14. A process for etching a metal comprising contact of the metal with the etchant of Claim 7.

15. A process for etching a metal comprising contact of the metal with the etchant of Claim 11.

16. The process of Claim 15 where the metal is copper.

17. In a process for the manufacture of a printed circuit board including the steps of applying a resist over a copper cladding and removing unwanted copper by etching, the improvement comprising etching said unwanted copper with the etchant of Claim 11 to leave a pattern of copper behind.

18. The process of Claim 17 including the steps of removing said resist and applying solder over the remaining patterned copper.