WARRANT TO PURCHASE STOCK

Exhibit 4.5

THIS WARRANT AND THE SHARES ISSUABLE HEREUNDER HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, PLEDGED, OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT OR PURSUANT TO RULE 144 OR AN OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE CORPORATION AND ITS COUNSEL, THAT SUCH REGISTRATION IS NOT REQUIRED.

WARRANT TO PURCHASE STOCK

Corporation: Rubicon Technology, Inc., a Delaware corporation

Number of Shares:                     

Class of Stock: Series A Preferred Stock

Initial Exercise Price:                     

Issue Date:                     

Expiration Date:                     

THIS WARRANT CERTIFIES THAT, for the agreed upon value of                      and for other good and valuable consideration,                                               (“Holder”) is entitled to purchase the number of fully paid and nonassessable shares of Series A Preferred Stock (the “Shares”) of Rubicon Technology, Inc. (the “Company”) at the initial exercise price per Share of                      (the “Warrant Price”) all as set forth above and as adjusted pursuant to Article 2 of this Warrant, subject to the provisions and upon the terms and conditions set forth in this Warrant.

ARTICLE 1. EXERCISE OF WARRANT, DURATION AND WARRANT PRICE.

1.1 Method of Exercise. Holder may exercise this Warrant by delivering a duly executed Notice of Exercise in substantially the form attached as Appendix 1 to the principal office of the Company at any time until 5:00 p.m., Chicago time, on                     , 20    . Unless Holder is exercising the conversion right set forth in Section 1.2, Holder shall also deliver to the Company a check for the aggregate Warrant Price for the Shares being purchased.

1.2 Conversion Right. In lieu of exercising this Warrant as specified in Section 1.1, Holder may from time to time convert this Warrant, in whole or in part, into a number of Shares determined by dividing (a) the aggregate fair market value of the Shares or other securities otherwise issuable upon exercise of this Warrant minus the aggregate Warrant Price of such Shares by (b) the fair market value of one Share. The fair market value of the Shares shall be determined pursuant to Section 1.3.

1.3 Fair Market Value, If the Shares are traded in a public market, the fair market value of the Shares shall be the closing price of the Shares (or the closing price of the

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Company’s stock into which the Shares are convertible) reported for the business day immediately before Holder delivers its Notice of Exercise to the Company. If the Shares are not traded in a public market, the Board of Directors of the Company shall determine fair market value in its reasonable good faith judgment. The foregoing notwithstanding, if Holder advises the Board of Directors in writing within three days of Holder having received notice from the Company of the fair market value determination of the Board of Directors that Holder disagrees with such determination, then the Company and Holder shall promptly agree upon a reputable investment banking firm to undertake such valuation. If the valuation of such investment banking firm is greater than that determined by the Board of Directors, then all fees and expenses of such investment banking firm shall be paid by the Company. In all other, circumstances, such fees and expenses shall be paid by Holder.

1.4 Delivery of Certificate and New Warrant. Promptly after Holder exercises or converts this Warrant, the Company shall deliver to Holder certificates for the Shares acquired and, if this Warrant has not been fully exercised or converted and has not expired, a new Warrant representing the Shares not so acquired.

1.5 Replacement of Warrants. On receipt of evidence reasonably satisfactory to the Company of the loss, theft, destruction or mutilation of this Warrant and, in the case of loss, theft or destruction, on delivery of an indemnity agreement reasonably satisfactory in form and amount to the Company or, in the case of mutilation, on surrender and cancellation of this Warrant, the Company shall execute and deliver, in lieu of this Warrant, a new warrant of like tenor.

1.6 Assumption Upon Sale, Merger, or Consolidation of The Company.

1.6.1. “Acquisition”. For the purpose of this Warrant, “Acquisition” means any sale, license, or other disposition of all or substantially all of the assets or stock of the Company, or any reorganization, consolidation, or merger of the Company where the holders of the Company’s securities before the transaction beneficially own less than 50% of the outstanding voting securities of the surviving entity after the transaction.

1.6.2. Assumption of Warrant. Upon the closing of any Acquisition the successor entity shall assume the obligations of this Warrant, and this Warrant shall be exercisable for the same securities, cash, and property as would be payable for the Shares issuable upon exercise of the unexercised portion of this Warrant as if such Shares were outstanding on the record date for the Acquisition and subsequent closing. The Initial Exercise Price and/or number of shares shall be adjusted accordingly.

ARTICLE 2. ADJUSTMENTS TO THE SHARES.

2.1 Stock Dividends, Splits, Etc. In case at any time or from time to time while this Warrant remains outstanding, the Company shall, by reclassification, by stock split or reverse stock split, by the issuance of a stock dividend on shares for which this Warrant is then exercisable payable in such shares, or other similar means, subdivide or combine the then

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outstanding shares of stock for which this Warrant is then exercisable into a greater or lesser number of such shares, then the number of shares which may be purchased hereunder shall be increased or decreased proportionately (as determined by the Board of Directors of the Company) effective upon consummation of such reclassification, stock split or reverse stock split or stock dividend. When any adjustment is required to the number of shares for which this Warrant is exercisable hereunder, the Warrant Price shall be decreased or increased proportionately (as determined by the Board of Directors of the Company) effective upon such adjustment.

2.2 Reclassification, Exchange or Substitution. Upon any reclassification, exchange, substitution, or other event that results in a change of the number and/or class of the securities issuable upon exercise or conversion of this Warrant, Holder shall be entitled to receive, upon exercise or conversion of this Warrant, the number and kind of securities and property that Holder would have received for the Shares if this Warrant had been exercised immediately before such reclassification, exchange, substitution, or other event. Such an event shall include any mandatory conversion of the outstanding or issuable securities of the Company of the same class or series as the Shares to common stock pursuant to the terms of the Company’s Certificate of Incorporation in effect from time to time (the “Certificate”). The Company or its successor shall promptly issue to Holder a new Warrant for such new securities or other property. The new Warrant shall provide for adjustments which shall be as nearly equivalent as may be practicable to the adjustments provided for in this Article 2 including, without limitation, adjustments to the Warrant Price and to the number of securities or property issuable upon exercise of the new Warrant. The provisions of this Section 2.2 shall similarly apply to successive reclassifications, exchanges, substitutions, or other events.

2.3 Adjustments for Diluting Issuances. In the event of issuance by the Company, after the date of original issuance of this Warrant, of securities at a price per share less than the Warrant Price, then the number of shares of common stock issuable upon conversion of the Shares shall be subject to adjustment, from time to time in the manner set forth in the Certificate. The provisions set forth for the Shares in the Certificate relating to the above in effect as of the Issue Date may not be amended, modified or waived, in any manner which adversely affects the Holder in a manner different than the other holders of Series A Preferred Stock (other than differences resulting solely from the number of shares held) without the prior written consent of Holder.

2.4 No Impairment. The Company shall not, by amendment of its Articles of Incorporation or through a reorganization, transfer of assets, consolidation, merger, dissolution, issue, or sale of securities or any other voluntary action, avoid or seek to avoid the observance or performance of any of the terms to be observed or performed under this Warrant by the Company, but shall at all times in good faith assist in carrying out of all the provisions of this Article 2 and in taking all such action as may be necessary or appropriate to protect Holders rights under this Article against impairment. If the Company takes any action affecting the Shares or its common stock other than as described above that adversely affects Holder’s rights under this Warrant, the Warrant Price shall be adjusted downward and the number of Shares issuable upon exercise of this Warrant shall be adjusted upward in such a manner that the aggregate Warrant Price of this Warrant is unchanged.

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2.5 Fractional Shares. No fractional shares shall be issuable upon exercise or conversion of the Warrant and the number of Shares to be issued shall be rounded down to the nearest whole Share. If a fractional share interest arises upon any exercise or conversion of the Warrant, the Company shall eliminate such fractional share interest by paying Holder an amount computed by multiplying the fractional interest by the fair market value of a full Share as determined under Section 1.3.

2.6 Certificate as to Adjustments. Upon each adjustment of the Warrant Price, the Company at its expense shall promptly compute such adjustment, and furnish Holder with a certificate of its Chief Financial Officer (or in the absence, its President) setting forth such adjustment and the facts upon which such adjustment is based. The Company shall, upon written request, furnish Holder a certificate setting forth the Warrant price in effect upon the date thereof and the series of adjustments leading to such Warrant Price.

ARTICLE 3. REPRESENTATIONS AND COVENANTS OF THE COMPANY.

3.1 Representations and Warranties. The Company hereby represents and warrants to the Holder as follows:

(a) The initial Warrant Price referenced on the first page of this Warrant is not greater than (i) the price per share at which, the Shares were last issued in an arms-length transaction in which at least $500,000 of the Shares were sold and (ii) the fair market value of the Shares as of the date of this Warrant.

(b) All Shares which may be issued upon the exercise of the purchase right represented by this Warrant, and all securities, if any, issuable upon conversion of the Shares, shall, upon issuance, be duly authorized, validly issued, fully paid and nonassessable, and free of any liens and encumbrances except for restrictions on transfer provided for herein or under applicable federal and state securities laws.

(c) The Capitalization Table attached hereto is true and correct.

3.2 Notice of Certain Events. If the Company proposes at any time (a) to declare any dividend or distribution upon its common stock, whether in cash, property, stock, or other securities and whether or not a regular cash dividend; (b) to offer for subscription pro rata to the holders of any class or series of its stock any additional shares of stock of any class or series or other rights; (c) to effect any reclassification or recapitalization of common stock; (d) to merge or consolidate with or into any other corporation, or sell, lease, license, or convey all or substantially all of its assets, or to liquidate, dissolve or wind up; or (e) offer holders of registration rights the opportunity to participate in an underwritten public offering of the Company’s securities for cash then, in connection with each such event, the Company shall give Holder (1) at least 20 days prior written notice of the date on which a record will be taken for

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such dividend, distribution, or subscription rights (and specifying the date on which the holders of common stock will be entitled thereto) or for, determining rights to vote, if any, in respect of the matters referred to in (c) and (d) above; (2) in the case of the matters referred to in (c) and (d) above at least 20 days prior written notice of the date when the same will take place (and specifying the date on which the holders of common stock will be entitled to exchange their common stock for securities or other property deliverable upon the occurrence of such event); and (3) in the case of the matter referred to in (e) above, the same notice as is given to the holders of such registration rights. The Holder is not entitled to vote and is not entitled to any rights as a shareholder of the Company until this Warrant is exercised.

3.3 Information Rights. So long as the Holder holds this Warrant and/or any of the Shares, the Company shall deliver to the Holder (a) promptly after mailing, copies of all notices or other written communications to all of any class or series of any class of the stockholders of the Company, (b) within 150 days after the end of each fiscal year of the Company, the annual audited financial statements of the Company certified by independent public accountants of recognized standing and (c) such other financial statements required under and in accordance with any loan documents between Holder and the Company (or if there are no such requirements or if the subject loan(s) no longer are outstanding), then within 30 days after the end of each of the first three quarters of each fiscal year, the Company’s quarterly, unaudited financial statements.

3.4 Registration Under Securities Act of 1933, as amended. The Company agrees that the Shares or, if the Shares are convertible into common stock of the Company, such common stock, shall be subject to the registration rights set forth on Exhibit “A” attached hereto.

ARTICLE 4. REPRESENTATIONS, WARRANTIES OF THE HOLDER. The Holder represents and warrants to the Company as follows:

4.1 Purchase for Own Account. Except for transfers to Holder’s affiliates, this Warrant and the securities to be acquired upon exercise of this Warrant by the Holder will be acquired for investment for the Holder’s account, not as a nominee or agent, and not with a view to the public resale or distribution within the meaning of the 1933 Act, and the Holder has no present intention of selling, granting any participation in, or otherwise distributing the same. If not an individual, the Holder also represents that the Holder has not been formed for the specific purpose of acquiring this Warrant or the Shares.

4.2 Disclosure of Information. The Holder has received or has had full access to all the information it considers necessary or appropriate to make an informed investment decision with respect to the acquisition of this Warrant and its underlying securities. The Holder further has had an opportunity to ask questions and receive answers from the Company regarding the terms and conditions of the offering of this Warrant and its underlying securities and to obtain additional information (to the extent the Company possessed such information or could acquire it without unreasonable effort or expense) necessary to verify any information furnished to the Holder or to which the Holder has access.

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4.3 Investment Experience. The Holder understands that the purchase of this Warrant and its underlying securities involves substantial risk. The Holder: (i) has experience as an investor in securities of companies in the development stage and acknowledges that the Holder is able to fend for itself, can bear the economic risk of such Holder’s investment in this Warrant and its underlying securities and has such knowledge and experience in financial or business matters that the Holder is capable of evaluating the merits and risks of its investment in this Warrant and its underlying securities and/or (ii) has a preexisting personal or business relationship with the Company and certain of its officers, directors or controlling persons of a nature and duration that enables the Holder to be aware of the character, business acumen and financial circumstances of such persons.

4.4 Accredited Investor Status. The Holder is an “accredited investor” within the meaning of Regulation D promulgated under the 1933 Act.

4.5 Stockholders’ Agreement. In connection with any exercise of this Warrant, the Company may require Holder to enter into and become bound by that certain Stockholders’ Agreement dated as of February 16, 2001 among the Company and its stockholders, as the same may be amended from time to time or superceded by any other agreement among the Company and all of its stockholders.

ARTICLE 5. MISCELLANEOUS.

5.1 Term. This Warrant is exercisable, in whole or in part, at any time and from time to time on or before the Expiration Date set forth above,

5.2 Legends. This Warrant and the Shares (and the securities issuable, directly or indirectly, upon conversion of the Shares, if any) shall be imprinted with a legend in substantially the following form:

THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, PLEDGED OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT OR PURSUANT TO RULE 144 OR AN OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE CORPORATION AND ITS COUNSEL THAT SUCH REGISTRATION IS NOT REQUIRED.

5.3 Compliance with Securities Laws on Transfer. This Warrant and the Shares issuable upon exercise of this Warrant (and the securities issuable, directly or indirectly, upon conversion of the Shares, if any) may not be transferred or assigned in whole or in part without compliance with applicable federal and state securities laws by the transferor and the transferee (including, without limitation, the delivery of investment representation letters and legal opinions reasonably satisfactory to the Company, as reasonably requested by the Company). The Company shall not require Holder to provide an opinion of counsel if the transfer is to an affiliate of Holder or if there is no material question as to the availability of

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current information as referenced in Rule 144(c), Holder represents that it has complied with Rule 144(d) and (e) in reasonable detail, the selling broker represents that it has complied with Rule 144(d) and the Company is provided with a copy of Holder’s notice of proposed sale.

5.4 Transfer Procedure. Subject to the provisions of Section 4.3. Holder may transfer all or part of this Warrant or the Shares issuable upon exercise of this Warrant (or the securities issuable, directly or indirectly, upon conversion of the Shares, if any) at any time to Silicon Valley Bancshares or The Silicon Valley Bank Foundation, or to any affiliate of Holder, or, to any other transferee by giving the Company notice of the portion of the Warrant being transferred setting forth the name, address and taxpayer identification number of the transferee and surrendering this Warrant to the Company for reissuance to the transferee(s) (and Holder if applicable). Unless the Company is filing financial information with the Securities and Exchange Commission pursuant to the Securities Exchange Act of 1934, the Company shall have the right to refuse to transfer any portion of this Warrant to any person who directly competes with the Company.

5.5 Notices. All notices and other communications from the Company to the Holder, or vice versa, shall be deemed delivered and effective when given personally or mailed by first-class registered or certified mail, at such address as may have been furnished to the Company or the Holder, as the case may be, in writing by the Company or such holder from time to time. All notices to be provided under this Warrant shall be sent to the following address:

5.6 Waiver. This Warrant and any term hereof may be changed, waived, discharged or terminated only by an instrument in writing signed by the party against which enforcement of such change, waiver, discharge or termination is sought.

5.7 Attorneys Fees. In the event of any dispute between the parties concerning the terms and provisions of this Warrant, the party prevailing in such dispute shall be entitled to collect from the other party all costs incurred in such dispute, including reasonable attorneys’ fees.

5.8 Governing Law. This Warrant shall be governed by and construed in accordance with the laws of the State of Illinois, without giving effect to its principles regarding conflicts of law.

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APPENDIX 1

NOTICE OF EXERCISE:

1. The undersigned hereby elects to purchase                      shares of the Preferred Series A Stock of Rubicon Technology, Inc. pursuant to the terms of the attached Warrant, and tenders herewith payment of the purchase price of such shares in full.

or

l. The undersigned hereby elects to convert the attached Warrant into Shares in the manner specified in the Warrant. This conversion is exercised with respect to of the Shares covered by the Warrant.

2. Please issue a certificate or certificates representing said shares in the name of the undersigned or in such other name as is specified below:

3. The undersigned represents it is acquiring the shares solely for its own account and not as a nominee for any other party and not with a view toward the resale or distribution thereof except in compliance with applicable securities laws.

    (Date)

EXHIBIT A

Registration Rights

The Shares (if common stock), or the common stock issuable upon conversion of the Shares, shall be entitled to registration rights in accordance with the terms of the following agreement (the “Agreement’) between the Company and its investor(s):

The Company agrees that no amendments will be made to the Agreement which would have an adverse impact on Holder’s registration rights in a manner different that the other parties to the Agreement without the consent of Holder. By exercise of this Warrant to which this Exhibit A is attached, Holder agrees to be bound by, and shall be deemed to be a party to, the Agreement.