Thanks alot Anthony,
I found a solution myself (based on yours):
I needed some time to find out that the first character of a string
doen`s have the index '0'  ( compared to common programming languages)
but has the index '1'.
This is the reason I pass '1' in next_char_index as initial parameter
for the
string _to_char template.

The "calling template":
<xsl:template ... >
 .....
  <xsl:call-template name="string_to_char">
        <xsl:with-param name="next_char_index" select="'1'"/>
  </xsl:call-template>
.....
</xsl:template ... >


  <xsl:template name="string_to_char">
    <xsl:param name="next_char_index" />
    <xsl:variable name="string_length" select="string-length(@value)"/>
    <xsl:variable name="string" select="@value"/>
    <xsl:if test="$string_length >= $next_char_index">
      <xsl:element name="char">
        <xsl:attribute name="value">
          <xsl:value-of
select="substring($string,$next_char_index,1)"/>
        </xsl:attribute>
      </xsl:element>
      <xsl:call-template name="string_to_char">
        <xsl:with-param name="next_char_index"
select="$next_char_index+1"/>
      </xsl:call-template>
    </xsl:if>
  </xsl:template>

cheers,
Boris