WARRANT NO to purchase Series E Convertible Preferred Stock of RUBICON TECHNOLOGY, INC.

Exhibit 4.15

THE ISSUANCE OF THIS WARRANT AND THE SECURITIES ISSUABLE PURSUANT TO THE TERMS HEREOF HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR UNDER ANY STATE OR OTHER SECURITIES LAWS. NEITHER THE SECURITIES NOR ANY INTEREST THEREIN MAY BE OFFERED, SOLD, TRANSFERRED, ASSIGNED OR OTHERWISE DISPOSED OF EXCEPT (I) PURSUANT TO EFFECTIVE REGISTRATIONS UNDER APPLICABLE SECURITIES LAWS OR (II) UPON DELIVERY TO THE COMPANY OF AN OPINION OF COUNSEL (OR OTHER EVIDENCE SATISFACTORY TO THE COMPANY) ACCEPTABLE TO THE COMPANY, THAT THE PROPOSED DISPOSITION MAY BE EFFECTED IN COMPLIANCE WITH APPLICABLE SECURITIES LAWS WITHOUT REGISTRATION. ADDITIONAL RESTRICTIONS ON TRANSFER ARE SET FORTH HEREIN.

WARRANT NO. 29

(the “ Warrant ”)

to purchase Series E Convertible Preferred Stock

of

RUBICON TECHNOLOGY, INC.

THIS IS TO CERTIFY THAT, for value received, including the payment of Ten Dollars ($10.00) and other good and valuable consideration the amount and sufficiency is hereby acknowledged, Lighthouse Capital Partners IV, L.P., or its registered assigns (the “ Holder ”), is (subject to the restrictions provided herein) entitled to purchase from Rubicon Technology, Inc., a Delaware corporation (the “ Company ”), at any time or from time to time and prior to 5:00 p.m. local time at the location of the Warrant Office (as defined in Section 2.1) on December 20, 2015, at the Warrant Price, Three Hundred and Seventeen Thousand Six Hundred and Ninety-six (317,696) shares of Series E Convertible Preferred Stock, par value $.001 per share (“ Series E Convertible Preferred Stock ”), subject to Section 1.3 below with respect to any fractional amount and subject to adjustment as provided herein, and is also entitled to exercise the other rights, powers and privileges hereinafter set forth. Capitalized terms used in this Warrant but not defined in the context thereof shall have the meanings specified in Article V.

ARTICLE I

EXERCISE OF WARRANT

1.1 Method of Exercise . To exercise this Warrant in whole or in part, the Holder shall deliver to the Company at the Warrant Office (i) this Warrant together with the subscription notice attached hereto as Exhibit A (the “ Subscription Notice ”) filled out and duly executed by the Holder indicating the Holder’s election to exercise this Warrant and specifying the number of shares of Series E Convertible Preferred Stock to be purchased; and (ii) a certified or bank cashier’s check payable to the order of the Company in an amount equal to the aggregate Warrant Price for the number of shares of Series E Convertible Preferred Stock being purchased. Subject to the restrictions provided herein, the Company shall as promptly as practicable, and in any event within 14 days thereafter, execute and deliver or cause to be executed and delivered, in accordance with the Subscription Notice, a single certificate in the name of the Holder representing the aggregate number of shares of Series E Convertible Preferred Stock specified in the Subscription Notice. Such certificate shall be deemed to have been issued, and the Holder shall be deemed for all purposes to have become a holder of record of such shares, as of the date the Subscription Notice is actually received by the Company with payment as provided above. If this Warrant shall have been exercised only in part, the Company shall, at the time of delivery of such certificate or certificates, deliver to the Holder a new Warrant evidencing the rights of the Holder to purchase the remaining shares of Series E Convertible Preferred Stock called for by this Warrant, which new Warrant shall in all other respects be identical with this Warrant, or at the request of the Holder, appropriate notation may be made on this Warrant and the same returned to the Holder. The Company shall pay all expenses, taxes and other charges payable in connection with the preparation, issuance and delivery of such stock certificates and new Warrants.

1.2 Right to Convert Warrant into Stock: Net Issuance .

(a) Right to Convert . In addition to and without limiting the rights of the Holder under the

terms of this Warrant, the Holder shall have the right to convert this Warrant or any portion hereof into shares of Series E Convertible Preferred Stock as provided in this Section 1.2 at any time that this Warrant is otherwise exercisable during the term of this Warrant (the “ Conversion Right ”). Upon exercise of the Conversion Right, the Company shall deliver to the Holder (without payment by the Holder of the Warrant Price) that number of fully paid and nonassessable shares of Series E Convertible Preferred Stock equal to the quotient of (i) the number of shares of Series E Convertible Preferred Stock purchasable under this Warrant (or the portion thereof being exercised); multiplied by the difference of (A) the Fair Market Value of one (1) share of Series E Convertible Preferred Stock; minus (B) the Warrant Price; divided by (ii) the Fair Market Value of one (1) share of Series E Convertible Preferred Stock.

(b) Method of Exercise . The Conversion Right may be exercised by the Holder by the surrender of this Warrant, at the Warrant Office, together with the Subscription Notice specifying that the Holder thereby intends to exercise the Conversion Right and indicating the number of shares of Series E Convertible Preferred Stock with respect to which the Holder intends to exercise this Warrant. The Holder shall also deliver therewith additional consideration, if any, such that the aggregate consideration received by the Company in respect of any shares of Series E Convertible Preferred Stock is at least equal to the par value of the shares of Series E Convertible Preferred Stock. The conversion shall be effective on the date which is the later of (i) receipt by the Company of the items described above; or (ii) a date specified in the Holder’s notice to the Company.

1.3 No Fractional Shares to be Issued . The Company shall not be required upon any exercise or conversion of this Warrant to issue a certificate representing any fraction of a share, but, in lieu thereof, may pay to the Holder cash in an amount equal to a corresponding fraction (calculated to the nearest 1/100th of a share) of the Fair Market Value of one share of Series E Convertible Preferred Stock as of the date of receipt by the Company of notice of exercise of this Warrant.

1.4 Legend on Shares . Each certificate for shares of Series E Convertible Preferred Stock initially issued upon exercise of this Warrant, unless at the time of exercise such shares of Series E Convertible Preferred Stock are registered under the Securities Act of 1933, as amended (the “ Act ”) and applicable state securities laws, shall bear a legend in substantially the following form (and any additional legend required by law or by any securities exchange upon which such shares of Series E Convertible Preferred Stock may, at the time of such exercise, be listed):

THE ISSUANCE OF THE SHARES EVIDENCED HEREBY WAS NOT REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR UNDER ANY STATE OR OTHER SECURITIES LAWS. NEITHER THE SHARES NOR ANY INTEREST THEREIN MAY BE OFFERED, SOLD, TRANSFERRED, ASSIGNED OR OTHERWISE DISPOSED OF EXCEPT (I) PURSUANT TO EFFECTIVE REGISTRATIONS UNDER APPLICABLE SECURITIES LAWS OR (II) UPON DELIVERY TO THE CORPORATION OF AN OPINION OF COUNSEL (OR OTHER EVIDENCE SATISFACTORY TO THE CORPORATION) ACCEPTABLE TO THE CORPORATION, THAT THE PROPOSED DISPOSITION MAY BE EFFECTED IN COMPLIANCE WITH APPLICABLE SECURITIES LAWS WITHOUT REGISTRATION.

THE CORPORATION WILL FURNISH WITHOUT CHARGE TO EACH STOCKHOLDER WHO SO REQUESTS, A FULL SUMMARY OR STATEMENT OF ALL OF THE POWERS, DESIGNATIONS, PREFERENCES AND RELATIVE, PARTICIPATING, OPTIONAL, OR OTHER SPECIAL RIGHTS OF EACH CLASS OF STOCK OR SERIES THEREOF AND THE QUALIFICATIONS, LIMITATIONS OR RESTRICTIONS OF SUCH POWERS, PREFERENCES AND/OR RIGHTS. ANY SUCH REQUEST SHOULD BE ADDRESSED TO THE SECRETARY OF THE CORPORATION.

Any certificate issued at any time in exchange or substitution for any certificate bearing such legend (except a new certificate issued upon completion of a public distribution pursuant to a registration statement which has become effective under the Act of the securities represented thereby) shall also bear such legend unless, in the opinion of such counsel as shall be approved by the Company, the securities represented thereby need no longer be subject to the restrictions contained in Article III. The provisions of Article III shall be binding upon all subsequent holders of this Warrant.

1.5 Automatic Exercise. Immediately before the expiration or termination of this Warrant, to the extent this Warrant is not previously exercised, and if the Fair Market Value of one share of whichever is applicable of either (i) the Preferred Stock subject to this Warrant or (ii) the Company’s Common Stock issuable upon conversion of the Preferred Stock subject to this Warrant, is greater than the Warrant Price, then in effect as adjusted pursuant to this Warrant, then this Warrant shall be deemed automatically exercised pursuant to Section 1.2(a) above, even if not surrendered. To the extent this Warrant or any portion thereof is deemed automatically exercised pursuant to this Section, the Company agrees to promptly notify the Holder of the number of shares of Series E Convertible Preferred Stock, if any, the holder hereof is to receive by reason of such automatic exercise.

1.6 Registration Rights Agreement . In connection with any exercise of this Warrant, the parties will enter into and become bound by the terms and conditions of the registration rights agreement between the Company and the holders of Series E Convertible Preferred Stock.

1.7 Stockholders’ Agreement . In connection with any exercise of this Warrant, the Company may require that the Holder enter into and become bound by the terms and conditions of any stockholders’ agreement by and among the Company and the stockholders of the Company, provided such terms and conditions are applied to Holder in the same manner as any other stockholder of the Company. Company, within thirty days of Grant Date, and within thirty days of any subsequent revisions to stockholders’ agreement, shall deliver to Holder each stockholders’ agreement as amended and restated from time to time.

1.8 Information. Notwithstanding Section 1.7 above, beginning on the date whereby Company no longer has any indebtedness owed to Holder under any Loan and Security Agreement or Revolver or Other Agreement, and until the earlier of the Expiration Date or the tenth anniversary of the Grant Date, Company shall cause to be furnished to Holder, (i) the audited fiscal year end financial statements of Company no later 120 days after the related fiscal year end, and (ii) internal quarterly financial statements of Company no later than 45 days after the related quarter end. Each financial statement to be furnished to Holder must be prepared in accordance with generally accepted accounting principles, consistently applied.

ARTICLE II

WARRANT OFFICE; TRANSFER

DIVISION OR COMBINATION OF WARRANTS

2.1 Warrant Office . The Company shall maintain an office for certain purposes specified herein (the “ Warrant Office ”), which office shall initially be the Company’s office at 9931 Franklin Avenue, Franklin Park, Illinois 60131 and may subsequently be such other office of the Company or of any transfer agent in the continental United States as to which written notice has been given to the Holder.

2.2 Warrant Non-Transferable; Ownership of Warrant . This Warrant and all rights hereunder may not be transferred, sold, hypothecated or assigned, without the prior written consent of the Company which shall be promptly provided on the condition that Holder has complied with the provisions of Article III hereof. The Company may deem and treat the Person in whose name this Warrant is registered as the holder and owner hereof (notwithstanding any notations of ownership or writing hereon made by anyone other than the Company) for all purposes and shall not be affected by any notice to the contrary, until presentation of this Warrant for registration of transfer as provided in this Article II.

2.3 Warrant Register . Subject to Section 2.2 and Article III hereof, the Company shall maintain at the Warrant Office books for the registration of warrants and the registration of transfers of warrants. To effect a transfer of this Warrant upon satisfaction of the provisions of Section 2.2 and Article III the Holder shall surrender this Warrant at the Warrant Office, together with a written assignment of this Warrant duly executed by the Holder or the Holder’s duly authorized agent or attorney and funds sufficient to pay any transfer taxes payable upon the making of such transfer. Upon such surrender and payment the Company shall execute and deliver a new Warrant or Warrants in the name of the assignee or assignees and in the denominations specified in such instrument of assignment, and this Warrant shall be canceled.

2.4 Division or Combination of Warrants . This Warrant may be divided or combined with other warrants exercisable at the same Warrant Price upon presentation hereof and of any warrant or warrants with which this warrant is to be combined at the Warrant Office, together with a written notice specifying the names and denominations in which new warrants are to be issued, signed by the Holder and the holders thereof or their respective duly authorized agents or attorneys. Subject to compliance with Sections 2.2 and 2.3 and Article III as to any transfer which may be involved in such division or combination, the Company shall execute and deliver a new warrant or warrants in exchange for the warrant or warrants to be divided or combined in accordance with such notice.

2.5 Expenses of Delivery of Warrants . The Company shall pay all expenses, taxes (other than transfer taxes) and other charges payable in connection with the preparation, issuance and delivery of warrants hereunder.

ARTICLE III

RESTRICTIONS ON TRANSFER

3.1 Restrict