Process hangs in background

Question

I am bit of a confusion here. I let the user manually close Excel but still a process remains on the background? May you kindly help me resolve this issue. In my own understanding, I tried releasing COM objects as well with it? Also I would like to know what errors have I've been doing with it. I appreciate any help. Thank you so much.

Private Sub releaseComObject(ByVal obj As Object)
    Try
        System.Runtime.InteropServices.Marshal.FinalReleaseComObject(obj)
        obj = Nothing
    Catch ex As Exception
        obj = Nothing
    Finally
        GC.Collect()
    End Try
End Sub


Private Sub BtnExport_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnExport.Click
    'verfying the datagridview having data or not
    If ((DataGridView1.Columns.Count = 0) Or (DataGridView1.Rows.Count = 0)) Then
        Exit Sub
    End If
    BtnExport.Text = "Please Wait..."
    BtnExport.Enabled = False
    'Creating dataset to export
    Dim dset As New DataSet
    'add table to dataset
    dset.Tables.Add()
    'add column to that table
    For i As Integer = 0 To DataGridView1.ColumnCount - 1
        dset.Tables(0).Columns.Add(DataGridView1.Columns(i).HeaderText)
    Next
    'add rows to the table
    Dim dr1 As DataRow
    For i As Integer = 0 To DataGridView1.RowCount - 1
        dr1 = dset.Tables(0).NewRow
        For j As Integer = 0 To DataGridView1.Columns.Count - 1
            dr1(j) = DataGridView1.Rows(i).Cells(j).Value
        Next
        dset.Tables(0).Rows.Add(dr1)
    Next
    Dim excel As New Excel.Application
    Dim wBook As Excel.Workbook
    Dim wSheet As Excel.Worksheet

    wBook = excel.Workbooks.Add()
    wSheet = CType(wBook.ActiveSheet(), Microsoft.Office.Interop.Excel.Worksheet)
    Dim dt As System.Data.DataTable = dset.Tables(0)
    Dim dc As System.Data.DataColumn
    Dim dr As System.Data.DataRow
    Dim colIndex As Integer = 0
    Dim rowIndex As Integer = 0
    For Each dc In dt.Columns
        colIndex = colIndex + 1
        excel.Cells(1, colIndex) = dc.ColumnName
    Next
    For Each dr In dt.Rows
        rowIndex = rowIndex + 1
        colIndex = 0
        For Each dc In dt.Columns
            colIndex = colIndex + 1
            excel.Cells(rowIndex + 1, colIndex) = dr(dc.ColumnName)
        Next
    Next
    wSheet.Columns.AutoFit()
    excel.Visible = True
    BtnExport.Text = "    Export"
    BtnExport.Enabled = True

    releaseComObject(wSheet)
    releaseComObject(wBook)
    releaseComObject(excel)

End Sub