EXHIBIT 10.18

                                                    LOAN MODIFICATION AGREEMENT


         This  agreement  amends the  Promissory  Note dated  September 30, 1996
("Note")  executed by  SEMITOOL,  INC  ("Borrower")  in favor of Bank of America
National  Trust  and  Savings  Association,  doing  business  as  Seafirst  Bank
("Bank"),  regarding a loan in the maximum  principal  amount of  $15,000,000.00
(the  "Loan").  For mutual  consideration,  Borrower and Bank agree to amend the
above loan documents as follows:

         1. Payments. Beginning on the 31st day of October, 1997, Borrower shall
make monthly  interest-only  payments  equal to all accrued  interest  under the
Note, with all other payment-related provisions of the Note remaining unchanged.

         2.       Maturity  Date.  The maturity date of the Note is changed to 
December 31, 1999.  Bank's  commitment  to make advances to Borrower  under its
line of credit is also extended to December 31, 1999.

         3. Other Terms. Except as specifically amended by this agreement or any
prior amendment,  all other terms,  conditions,  and definitions of the Note and
all other security agreements,  guaranties, deeds of trust, mortgages, and other
instruments  or agreements  entered into with regard to the Loan shall remain in
full force and effect.

     4. Commitment Fee. A Commitment fee of .125 percentage points of the unused
portion of the commitment shall be collected quarterly.

         DATED SEPTEMBER 29, 1997.


Bank:                                          Borrower:

SEAFIRST BANK                                  SEMITOOL, INC

By     /s/Joe Poole                            By: /s/Raymon F. Thompson
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Title    VP                                       Raymon F. Thompson, President