<PAGE>
                                                             Exhibit 10.15


                     AMENDMENT NO. 3 TO OPTION AGREEMENT II

* Portions of this document have been omitted and filed separately with the
Securities and Exchange Commission pursuant to a request for confidential
treatment.

                                    AMENDMENT

This Amendment No. 3 to Option Agreement II (the "Amendment No. 3") is effective
as of June 16, 2000 (the "Effective Date") by and between Adaptec Manufacturing
(S) Pte. Ltd., a company organized under the laws of Singapore, with its current
registered address at 6 Battery Road, 532-00, Singapore 049909 ("Customer"), and
Taiwan Semiconductor Manufacturing Co., Ltd., a company organized under the laws
of the Republic of China, with its registered address at No. 121 Park Avenue 3,
Science-Based Industrial Park, Hsin-Chu, Taiwan, Republic of China ("TSMC"), for
the purpose of amending the Option Agreement II entered into by Customer and
TSMC on October 23, 1995 (the "Option Agreement II") as follows:

1.       Replace Section 7(a) with the followings:

         "The term of this Agreement  shall  commence from the Effective  Date,
         and continue until December 31, 2004."

2.       Add the followings at Section 20:

         "Section 20       Non-Publicity

         (a) No publicity or information regarding the existence or contents of
         this Agreement shall be given or released by either party in any case,
         other than as required by law. In the event that any applicable law or
         regulation  requires the disclosure of this Agreement,  the disclosing
         party  must  provide  details  of  the  disclosure  request  prior  to
         submission of such information to the requesting authority.

         (b) The  exchange of  information  by either party for the purposes of
         review  of the  contents  prior  to  disclosure  as  described  in the
         foregoing  Section  20(a)  will be  handled  directly  by the TSMC and
         Customer    representatives    responsible   for   official    company
         communications and public relations:

         For TSMC

         Name:                      Ronald C. Norris
         Title:                     Senior Vice President, Worldwide Marketing
                                    and Sales
         Address:                   No. 121, Park Avenue 3, Science-Based
                                    Industrial Park, Hsin-Chu,
                                    Taiwan, Republic of China
         Telephone No:              011-886-3-578-0221
         Fax No:                    011-886-3-578-1545

                                    Cc to:

         Name:                      David Keller
         Title:                     Senior Director, Account Management, TSMC NA
         Address:                   1740 Technology Drive, Suite 660, San Jose,
                                    CA 95110, USA
         Telephone No:              408-487-4139
         Fax No:                    408-441-7713

         For Customer


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<PAGE>

         Name:                      Dolores Marciel
         Title:                     Vice President of Worldwide Material
         Address:                   691 South Milpitas Boulevard, Milpitas,
                                    CA 95035, USA
         Telephone No:              408-945-8600
         Fax No:                    408-957-8227

         (c) In the event that  either  party  breaches  Section  20(a) of this
         Agreement,  the  non-breaching  party  may  terminate  this  Agreement
         immediately  with a 180 days  written  notice to the  breaching  party
         pursuant to Section 7(c) of this Agreement."

3.       Replace Exhibits B and D with the new Exhibits B and D attached hereto.

4.       Customer  agrees to pay to TSMC in cash the entire  Option Fee for the
         year 2003 and 2004,  total in the amount of US$20,000,000 as set forth
         in the new Exhibit D, by June 16, 2000.

5.       All the other terms and  conditions  of the Option  Agreement  II shall
         remain  unchanged  to the  extent  not in  conflict  with the terms and
         conditions in this Amendment No. 3.


Taiwan Semiconductor                        Adaptec Manufacturing (S) Pte. Ltd.
Manufacturing Co., Ltd.




/s/ Ronald C. Norris                        /s/ Kok Yong Lim
------------------------------------        ------------------------------------
Name:    Ronald C. Norris                   Name:    Kok Yong Lim
Title:   Senior Vice President              Title:   Vice President
         Worldwide Marketing & Sales                 Manufacturing



                                       2
<PAGE>


                                    Exhibit B
                        CUSTOMER/TSMC COMMITTED CAPACITY


                           Unit: K 6" Wafer Equivalent

<TABLE>
<CAPTION>
                                                     2000       2001       2002       2003       2004
                                                     ----       ----       ----       ----       ----
<S>                                                  <C>        <C>        <C>        <C>        <C>

Base Capacity                                        [*]        [*]        [*]        [*]        [*]
(For Options)

X% of Base                                           [*]        [*]        [*]        [*]        [*]
Capacity

Option                                               [*]        [*]        [*]        [*]        [*]

TSMC Committed Capacity                              [*]        [*]        [*]        [*]        [*]
(Base Capacity + Option Capacity)

Customer Committed Capacity                          [*]        [*]        [*]        [*]        [*]
(X% Base Capacity + Option Capacity)

</TABLE>

Option Capacity [*] wafers times [*] per wafer = US $33.6 Million for years 1999
- 2002
Option Capacity [*] wafers times [*] per wafer = US $20 Million for years 2003
and 2004

[*]  Confidential  information  has been omitted and filed  separately  with the
Securities  and  Exchange  Commission  pursuant  to a request  for  confidential
treatment.




                                       3
<PAGE>

                                    Exhibit D
                                   OPTION FEE

<TABLE>
<CAPTION>
Year          Option Capacity                    Option Fee                     Due Date
              (Unit: Wafer Equivalent)           (Unit: US$)
----------------------------------------------------------------------------------------------
<S>           <C>                                <C>                            <C>
2000          [*]                                $8.4M                          Paid

2001          [*]                                $8.4M                          Paid

2002          [*]                                $8.4M                          Paid

2003          [*]                                $10M                           June 16, 2000

2004          [*]                                $10M                           June 16, 2000

</TABLE>

[*]  Confidential  information  has been omitted and filed  separately  with the
Securities  and  Exchange  Commission  pursuant  to a request  for  confidential
treatment.



                                       4