Murray Hill. New Jersey 07974 I

Fitzgerald B. Brarnwell, Richard E. Zadjura,

and Charles Paley Brooklyn College, CUNY

Brooklyn. New York 11210 and Susan R. Fahrenholtr

Bell Laborator~es

In the last ten years, the use of photoresists has been of great significance in the electronici and communications industries. They are universally used in the fabrication of miniaturized inteerated circuits needed for the oroduction of oocket cal-

A Positive Photoresist

The photochemical Wolff rearrangement

" culators, computers, telecommunications devices, and a host of consumer oriented products and services (1,2). A photo- resist is a mixture of chemicals generally containing a film- forming polymer and one or more photosensitive compounds. The term photoresist arises from the essential photochemical behavior and chemical resistance of this mixture. When ex- posed to light of the proper wavelength, a photoresist under- goes photochemical reactions which alter the molecular structure of some of its components and change its solubility. If exposure to light imparts increased chemical solubility, the photoresist is called a positive photoresist. Conversely, if ex- posure to light imparts reduced solubility, the photoresist is called a negative photoresist.

The fahrication of an integrated circuit is a complex process which depends on the construction of a precise pattern of electronic components, for example, metal-oxide-semicon- ductor (MOS) devices. Critical distances in commercially available integrated circuits are often measured in microme- ters.

The figure schematically shows the key role played by photoresists in the fabrication process for MOS devices. In this example, the photoresist is used to form a well-defined image of silicon dioxide on silicon. The photoresist is uniformly de- posited on a thin layer of silicon dioxide above a silicon suh- strate and irradiated through a mask. Illuminated areas un- dergo a photochemically initiated solubility change. The in- soluble nortion of the ohotoresist Drotects the silicon diox- ide-silicon substrate from chemical attack. The soluble portion of the ohotoresist is chemicallv removed usina the appropriate solvent to leave areas of the &con dioxide-~ilicon&h.&ate which are subject to chemical attack. The exposed silicon dioxide layer can be removed by processing methods such as acid etching, plasma etching, or ion milling to form a well- defined image of silicon dioxide on the silicon suhstrate. Subsequent processing steps in the fahrication of an MOS integrated circuit include doping the silicon dioxide-silicon substrate, e.g. with phosphorous, and vacuum deposition of aluminum andlor other metals.

The use of photoresists and subsequent processing steps is repeated many times in the manufacture of a typical inte- grated circuit which may contain as many as 10,000 electronic components on a few millimeters square silicon wafer. Pres- ently, the image resolution of such photoresists is 2 pm.

We have develoved an undergraduate experiment in which thv formulation and phutorhemicnl twhariur o i a pusiiive photore<ist are used t u illuitri~w the Schorten-Haumann es- teriiiration technique and the Wolft rearrangement. The ohotoresist that i* ftmnul:~ted in this experiment consists of H three component mixture: (1) a solu&on inhihitor; (2) a film-forming resin; (3) a solvent capable of dissolving both the solution inhibitor and the resin. The roles that each of these components plays in determining the photochemical and physical properties of the photoresist are described below.

Solution Inhibitor The solution inhibitor used in positive photoresists is gen-

erally synthesized from derivatives of a diazonaphthoquinone

LIGHT

mILLUMINATED l11M iMASK PHOTORESIST

AREAS SILICON DIOXIDE

NEGATIVE RESIST: POSITIVE RESIST: RENDERED INSOLUBLE RENDEREDSOLUBLE

I I

ETCHED FILM PATTERNS L I

I RESIST REMOVED I Schematic showing me key role played by photaesists in Uw fathcation p m s s tor MOS devices.

sulfonate using the Schotten-Baumann esterification tech- nique. The reaction sequence for the preparation of a typical solution inhibitor, a 1-diazo-1,2 naphthoquinone-5-sulfonate, is

So,a SO,R (11 (11) (1111

Volume 55, Number 6. June 1978 / 403

The reaction seauence used in the experiment begins with - (I).

The Schotten-Baumann esterification technique involves the preparation of an intermediate sulfonyl chloride, (II), which then is converted to an ester, (111). The preparation of the sulfonyl chloride is necessary since sulfonic acids are not reactive enough to he esterfied directly. In the key step of this technique, the sulfonyl chloride reacts with an alcohol in the presence of a hase (e.g., pyridine, aqueous NaOH, or Na~C03) to yield the ester (3).

The Wolff rearrangement is characteristic of diazoketones which in the presence of uv light or heat yield molecular ni- trogen and a reactive ketene. The reactive ketene readily adds water to form a carhoxylic acid (4 ,5 ) .

The photochemical behavior of l-diazo-13-naphthoqui- none-5-sulfonate solution inhihiton or the equivalent 2-diazo compound makes use of this Wolff rearrangement and is de- scribed in the reaction sequence proposed by Siis ( 6 )

Note that upon exposure to light and in the presence of ambient moisture, the hydrophobic solution inhibitor, (III), . -

yields a stoichiometric amount of molecular nitrogen, undergoes a ring contraction, and reacts with water to form a hydrophilic, hase soluble carhoxylic acid, (IV). This photo- chemical reaction is the key step in image formation for a positive photoresist.

Solvent System and Resin

The solvent system is the medium by which the mixture of film forming resin and solution inhibitor are applied to a suhstrate (e.g., quartz, aluminum, zinc). Solvents commonly used include acetone, methoxyethyl acetate (methyl cellosolve acetate), or mixtures of similar solvents.

The resins used in positive photoresists are water insoluble polymers usually made from mixtures of one or more phenols or cresols and formaldehyde. Such polymers are called No- volaks and are commercially prepared following the reaction sequence

These materials are soluble in strongly alkaline solutions (pH 2131, resistant to acids, and generally not photosensi- tive.

Photoresist Photochemical Behavior

The photoresist is formulated hy mixing together solution inhihitor (from 2 3 0 0 0 ) and resin in an appropriate sulvent to form a homogeneous, viscous, tacky material. This material

is uniformly deposited upon a suhstrate and the solvent is evaporated to create a hard, film-forming photoresist. The photoresist is then exposed to a specific pattern of uv light created by a mask (e.g., stencil, key, or token).

The high percentage of solution inhibitor in the photoresist allows photochemical reactions to occur which change the solubility characteristics of the photoresist. Those areas of photoresist which were exposed to light are more readily hase soluble than those areas which were not exposed. Exposed areas consist of phenolic resin and a carboxylic acid, (IV). Unexposed areas consist of the phenolic resin and hydro- phobic diazoketone sulfonate (solution inhibitor), and therefore are more resistant to hase attack.

A photoresist pattern, consequently, is developed by an aqueous hase solution which is strong enough to dissolve ex- posed areas without removina unexposed areas. A deposit of "nexpowd photoresist is l e f ~ u ~ u n ;he auhrtratr. u,hich will duplicate the image of the mask. The suhstratt. may now he

without significant damage to the protected areas.

Experimental

The solution inhibitor, (2-octyl l-diazo-l.2-naphthoquinone-5- sulfonate, (III), (R = 2-oetyl) is synthesized from the commercially available sadium salt, (sodium 1-diazo-1,2-naphthaquinone-5-sul- fonate, (I)),' using the Schotten-Baumann esterification technique as shown in the reaction sequence of eqn. (1).

The ohotoresist is oreoared bv mixine the freshlv svnthesized hv- . . drophohlr dlnmk~rone sulhnate with a commercially availnhlc No- v c h k rrim! The muulrr is unifurmlg dcpwiud on a inirroaopcslide nnd then rrradmtcd wlth amaik in place it..:., strncil, key, r#.ken,or photographic negative), to create a positive image of the ohject.

Procedure Solution Inhibitor

CAUTION! Chlorosulfonie acid is corrosive and reacts vio- lently with water.

The entire procedure must he run in subdued 1ighk3 In a foil cov- ered flask containing 3.0 g of (I),4 carefully add 15 ml of chlarosulfonic acid. The rate of addition should he controlled soas to maintain the reaction temperature below 60°C. Heat the reaction mixture with gentle stirring for 15 min at 65-70% Do not let the temperalure af the reaction mixture rise above 75°C at any time during the experi- ment.5 Chill the flask containing the reaction mixture to below PC.

Dropwise, with gentle stirring, add 100 ml of chilled, deionized water do- the thermometer at a rate such that the temperature of the reaction mixture does not exceed 7 5 T 6

CAUTION! The addition of water to chlorosulfonie acid is violently exothermic and liberates large quantities of gaseous HCI and concentrated HzSOI.

Collect the solid yellow precipitate (II), and carefully dispose of the filtrate. Add to the solid 15 ml of 2-adand and 20 ml of a 10% aqueous snlution nf NaoCOl. Gentlv stir the heteroeeneous reaction mixture . ~~~ ~~ . ~ .. at wtrm rernprrnturc iur!)~~ min. I.rt the rcsc.tim mixture stand i n an icr bath for smeral minute.;, ur u n l d n ?rllc,w suspended d i d 1111) I H = 2-orty11 formi. ('allect rhcsolid,wih ir withcold (-ST1 wawr. air dry, and then wash with petroleum ether.

This salt or the equivalent 2-diazo salt may he purchased from Fairmont Chemical Co., 117 Blanchard Street. Newark, N.J. 07105.

Novolak polymers are available from Haven Chemical, Monmto, and Union Carbide. amone others. ' 1)iamkrtwwi~w pw11~-11IarI? hght smiitiw i n the liquid phale.

Expwurr t a r strong sunlight or t n nl~livrl) intenic fluwwcent lghtmg will produce undesirable photochemical reactions.

Long term storage of the sensitizer and its precursors should be done in the absence of light in a refrigerator. Photoresist solutions shouldnot hestored insealed containers for extended periods of time because of the danger of nitrogen pressure huild-up from dimketone rlernmnosition . . . : . . . . , . . . . . . . . .

'Dlnzokrrmes wdl undrrgu a thermally acrivatrd \Vullf resr- rnnremmt at n slpifirnnt rnre iftheir tempcrnttireexeeedsRO°C. I n t h ~ rwnt, the s v m h ~ q ufthe wluricm inhhitorml~zt he z t ~ m d ngnin with fresh materials.

"he addition is done in this manner to shield the reaction from light.

404 1 Journal of Chemical Education

Preparation of the Photoresist

Combine 0.50 g of Novolak resin and 10.0 mlaf acetone. Gently stir the mixture until a homogeneous solution is achieved. Add 0.20 g of solution inhibitor end continue stirring for several minutes or until the mixture is homogeneous. This reaction mixture must he well shielded from light during and after the addition of the solution in- hibitor.

In subdued lieht. uniformlv snread a thin laver of nhotoresist on .. . . . several glnis slides'nnd allou, rhcm ro thoroughly dry in the ahsen< e of light.^ Hare a mask irurh a* a key, coin, or photogrnphir negnrlr~J

Use etched (frosted) slides if possible. The photoresist appears to adhere to a rough surface better than to a smooth surface.

8 We have used the following procedures for coating the slide (substrate) with photoresist. (1) Dip the slide in asolution of photo- resist end allow it to dry in the absence of light in an oven at 60°C for at least hr. It is important that the slides he completely solvent free before proceeding further. The method is similar to the preparation of thin layer chromatography plates. (2) Spin-cast the slides by mounting the flat side of the slide with adhesive on a motor drive shaft, (e.g., the top of a student centrifuge). Start the slide spinning and add the photoresist dropwise on to the slide. Centrifugal motion will produce a uniform layer of photoresist on the slide. The entire apparatus must be covered with a box to protect the photoresist from stray light and to protect the student from the resultant spin-casting spray. If necessary, dry the slide in an oven at 60°C for about 5 min. We have found this method far superior to the slide dipping meth- od.

Exposure times vary according to the intensity and wavelength of the source and the thickness of the photoresist. A 15-20-min ex- posure from a medium pressure Hg lamp (-70 W) held 3 in. from spin-coated samples will produce sharp images.

'O The rate of development is a function of the thickness of the photoresist which may vary greatly depending on the method of coating. However, with a little practice, excellent images and con- trolled development times may be obtained. If the pH is toolow, the developer will require inordinately long development times. If the pH is too high, the developer will remove the entire photoresist. Images of relatively high resolution can he achieved through judicious choice of developer strength.

l1 A more effective method of development is to place the exposed slide on a spin-casting apparatus, and then coat the slide with the NaOH developer. After 15 see development, spin-east the slide for 20 see to dry it. Repeat until the desired image is obtained.

aver the dry photoresist. Expose each slide (with the mask in place) to a uv s ~ u r c e . ~ Prepare 100 ml of developer-an aqueous NaOH so- lution at 12 < pH < 13. Dip and gently swirl an exposed slide, ad- justing the pH of the developer until development times on the order of 4 min are aehie~ed.'~J' Rinse the slide in dilute acid (pH < 5) to remove any excess base and stop development reactions. Repeat the development procedure for the remaining slides.

Discussion Students with a t least one semester of organic chemistry

lahoratorv experience can comolete this experiment in six hours if ahechanical stirrer a n d a properly shielded uv source are available. An interesting spin-off from this experiment is found in the formulation of experiments which investigate the Wolff rearraneement. Students are encouraged to design, obtain approv& for, and t o execute experiments which (1) quantitatively measure nitrogen evolution from the photolysis of a dilute mixture of solution inhibitor in a ~lvmelwater sol- -. vent; (2) volumetrically analyze this solution for carhoxylic acid. Students are further encouraged to design experiments which offer a more detailed analysis of photolysis products and reauire the use of more so~hist icated soectrosco~ic instru- ments and chromatographic apparatus.

Acknowledgment - We would like t o thank Dr. E. D. Feit of Bell Laboratories

for reading the manuscript in draft form and Professors L. Gortler and I. Kaye for assistance in testing this experiment in the undergraduate oreanic chemistry course a t Brooklyn College of CUNY. Literature Cited ( 1 , l l t l~ingcr. W C.. Yr. .+n<r. A w w 45 ,197.1 'lbompon.L P. snd Kmum.R L A n n

hi8 Mot C.. .fi- 1976 . Clrnr. H 8n.l Cwwlr. I. V , 'Hanrllllrnlk rd'l'hln F.lm T % r h l.<m " ~ E o r o r s hln8rrel.L I e n d l ~ l a n r . K I . Mclirsu H111. Z l w York 1970. Chap 7.

(2) DeForest, W. S.,"PhotorosistMaterL1Isand Pr-aes," Mffimw-HiU, NewYork, 1975, pp. 47-63 and references cited therein.

(3) Morrison. R. T., and Boyd, R. N.,"OwnieChemiatry," 3rd Ed., AUynsnd Bacon, Ine. B o s h . 1973, p p 666.

(4 Smith, P. A. S. "MoleeuleuRearran@menta," (Editor: de Mayo, P.I. Intencience, New York. 1963, Vol. 1. p p 528550and 55&568.

( 5 ) Kirmse, W., "Carbene Chemietrv? 2nd Ed., Academic Press, New York, 1971, pp. 475492.

(6) S&, 0.. JultuaLiebigs Ann. Cham., 656.65 (1944).

Volume 55. Number 6. June 1978 1 405