c#, Excel + csv: how to get the correct encoding?

I've been trying this for quite a while now, but can't figure it out. I'm trying to export data to Excel via a *.csv file. It works great so far, but I have some encoding problems when opening the files in Excel.

(original string on the left, EXCEL result on the right):

Messwert(µm / m) ==> Messwert(µm / m)

Dümme Mässöng ==> Dümme Mässöng

Notepad++ tells me that the file is encoded "ANSI as UTF8"(WTF?)

So here are different ways I tried to get a valid result: obvious implementation:

tWriter.Write(";Messwert(µm /m)");

more sophisticated one (tried probably a dozen or more encoding combinations:)

tWriter.Write(Encoding.Default.GetString(Encoding.Unicode.GetBytes(";Messwert(µm /m)")));
tWriter.Write(Encoding.ASCII.GetString(Encoding.Unicode.GetBytes(";Messwert(µm /m)")));

and so on

Whole source code for the method creating the data:

    MemoryStream tStream = new MemoryStream();
    StreamWriter tWriter = new StreamWriter(tStream);
    tWriter.Write("\uFEFF");

    tWriter.WriteLine(string.Format("{0}", aMeasurement.Name));
    tWriter.WriteLine(aMeasurement.Comment);
    tWriter.WriteLine();
    tWriter.WriteLine("Zeit in Minuten;Messwert(µm / m)");

    TimeSpan tSpan;
    foreach (IMeasuringPoint tPoint in aMeasurement)
    {
        tSpan = new TimeSpan(tPoint.Time - aMeasurement[0].Time);
        tWriter.WriteLine(string.Format("{0};{1};", (int)tSpan.TotalMinutes, getMPString(tPoint)));
    }

    tWriter.Flush();
    return tStream;

Generated CSV file:

Dümme Mössäng
Testmessung die erste

Zeit in Minuten;Messwert(µm / m)
0;-703;
0;-381;
1;1039;
1;1045;
2;1457;
2;1045;