Exhibit 10.11

LOCK-UP AGREEMENT

June 29, 2006

Ultra Clean Holdings, Inc.
150 Independence Drive
Menlo Park, California 94025
Attention: Clarence L. Granger,
President and Chief Executive Officer

Ladies and Gentlemen:

     Pursuant to the terms of the Agreement and Plan of Merger and
Reorganization dated as of June 29, 2006 (the “Merger Agreement”) among Sieger
Engineering, Inc. (the “Company”), Leonid Mezhvinsky (“Mezhvinsky”), Ultra Clean
Holdings, Inc. (“Parent”), Bob Acquisition Inc., Pete Acquisition LLC, Leonid
and Inna Mezhvinsky as trustees of the Revocable Trust Agreement of Leonid
Mezhvinsky and Inna Mezhvinsky dated April 26, 1988 (the “Mezhvinsky Trust”),
Joe and Jenny Chen as trustees of the Joe Chen and Jenny Chen Revocable Trust
dated 2002, Victor Mezhvinsky, Victor Mezhvinsky as trustee of the Joshua
Mezhvinsky 2004 Irrevocable Trust under Agreement dated June 4, 2004 (the
“Joshua Trust”), David Hongyu Wu and Winnie Wei Zhen Wu as trustees of the Chen
Minors Irrevocable Trust, Frank Moreman, and Leonid Mezhvinsky, as Sellers’
Agent, the undersigned will receive cash and shares of Common Stock, par value
$0.001 per share, of Parent (the “Shares”), in exchange for shares of common
stock of the Company, a majority of which is owned by the undersigned. In order
to induce Parent to enter into the Merger Agreement and in connection with this
letter agreement (this “Agreement”), Parent and the undersigned are also
entering into the Amended and Restated Registration Rights Agreement and the
Amended and Restated Stockholders’ Agreement dated as of the date hereof. Each
of Mezhvinsky, the Mezhvinsky Trust, Victor Mezhvinsky and the Joshua Trust
shall hereinafter be referred to as a “Mezhvinsky Stockholder,” and collectively
as the “Mezhvinsky Stockholders.” Each of Parent and the other parties hereto
hereby agrees as follows:

     1. Until December 26, 2006 (the “Initial Release Date”), each of the
parties hereto (other than Parent) agrees not to sell, offer to sell, contract
to sell, sell any option or contract for the sale or purchase of, lend, enter
into any swap or other arrangement that transfers to another any of the economic
consequences of ownership of, or otherwise dispose of (collectively, “Transfer”)
any Shares.

     2. From the Initial Release Date until June 29, 2008 (the “Second Release
Date”), each Mezhvinsky Stockholder agrees not to Transfer any Shares unless (i)
each such Transfer during that period complies with the volume limitations of
Rule 144 (“Rule 144”) under the Securities Act of 1933, as amended (the
“Securities Act”) and (ii) Transfers made during any consecutive 90-day period
do not exceed 25% of the Shares.

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     3. Notwithstanding anything herein to the contrary, in the event Mezhvinsky
ceases to be (i) an employee or (ii) a director (provided that he has not ceased
to be a director either because he was not elected by the Company's stockholders
after having been nominated to stand for election or because he declined to
stand for election) of Parent, the Mezhvinsky Stockholders may Transfer up to
25% of the Shares in any 90-day period beginning on the later of (i) the Initial
Release Date and (ii) the date Mezhvinsky ceases to be an employee or a director
of Parent.

     4. Beginning on the Second Release Date and thereafter the Stockholders
shall not be subject to any contractual limitation hereunder on their ability to
Transfer any Shares.

     5. Notwithstanding the foregoing, Mezhvinsky acknowledges and agrees that
at all times that he remains an “affiliate” (as defined in Rule 405 under the
Securities Act) of Parent by virtue of his position on the Board of Directors of
Parent or otherwise, he shall be subject to Parent’s insider trading and Section
16 compliance policies.

     6. The undersigned acknowledge that Parent may impose stock transfer
restrictions on the Shares (including placing legends on the Shares indicating
that such Shares are subject to this Agreement) to enforce the provisions of
this Agreement or to the extent that restrictions exist under the Securities
Act. The undersigned further acknowledge that the restrictions imposed by this
Agreement are in addition to any other restrictions imposed on the Transfer of
the Shares pursuant to any other agreement in effect between Parent and the
undersigned or pursuant to applicable law; provided that, if any Shares cease to
be subject to any restrictions on Transfer under the Securities Act or this
Agreement, upon the written request of a Stockholder and the submission of
evidence reasonably satisfactory to Parent of that fact, Parent shall issue to
the Stockholder a new certificate or certificates evidencing those Shares
without any legend or stock transfer restriction that may have been placed
thereon with respect thereto.

     7. For purposes of this Agreement, the Shares shall be deemed to include
all of the Shares received by Leonid and Inna Mezhvinsky as trustees of the
Revocable Trust Agreement of Leonid Mezhvinsky and Inna Mezhvinsky dated April
26, 1988, Victor Mezhvinsky and Victor Mezhvinsky as trustee of the Joshua
Mezhvinsky 2004 Irrevocable Trust under Agreement dated June 4, 2004 (the
“Receiving Parties”) pursuant to the Merger Agreement and all of the Shares
received and Transferred by the Receiving Parties shall be aggregated for
purposes of determining the number of Shares that may be Transferred during any
particular period.

     8. Notwithstanding anything contained herein to the contrary, any party
hereto may, at any time, Transfer any number of Shares to such party’s
“affiliates” (as defined in Rule 405 under the Securities Act), spouse,
ex-spouse, domestic partner, lineal descendant or antecedent, brother or sister,
the adopted child or adopted

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grandchild, or the spouse or domestic partner of any child, adopted child,
grandchild or adopted grandchild of such party, or a trust or trusts for the
benefit of such party or those members of such party’s family specified in this
Section 8; provided that any such transferee shall be subject to the terms of
this Agreement.

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     If the above reflects our agreement with you, please sign in the place
indicated below.

LEONID MEZHVINSKY         /s/ Leonid Mezhvinsky  

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        LEONID AND INNA MEZHVINSKY
AS TRUSTEES OF THE REVOCABLE
TRUST AGREEMENT OF LEONID
MEZHVINSKY AND INNA
MEZHVINSKY DATED
APRIL 26, 1988             By: /s/ Leonid Mezhvinsky  

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  Name: Leonid Mezhvinsky    Title: Trustee             VICTOR MEZHVINSKY      
      By: /s/ Victor Mezhvinsky  

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            VICTOR MEZHVINSKY AS TRUSTEE
OF THE JOSHUA MEZHVINSKY 2004
IRREVOCABLE TRUST UNDER
AGREEMENT DATED JUNE 4, 2004             By: /s/ Victor Mezhvinsky  

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  Name: Victor Mezhvinsky   Title: Trustee

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JOE AND JENNY CHEN AS TRUSTEES OF
THE JOE AND JENNY CHEN REVOCABLE
TRUST DATED 2002             By: Joe Chen  

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  Name: Joe Chen   Title: Trustee             THE CHEN MINOR IRREVOCABLE
TRUST             By: /s/ David Hongyu Wu  

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  Name: David Hongyu Wu   Title: Trustee                   By: /s/ Winnie Wei
Zhen Wu  

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  Name: Winnie Wei Zhen Wu   Title: Trustee                   FRANK MOREMAN    
    /s/ Frank Moreman      

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ACCEPTED AND AGREED TO:   ULTRA CLEAN HOLDINGS, INC.           By: /s/ Clarence
L. Granger  

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  Name: Clarence L. Granger     Title: President, Chief Executive Officer
and Chief Operating Officer

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