How to modify this code to make my StreamWriter file READ ONLY

' This Sub uses a StreamWriter object to create a .txt file 
    'With Text coming out of the Serial port. The name of the file
    'is composed of the Machine Serial#, Date and Time String (time ‘includes seconds)
    'Resulting file name 4025+8_4_2003+15_22_56

    Sub SerialCopyWriter()

        Dim myStreamWriter As StreamWriter
        Dim myPath As String = "..\Readings Backup\"
        Dim myMachine As Integer = 3035
        Dim myName As String = myMachine & "+" & Now.Month & "_" & Now.Day & "_" _
        & Now.Year.ToString & "+" & Now.Hour & "_" & Now.Minute & "_" & Now.Second & ".txt"
        Dim myFileName As String = myPath + myName

        Try
            ' Create the StreamWriter
            myStreamWriter = File.CreateText(myFileName)
            myStreamWriter.Write(txtFileText.Text)
            myStreamWriter.Flush()

        Catch exc As Exception
            ' Show the error to the user.
            MsgBox("A copy of the serial port Data-Reading for this Coffee Machine could not be Created:" + _
                vbCrLf + vbCrLf + "Exception: " + exc.Message)
        Finally

            If Not myStreamWriter Is Nothing Then
                myStreamWriter.Close()
            End If
        End Try
    End Sub